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Condensed Matter

New submissions

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New submissions for Fri, 23 Feb 18

[1]  arXiv:1802.07729 [pdf, other]
Title: Facilitated quantum cellular automata as simple models with nonthermal eigenstates and dynamics
Comments: 9 pages, 7 figures; comments welcome
Subjects: Statistical Mechanics (cond-mat.stat-mech); Disordered Systems and Neural Networks (cond-mat.dis-nn); Quantum Physics (quant-ph)

We introduce and describe a class of simple facilitated quantum spin models in which the dynamics is due to the repeated application of unitary gates. The gates are applied periodically in time, so their combined action constitutes a Floquet unitary. The dynamics of the models we discuss can be classically simulated, and their eigenstates classically constructed (although they are highly entangled). We consider a variety of models in both one and two dimensions, involving Clifford gates and Toffoli gates. For some of these models, we explicitly construct conserved densities; thus these models are "integrable." The other models do not seem to be integrable; yet, for some system sizes and boundary conditions, their eigenstate entanglement is strongly subthermal. Some of the models have exponentially many eigenstates in which one or more sites are "disentangled" from the rest of the system, as a consequence of reflection symmetry.

[2]  arXiv:1802.07733 [pdf, other]
Title: Magnetotransport in multi-Weyl semimetals: A kinetic theory approach
Comments: 21 pages including 3 Appendices, 5 figures
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); High Energy Physics - Theory (hep-th)

We study longitudinal magnetotransport in three-dimensional multi-Weyl semimetals, constituted by pairs of (anti)-monopole of arbitrary integer charge $n$, with $n=1,2$ and $3$ in crystalline environment. For any $n>1$, even though the distribution of the underlying Berry curvature in the medium is \emph{anisotropic}, the corresponding intrinsic component of the longitudinal magnetoconductivity (bearing the signature of the chiral anomaly) is \emph{insensitive} to the direction of the external magnetic field ($B$) and increases as $B^2$, at least when it is sufficiently weak. In addition, the magnetoconductivity scales (roughly) as $n^3$ with the monopole charge. However, the net \emph{positive} longitudinal magnetoconductivity receives correction from other sources as well (such as the metallic contribution), which ultimately turns it into a direction dependent quantity for any $n>1$ (neglecting a velocity anisotropy for $n=1$), stemming from the underlying anisotropic quasiparticle dispersion in the system. The predicted scaling of the longitudinal magnetoconductivity and signature of the chiral anomaly can, in principle, be observed in recently proposed candidate materials for multi-Weyl semimetals in various solid state compounds.

[3]  arXiv:1802.07736 [pdf, other]
Title: Quantum field theory of nematic transitions in spin orbit coupled spin-1 polar bosons
Comments: 5 pages main text, 4 figures. 6 pages of supplement
Subjects: Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el)

We theoretically study an ultra-cold gas of spin-1 polar bosons in one spatial dimension which are subject to a quadratic Zeeman field and a Raman induced spin-orbit coupling. Concentrating on the regime in which the background fields can be treated perturbatively we analytically solve the model in its low-energy sector, i.e. we characterize the relevant phases and the quantum phase transitions between them. Depending on the sign of the effective quadratic Zeeman field {\epsilon}, two superfluid phases with distinct nematic order appear. In addition, we uncover a spin-disordered superfluid phase at strong coupling. We employ a combination of renormalization group calculations and duality transformations to access the nature of the phase transitions. At ${\epsilon} =0$, a line of spin-charge separated pairs of Luttinger liquids divides the two nematic phases and the transition to the spin disordered state at strong coupling is of the Berezinskii-Kosterlitz-Thouless type. In contrast, at ${\epsilon} \neq 0$, the quantum critical theory separating nematic and strong coupling spin disordered phases contains a Luttinger liquid in the charge sector that is coupled to a Majorana fermion in the spin sector (i.e. the critical theory at finite {\epsilon} maps to a quantum critical Ising model that is coupled to the charge Luttinger liquid). Due to an emergent Lorentz symmetry, both have the same, logarithmically diverging velocity. We discuss the experimental signatures of our findings that are relevant to ongoing experiments in ultra-cold atomic gases of $^{23}$Na.

[4]  arXiv:1802.07755 [pdf, other]
Title: Emergent light crystal from frustration and pump engineering
Comments: 6 pages, 3 figures
Subjects: Quantum Gases (cond-mat.quant-gas)

We demonstrate how pump engineering drives the emergence of frustration-induced quasi-long-range order in a low-dimensional photonic cavity array. We consider a Lieb chain of nonlinear cavities as described by the Bose-Hubbard model and featuring a photonic flat band in the single-particle spectrum. Incoherent pumping of the Lieb lattice leads to a photonic density-wave which manifests an algebraic decay of correlations with twice the period of the lattice unit cell. This work opens up new directions for the emergence of strongly-correlated phases in quantum optical frustrated systems through pump design.

[5]  arXiv:1802.07764 [pdf]
Title: Integer Quantum Hall Effect in Graphene Channel with p-n Junction at Domain Wall in Ferroelectric Substrate
Comments: 12 pages, 4 figures
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

We revealed that 180 degree ferroelectrics domain walls (FDWs) in a ferroelectric substrate, which induce p-n junctions in a graphene channel, lead to the nontrivial temperature and gate voltage dependences of the perpendicular and parallel modes of the integer quantum Hall effect. In particular the number of perpendicular modes v, corresponding to the p-n junction across the graphene channel varies with gate voltage increase from small integers to higher fractional numbers, e.g.v=1, 1.5, 2, ... 5.1,..9.1, 23 in the vicinity of the transition from ferroelectric to paraelectric phase.These numbers and their irregular sequence principally differ from the sequence of fractional numbers v=1.5,2.5,... reported earlier. The origin of the unusual v-numbers is significantly different numbers of the edge modes, v1 and v2, corresponding to significantly different concentration of carriers in the left (n1) and right (n1) domains of p-n junction boundary. The concentrations n1 and n2 are determined by the gate voltage and polarization contributions, and so their difference originates from the different direction of spontaneous polarization in different domains of ferroelectric substrate. The difference between n1 and n2 disappears with the vanishing of spontaneous polarization in a paraelectric phase. The phase transition from the ferroelectric to paraelectric phase can take place either with the temperature increase (temperature-induced phase transition) or with the decrease of ferroelectric substrate thickness (thickness-induced phase transition).

[6]  arXiv:1802.07765 [pdf, other]
Title: Quantum dynamics in sine-square deformed conformal field theory: Quench from uniform to non-uniform CFTs
Comments: are welcome; 8 pages, 4 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el); High Energy Physics - Theory (hep-th)

In this work, motivated by the sine-square deformation (SSD) for (1+1)-dimensional quantum critical systems, we study the non-equilibrium quantum dynamics of a conformal field theory (CFT) with SSD, which was recently proposed to have continuous energy spectrum and continuous Virasoro algebra. In particular, we study the time evolution of entanglement entropy after a quantum quench from a uniform CFT, which is defined on a finite space of length $L$, to a sine-square deformed CFT. We find there is a crossover time $t^{\ast}$ that divides the entanglement evolution into two interesting regions. For $t\ll t^{\ast}$, the entanglement entropy does not evolve in time; for $t\gg t^{\ast}$, the entanglement entropy grows as $S_A(t)\simeq \frac{c}{3}\log t$, which is independent of the lengths of the subsystem and the total system. This $\log t$ growth with no revival indicates that a sine-square deformed CFT effectively has an infinite length, in agreement with previous studies based on the energy spectrum analysis. Furthermore, we study the quench dynamics for a CFT with M$\ddot{\text{o}}$bius deformation, which interpolates between a uniform CFT and a sine-square deformed CFT. The entanglement entropy oscillates in time with period $L_{\text{eff}}=L\cosh(2\theta)$, with $\theta=0$ corresponding to the uniform case and $\theta\to \infty$ corresponding to the SSD limit. Our field theory calculation is confirmed by a numerical study on a (1+1)-d critical fermion chain.

[7]  arXiv:1802.07783 [pdf]
Title: Multi-Terminal Memtransistors from Polycrystalline Monolayer MoS2
Comments: 22 pages, 4 figures
Journal-ref: Nature 2018
Subjects: Materials Science (cond-mat.mtrl-sci)

In the last decade, a 2-terminal passive circuit element called a memristor has been developed for non-volatile resistive random access memory and has more recently shown promise for neuromorphic computing. Compared to flash memory, memristors have higher endurance, multi-bit data storage, and faster read/write times. However, although 2-terminal memristors have demonstrated basic neural functions, synapses in the human brain outnumber neurons by more than a factor of 1000, which implies that multiterminal memristors are needed to perform complex functions such as heterosynaptic plasticity. Previous attempts to move beyond 2-terminal memristors include the 3-terminal Widrow-Hoff memistor and field-effect transistors with nanoionic gates or floating gates, albeit without memristive switching in the transistor. Here, we report the scalable experimental realization of a multi-terminal hybrid memristor and transistor (i.e., memtransistor) using polycrystalline monolayer MoS2. Two-dimensional (2D) MoS2 memtransistors show gate tunability in individual states by 4 orders of magnitude in addition to large switching ratios with high cycling endurance and long-term retention of states. In addition to conventional neural learning behavior of long-term potentiation/depression, 6-terminal MoS2 memtransistors possess gate-tunable heterosynaptic functionality that is not achievable using 2-terminal memristors. For example, the conductance between a pair of two floating electrodes (pre-synaptic and post-synaptic neurons) is varied by 10X by applying voltage pulses to modulatory terminals. In situ scanning probe microscopy, cryogenic charge transport measurements, and device modeling reveal that bias-induced MoS2 defect motion drives resistive switching by dynamically varying Schottky barrier heights.

[8]  arXiv:1802.07785 [pdf]
Title: Gate-tunable memristors from monolayer MoS2
Subjects: Materials Science (cond-mat.mtrl-sci)

We report here gate-tunable memristors based on monolayer MoS2 grown by chemical vapor deposition (CVD). These memristors are fabricated in a field-effect geometry with the channel consisting of polycrystalline MoS2 films with grain sizes of 3-5 um. The device characteristics show switching ratios up to 500, with the resistance in individual states being continuously gate-tunable by over three orders of magnitude. The resistive switching results from dynamically varying threshold voltage and Schottky barrier heights, whose underlying physical mechanism appears to be vacancy migration and/or charge trapping. Top-gated devices achieve reversible tuning of the threshold voltage, with potential utility in non-volatile memory or neuromorphic architectures.

[9]  arXiv:1802.07790 [pdf]
Title: Spin inversion in graphene spin valves by gate-tunable magnetic proximity effect at one-dimensional contacts
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

Graphene has remarkable opportunities for spintronics due to its high mobility and long spin diffusion length, especially when encapsulated in hexagonal boron nitride (h-BN). Here, for the first time, we demonstrate gate-tunable spin transport in such encapsulated graphene-based spin valves with one-dimensional (1D) ferromagnetic edge contacts. An electrostatic backgate tunes the Fermi level of graphene to probe different energy levels of the spin-polarized density of states (DOS) of the 1D ferromagnetic contact, which interact through a magnetic proximity effect (MPE) that induces ferromagnetism in graphene. In contrast to conventional spin valves, where switching between high- and low-resistance configuration requires magnetization reversal by an applied magnetic field or a high-density spin-polarized current, we provide an alternative path with the gate-controlled spin inversion in graphene. The resulting tunable MPE employing a simple ferromagnetic metal holds promise for spintronic devices and to realize exotic topological states, from quantum spin Hall and quantum anomalous Hall effects, to Majorana fermions and skyrmions.

[10]  arXiv:1802.07808 [pdf, other]
Title: Scalable quantum nanowire networks grown by molecular beam epitaxy
Subjects: Materials Science (cond-mat.mtrl-sci); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

III-V semiconductor nanowires have shown great potential in a variety of quantum transport experiments, however it has been challenging to realize a scalable high quality nanowire platform that could lead to quantum information applications. Here, we study the potential of selective area epitaxy of InAs nanowire networks grown on GaAs-based buffer layers. The structures allow for substantial elastic strain relaxation and efficient carrier tuning. Moreover, we show that the nanowire networks possess strong spin-orbit coupling and quantum phase coherence with a signature of ballistic transport. With this, and the compatibility of the growth method with superconductor epitaxy, we conclude that this material platform fulfills the key requirements for engineering of scalable topological superconducting quantum networks.

[11]  arXiv:1802.07827 [pdf, ps, other]
Title: Method to identify parent Hamiltonians for trial states
Comments: 5 pages, no figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el)

We describe a general method to identify exact, local parent Hamiltonians for trial states like quantum Hall or spin liquid states, which we have used extensively during the past decade. It can be used to identify exact parent Hamiltonians, either directly or via the construction of simpler annihilation operators from which a parent Hamiltonian respecting all the required symmetries can be constructed. Most remarkably, however, the method provides approximate parent Hamiltonians whenever an exact solution is not available within the space of presumed interaction terms.

[12]  arXiv:1802.07835 [pdf, other]
Title: Hydrodynamic assembly and chiral synchronisation of active colloids
Subjects: Soft Condensed Matter (cond-mat.soft); Fluid Dynamics (physics.flu-dyn)

In out-of-equilibrium materials, the motility of the building blocks, such as bacteria or artificial swimmers, allows the realisation of active assemblies. This far, the formation of ordered dynamic structures, including living crystals, rotating clusters and chains has required specific interactions such as electro-magnetic, depletion or non-equilibrium surface forces. Using large-scale simulations, we show that hydrodynamic interactions, together with an aligning field, are sufficient to stabilise dynamic self-assembled structures: small chiral rotors consisting of two or three active particles as well as tunable dynamic crystals. Spontaneous chiral symmetry breaking within the particle dimer or trimer, leads to rotational motion. Our results demonstrate a chiral transfer mediated by hydrodynamic interactions, resulting in a synchronisation of the rotors with a dominant direction of rotation.

[13]  arXiv:1802.07848 [pdf]
Title: Influence of surface stoichiometry and quantum confinement on the electronic structure of small diameter InxGa1-xAs nanowires
Comments: To appear in: Materials Chemistry and Physics
Journal-ref: Materials Chemistry and Physics, Volume 206, 2018, Pages 35-39
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

Electronic structures for InxGa1-xAs nanowires with [100], [110], and [111] orientations and critical dimensions of approximately 2 nm are treated within the framework of density functional theory. Explicit band structures are calculated and properties relevant to nanoelectronic design are extracted including band gaps, effective masses, and density of states. The properties of these III-V nanowires are compared to silicon nanowires of comparable dimensions as a reference system. In nonpolar semiconductors, quantum confinement and surface chemistry are known to play a key role in the determination of nanowire electronic structure. InxGa1-xAs nanowires have in addition effects due to alloy stoichiometry on the cation sublattice and due to the polar nature of the cleaved nanowire surfaces. The impact of these additional factors on the electronic structure for these polar semiconductor nanowires is shown to be significant and necessary for accurate treatment of electronic structure properties.

[14]  arXiv:1802.07857 [pdf, ps, other]
Title: Tutorial: Magnetic resonance with nitrogen-vacancy centers in diamond---microwave engineering, materials science, and magnetometry
Comments: 16 pages, 15 figures. A tutorial article to appear in Journal of Applied Physics (Special Topic: Defects in Semiconductors 2017)
Subjects: Materials Science (cond-mat.mtrl-sci); Quantum Physics (quant-ph)

This tutorial article provides a concise and pedagogical overview on negatively-charged nitrogen-vacancy (NV) centers in diamond. The research on the NV centers has attracted enormous attention for its application to quantum sensing, encompassing the areas of not only physics and applied physics but also chemistry, biology and life sciences. Nonetheless, its key technical aspects can be understood from the viewpoint of magnetic resonance. We focus on three facets of this ever-expanding research field, to which our viewpoint is especially relevant: microwave engineering, materials science, and magnetometry. In explaining these aspects, we provide a technical basis and up-to-date technologies for the research on the NV centers.

[15]  arXiv:1802.07861 [pdf, ps, other]
Title: Statistical complexity without explicit reference to underlying probabilities
Comments: 10 pages, 14 figures, In Press, Accepted Manuscript
Journal-ref: Physica A, 2018
Subjects: Statistical Mechanics (cond-mat.stat-mech)

We show that extremely simple systems of a not too large number of particles can be simultane- ously thermally stable and complex. To such an end, we extend the statistical complexity's notion to simple configurations of non-interacting particles, without appeal to probabilities, and discuss configurational properties.

[16]  arXiv:1802.07864 [pdf, other]
Title: A tight-binding model for the band dispersion in rhombohedral topological insulators over the whole Brilluoin zone
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

We put forward a tight-binding model for rhombohedral topological insulators materials with the space group $D^{5}_{3d}(R\bar{3}m)$. The model describes the bulk band structure of these materials over the whole Brillouin zone. Within this framework, we also describe the topological nature of surface states, characterized by a Dirac cone-like dispersion and the emergence of surface projected bulk states near to the Dirac-point in energy. We find that the breaking of the $R_{3}$ symmetry as one moves away from the $\Gamma$ point has an important role in the hybridization of the $p_x$, $p_y$, and $p_z$ atomic orbitals. In our tight-binding model, the latter leads to a band mixing matrix element ruled by a single parameter. We show that our model gives a good description of the strategies/mechanisms proposed in the literature to eliminate and/or energy shift the bulk states away from the Dirac point, such as stacking faults and the introduction of an external applied electric field.

[17]  arXiv:1802.07867 [pdf, other]
Title: Interlayer Couplings Mediated by Antiferromagnetic Magnons
Comments: submitted to PRL
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Materials Science (cond-mat.mtrl-sci); Quantum Physics (quant-ph)

Collinear antiferromagnets (AFs) support two degenerate magnon excitations carrying opposite spin polarizations, by which magnons can function as electrons in spin transport. We explore the interlayer coupling mediated by antiferromagnetic magnons in an insulating ferromagnet (F)/AF/F trilayer structure. The internal energy of the AF depends on the orientations of the two Fs, which manifests as effective interlayer interactions JS1.S2 and K(S1.S2)^2. Both J and K are functions of temperature and the AF thickness. Interestingly, J is antiferromagnetic at low temperatures and ferromagnetic at high temperatures. In the high-temperature regime, J is estimated to be much larger than the interlayer dipole-dipole interaction, allowing direct experimental verification.

[18]  arXiv:1802.07868 [pdf]
Title: Magnetic-field enhanced high-thermoelectric performance in topological Dirac semimetal Cd$_3$As$_2$ crystal
Comments: 25 pages, 4 figures, supplementary materials file is not included
Subjects: Materials Science (cond-mat.mtrl-sci); Strongly Correlated Electrons (cond-mat.str-el)

Thermoelectric materials can be used to convert heat to electric power through the Seebeck effect. We study magneto-thermoelectric figure of merit (ZT) in three-dimensional Dirac semimetal Cd$_3$As$_2$ crystal. It is found that enhancement of power factor and reduction of thermal conductivity can be realized at the same time through magnetic field although magnetoresistivity is greatly increased. ZT can be highly enhanced from 0.17 to 1.1 by more than six times around 350 K under a perpendicular magnetic field of 7 Tesla. The huge enhancement of ZT by magnetic field arises from the linear Dirac band with large Fermi velocity and the large electric thermal conductivity in Cd$_3$As$_2$. Our work paves a new way to greatly enhance the thermoelectric performance in the quantum topological materials.

[19]  arXiv:1802.07871 [pdf, other]
Title: Atomistic mechanism of graphene growth on SiC substrate: Large-scale molecular dynamics simulation based on a new charge-transfer bond-order type potential
Comments: Accepted for Physical Review B
Subjects: Materials Science (cond-mat.mtrl-sci)

Thermal decomposition of silicon carbide is a promising approach for the fabrication of graphene. However, the atomistic growth mechanism of graphene remains unclear. This paper describes the development of a new charge-transfer interatomic potential. Carbon bonds with a wide variety of characteristics can be reproduced by the proposed vectorized bond-order term. Large-scale thermal decomposition simulation enables us to observe the continuous growth process of the multi-ring carbon structure. The annealing simulation reveals the atomistic process by which the multi-ring carbon structure is transformed to flat graphene involving only 6-membered rings. Also, it is found that the surface atoms of the silicon carbide substrate enhance the homogeneous graphene formation.

[20]  arXiv:1802.07872 [pdf, other]
Title: Surfing its own wave: hydroelasticity of a particle near a membrane
Comments: 4 pages, 5 figures
Subjects: Soft Condensed Matter (cond-mat.soft); Biological Physics (physics.bio-ph); Fluid Dynamics (physics.flu-dyn)

We show using theory and experiments that a small particle moving along an elastic membrane through a viscous fluid is repelled from the membrane due to hydro-elastic forces. The viscous stress field produces an elastic disturbance leading to particle-wave coupling. We derive an analytic expression for the particle trajectory in the lubrication limit, bypassing the construction of the detailed velocity and pressure fields. The normal force is quadratic in the parallel speed, and is a function of the tension and bending resistance of the membrane. Experimentally, we measure the normal displacement of spheres sedimenting along an elastic membrane and find quantitative agreement with the theoretical predictions with no fitting parameters. We experimentally demonstrate the effect to be strong enough for particle separation and sorting. We discuss the significance of these results for bio-membranes and propose our model for membrane elasticity measurements.

[21]  arXiv:1802.07894 [pdf, ps, other]
Title: Flexible paramagnetic membranes in fast precessing fields
Comments: 7 pages, 2 figures
Subjects: Soft Condensed Matter (cond-mat.soft)

Elastic membranes composed of paramagnetic beads offer the possibility of assembling versatile actuators operated autonomously by external magnetic fields. Here we develop a theoretical framework to study shapes of such paramagnetic membranes under the influence of a fast precessing magnetic field. The competition of the elastic and magnetic energies that arise as a result of their bending and the induced dipolar interactions between nearest neighbors beads, respectively, determine their shape. In the harmonic approximation, the elastic energy is quadratic in the surface curvatures. We account for the magnetic energy by introducing a continuum limit energy, quadratic in the projections of the surface tangents onto the precession axis. We derive the Euler-Lagrange equation governing the equilibria of these membranes, as well as the corresponding stresses. We apply this framework to examine paramagnetic membranes with quasi-planar, cylindrical and helicoidal geometries. In all cases we found that their shape, energy and stresses can be modified by means of the parameters of the magnetic field, mainly by the angle of precession.

[22]  arXiv:1802.07900 [pdf, other]
Title: Spin Ice vs. Thin Ice
Authors: Cristiano Nisoli
Comments: 5 pages, 5 figures
Subjects: Soft Condensed Matter (cond-mat.soft); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Statistical Mechanics (cond-mat.stat-mech)

Particle-based ices have often been considered essentially equivalent to magnet-based spin ices. Yet, the two differ essentially in frustration and energetics. We show that particle-based ices can be mapped exactly into spin ices coupled to a background field. In trivial geometries the field has no effect and at equilibrium the two systems are indeed equivalent. In non-trivial cases, however, the field controls a richer phenomenology, absent in magnetic ices, and still largely unexplored: ice rule fragility, topological charge transfer, radial polarization, decimation induced disorder, glassiness.

[23]  arXiv:1802.07906 [pdf]
Title: Stability of Granular Tunnel
Comments: 13 pages, 6 Figures, and 1 Table
Subjects: Soft Condensed Matter (cond-mat.soft)

We demonstrated the stability of tunnels made of granular matters is strongly dependent on the grain size, tunnel diameter, and water content in the granules. Larger tunnel radius, larger grain size, and too much water content tend to destabilize the tunnel. We also develop a model to describe such findings. We identified a phase diagram of stability which greatly controlled by granular bond order. For granular bond order of larger than unity, we can always made a stable tunnel. However, for granular bond order of less than unity, we obtain a general expression for maximum tunnel thickness that can be made. To best of our knowledge, this is the first exploration regarding the granular tunnel stability.

[24]  arXiv:1802.07907 [pdf, other]
Title: Intermediate valence in single crystalline Yb$_2$Si$_2$Al
Comments: Submitted to Physical Review B
Subjects: Strongly Correlated Electrons (cond-mat.str-el)

Yb$_2$Si$_2$Al may be a prototype for exploring different aspects of the Shastry-Sutherland lattice, formed by planes of orthogonally coupled Yb ions. Measurements of the magnetic susceptibility find incoherently fluctuating Yb$^{3+}$ moments coexisting with a weakly correlated metallic state that is confirmed by measurements of the electrical resistivity. Increasing signs of Kondo coherence are found with decreasing temperature, including an enhanced Sommerfeld coefficient and Kadowaki-Woods ratio that signal that the metallic state found at the lowest temperatures is a Fermi liquid where correlations have become significantly stronger. A pronounced peak in the electronic and magnetic specific heat indicates that the coupling of the Yb moments to the conduction electrons leads to an effective Kondo temperature that is approximately 30 K. The valence and ground state symmetry of Yb$_2$Si$_2$Al have been investigated with electron spectroscopy methods. Yb$_2$Si$_2$Al is found to be strongly intermediate valent ($v_F$=2.68(2) at 80 K). The crystal-field split Hund's rule ground state exhibits strong dichroism and possible crystal-field ground state symmetries are discussed. The temperature dependence of the dichroism puts a lower limit to the energy of the first excited crystal-field level ($\Delta_{CEF}$$\geq$ 10 meV). Taken together, these experimental data are consistent with a scenario where a coherent Kondo lattice forms in $\mathrm{Yb_2Si_2Al}$ from an incoherently fluctuating ensemble of Yb moments with incomplete Kondo compensation, and strong intermediate valence character.

[25]  arXiv:1802.07910 [pdf]
Title: Carrier-Resolved Photo Hall Measurement in World-Record-Quality Perovskite and Kesterite Solar Absorbers
Comments: 13 pages, 3 figures, supplementary materials (15 pages) including link to Movie S1
Subjects: Materials Science (cond-mat.mtrl-sci)

Majority and minority carrier properties such as type, density and mobility represent fundamental yet difficult to access parameters governing semiconductor device performance, most notably solar cells. Obtaining this information simultaneously under light illumination would unlock many critical parameters such as recombination lifetime, recombination coefficient, and diffusion length; while deeply interesting for optoelectronic devices, this goal has remained elusive. We demonstrate here a new carrier-resolved photo-Hall technique that rests on a new identity relating hole-electron mobility difference ($\Delta\mu$), Hall coefficient ($h$), and conductivity ($\sigma$): $\Delta\mu=(2+d\ln h/d\ln \sigma)\,h\,\sigma$, and a rotating parallel dipole line ac-field Hall system with Fourier/lock-in detection for clean Hall signal measurement. We successfully apply this technique to recent world-record-quality perovskite and kesterite films and map the results against varying light intensities, demonstrating unprecedented simultaneous access to the above-mentioned parameters.

[26]  arXiv:1802.07921 [pdf, ps, other]
Title: Magnetoresistance in organic semiconductors: including pair correlations in the kinetic equations for hopping transport
Comments: 17 pages 10 figures accepted for publication in Physical Review B
Subjects: Disordered Systems and Neural Networks (cond-mat.dis-nn)

We derive the kinetic equations for polaron hopping in organics that explicitly take into account the double occupation possibility and pair intersite correlations. The equations include simplified phenomenological spin dynamics and provide a self-consistent framework for the description of the bipolaron mechanism of the organic magnetoresistance. At low applied voltages the equations can be reduced to effective resistor network that generalizes the Miller-Abrahams network and includes the effect of spin relaxation on the system resistivity. Our theory discloses the close relationship between the organic magnetoresistance and the intersite correlations. Moreover, in the absence of correlations, as in ordered system with zero Hubbard energy, the magnetoresistance vanishes.

[27]  arXiv:1802.07933 [pdf, other]
Title: Modelling chemotaxis of microswimmers: from individual to collective behavior
Subjects: Soft Condensed Matter (cond-mat.soft); Pattern Formation and Solitons (nlin.PS); Biological Physics (physics.bio-ph)

We discuss recent progress in the theoretical description of chemotaxis by coupling the diffusion equation of a chemical species to equations describing the motion of sensing microorganisms. In particular, we discuss models for autochemotaxis of a single microorganism which senses its own secretion leading to phenomena such as self-localization and self-avoidance. For two heterogeneous particles, chemotactic coupling can lead to predator-prey behavior including chase and escape phenomena, and to the formation of active molecules, where motility spontaneously emerges when the particles approach each other. We close this review with some remarks on the collective behavior of many particles where chemotactic coupling induces patterns involving clusters, spirals or traveling waves.

[28]  arXiv:1802.07937 [pdf, ps, other]
Title: Phase transition of vortex states in two-dimensional superconductors under a oscillating magnetic field from the chiral helimagnet
Comments: 8 pages, 14 figures
Subjects: Superconductivity (cond-mat.supr-con)

We have investigated vortex states in two-dimensional superconductors under a oscillating magnetic field from a chiral helimagnet. We have solved the two-dimensional Ginzburg-Landau equations with finite element method. We have found that when the magnetic field from the chiral helimagnet increases, vortices appear all at once in all periodic regions. This transition is different from that under the uniform magnetic field. Under the composite magnetic field with the oscillating and uniform fields (down-vortices), vortices antiparallel to the uniform magnetic field disappear. Then, the small uniform magnetic field easily remove down-vortices.

[29]  arXiv:1802.07941 [pdf, other]
Title: Strain effect in highly-doped n-type 3C-SiC-on-glass substrate for mechanical sensors and mobility enhancement
Subjects: Materials Science (cond-mat.mtrl-sci)

This work reports the strain effect on the electrical properties of highly doped n-type single crystalline cubic silicon carbide (3C-SiC) transferred onto a 6-inch glass substrate employing an anodic bonding technique. The experimental data shows high gauge factors of -8.6 in longitudinal direction and 10.5 in transverse direction along the [100] orientation. The piezoresistive effect in the highly doped 3C-SiC film also exhibits an excellent linearity and consistent reproducibility after several bending cycles. The experimental result was in good agreement with the theoretical analysis based on the phenomenon of electron transfer between many valleys in the conduction band of n-type 3C-SiC. Our finding for the large gauge factor in n-type 3C- SiC coupled with the elimination of the current leak to the insulated substrate could pave the way for the development of single crystal SiC-on-glass based MEMS applications.

[30]  arXiv:1802.07950 [pdf, ps, other]
Title: Photoinduced enhancement of excitonic order in the two-orbital Hubbard model
Comments: 20 pages, 14 figures, to appear in Phys. Rev. B
Subjects: Strongly Correlated Electrons (cond-mat.str-el)

Photoinduced dynamics in an excitonic insulator is studied theoretically by using a two-orbital Hubbard model on the square lattice where the excitonic phase in the ground state is characterized by the BCS-BEC crossover as a function of the interorbital Coulomb interaction. We consider the case where the order has a wave vector $Q=(0,0)$ and photoexcitation is introduced by a dipole transition. Within the mean-field approximation, we show that the excitonic order can be enhanced by the photoexcitation when the system is initially in the BEC regime of the excitonic phase, whereas it is reduced if the system is initially in the BCS regime. The origin of this difference is discussed from behaviors of momentum distribution functions and momentum-dependent excitonic pair condensation. In particular, we show that the phases of the excitonic pair condensation have an important role in determining whether the excitonic order is enhanced or not.

[31]  arXiv:1802.07959 [pdf, other]
Title: Power law analysis for temperature dependence of magnetocrystalline anisotropy constants of Nd$_2$Fe$_{14}$B magnets
Comments: 5pages, 6 figures
Subjects: Materials Science (cond-mat.mtrl-sci)

Phenomenological analysis for the temperature dependence of the magnetocrystalline anisotropy (MA) in rare earth magnets is presented. We define phenomenological power laws applicable to compound magnets using the Zener theory, apply these laws to the magnetocrystalline anisotropy constants (MACs) of Nd$_2$Fe$_{14}$B magnets. The results indicate that the MACs obey the power law well, and a general understanding for the temperature-dependent MA in rare earth magnets is obtained through the analysis. Furthermore, to examine the validity of the power law, we discuss the temperature dependence of the MACs in Dy$_2$Fe$_{14}$B and Y$_2$Fe$_{14}$B magnets as examples wherein it is difficult to interpret the MA using the power law.

[32]  arXiv:1802.07964 [pdf, other]
Title: Topological phases of non-Hermitian systems
Comments: 27 pages, 16 figures, 2 tables
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Disordered Systems and Neural Networks (cond-mat.dis-nn); Quantum Gases (cond-mat.quant-gas); Statistical Mechanics (cond-mat.stat-mech); Quantum Physics (quant-ph)

Recent experimental advances in controlling dissipation have brought about unprecedented flexibility in engineering non-Hermitian Hamiltonians in open classical and quantum systems. A particular interest centers on the topological properties of non-Hermitian systems, which exhibit unique phases with no Hermitian counterparts. However, no systematic understanding in analogy with the periodic table of topological insulators and superconductors has been achieved. In this paper, we develop a coherent framework of topological phases of non-Hermitian systems. After elucidating the physical meaning and the mathematical definition of non-Hermitian topological phases, we start with one-dimensional lattices, which exhibit topological phases with no Hermitian counterparts and are found to be characterized by an integer topological winding number even with no symmetry constraint, reminiscent of the quantum Hall insulator in Hermitian systems. A system with a nonzero winding number, which is experimentally measurable from the wave-packet dynamics, is shown to be robust against disorder, a phenomenon observed in the Hatano-Nelson model with asymmetric hopping amplitudes. We also unveil a novel bulk-edge correspondence that features an infinite number of (quasi-)edge modes. We then apply the K-theory to systematically classify all the non-Hermitian topological phases in the Altland-Zirnbauer classes in all dimensions. The obtained periodic table unifies time-reversal and particle-hole symmetries, leading to highly nontrivial predictions such as the absence of non-Hermitian topological phases in two dimensions. We provide concrete examples for all the nontrivial non-Hermitian AZ classes in zero and one dimensions. In particular, we identify a Z2 topological index for arbitrary quantum channels. Our work lays the cornerstone for a unified understanding of the role of topology in non-Hermitian systems.

[33]  arXiv:1802.07968 [pdf, other]
Title: High-pressure phase diagram of hydrogen and deuterium sulfides from first principles: structural and vibrational properties including quantum and anharmonic effects
Subjects: Materials Science (cond-mat.mtrl-sci)

We study the structural and vibrational properties of the high-temperature superconducting sulfur trihydride and trideuteride in the high-pressure $Im\bar{3}m$ and $R3m$ phases by first-principles density-functional-theory calculations. On lowering pressure, the rhombohedral transition $Im\bar{3}m \rightarrow R3m$ is expected, with hydrogen bond desymmetrization and occurrence of trigonal lattice distortion. In hydrostatic conditions we find that, contrary to what suggested in some recent experiments, if the rhombohedral distortion exists it affects mainly the hydrogen-bonds, whereas the resulting cell distortion is minimal. We estimate that the occurrence of a stress anisotropy of approximately $10\%$ could explain this discrepancy. Assuming hydrostatic conditions, we calculate the critical pressure at which the rhombohedral transition occurs. Quantum and anharmonic effects, which are relevant in this system, are included at nonperturbative level with the stochastic self-consistent harmonic approximation (SSCHA). Within this approach, we determine the transition pressure by calculating the free energy Hessian. We find that quantum anharmonic effects are responsible for a strong reduction of the critical pressure with respect to the one obtained with the classical harmonic approach. Moreover, we observe a prominent isotope effect, as we estimate higher pressure transition for D${}_3$S than for H${}_3$S. Finally, within SSCHA we calculate the anharmonic phonon spectral functions in the $Im\bar{3}m$ phase. The strong anharmonicity of the system is confirmed by the occurrence of very large anharmonic broadenings leading to complex non-Lorentzian line shapes. However, for the vibrational spectra at zone center, accessible e.g. by infrared spectroscopy, the broadenings are very small (linewidth at most around 2~meV) and anharmonic phonon quasiparticles are well defined.

[34]  arXiv:1802.07977 [pdf, other]
Title: AFLOW-SYM: Platform for the complete, automatic and self-consistent symmetry analysis of crystals
Comments: 24 pages, 6 figures
Subjects: Materials Science (cond-mat.mtrl-sci)

Determination of the symmetry profile of structures is a persistent challenge in materials science. Results often vary amongst standard packages, hindering autonomous materials development by requiring continuous user attention and educated guesses. Here, we present a robust procedure for evaluating the complete suite of symmetry properties, featuring various representations for the point-, factor-, space groups, site symmetries, and Wyckoff positions. The protocol determines a system-specific mapping tolerance that yields symmetry operations entirely commensurate with fundamental crystallographic principles. The self consistent tolerance characterizes the effective spatial resolution of the reported atomic positions. The approach is compared with the most used programs and is successfully validated against the space group information provided for over 54,000 entries in the Inorganic Crystal Structure Database. Subsequently, a complete symmetry analysis is applied to all 1.7$+$ million entries of the AFLOW data repository. The AFLOW-SYM package has been implemented in, and made available for, public use through the automated, $\textit{ab-initio}$ framework AFLOW.

[35]  arXiv:1802.07988 [pdf, other]
Title: Bosonic quantum Hall states in single layer 2D optical lattices
Comments: 6 pages with 5 figures and 4 pages of supplementary material
Subjects: Quantum Gases (cond-mat.quant-gas); Atomic Physics (physics.atom-ph)

Quantum Hall (QH) states of 2D single layer optical lattices are examined using Bose-Hubbard model (BHM) in presence of artificial gauge field. We study the QH states of both the homogeneous and inhomogeneous systems. For the homogeneous case we use cluster Gutzwiller mean field (CGMF) theory with cluster sizes ranging from $2\times 2$ to $5\times 5$. We, then, consider the inhomogeneous case, which is relevant to experimental realization. In this case, we use CGMF and exact diagonalization (ED). The ED studies are using lattice sizes ranging from $3\times 3$ to $5\times 5$. Our results show that the geometry of the QH states are sensitive to the magnetic flux $\alpha$ and cluster sizes. For homogeneous system, among various combinations of $1/5\leqslant \alpha\leqslant 1/2$ and filling factor $\nu$, only the QH state of $\alpha=1/4$ with $\nu=1/2$, $1$, $3/2$ and $2$ occur as ground states. For other combinations, the competing superfluid (SF) state is the ground state and QH state is metastable. For BHM with envelope potential all the QH states observed in homogeneous system exist for box potentials, but none for the harmonic potential. The QH states also persist for very shallow Gaussian envelope potential. As a possible experimental signature we study the two point correlations of the QH and SF states.

[36]  arXiv:1802.07989 [pdf, other]
Title: Electronic properties of phosphorene and graphene nanoribbons with edge vacancies in magnetic field
Comments: 13 pages, 17 figures
Journal-ref: Physics Letters A 382, 846-854 (2018)
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

The graphene and phosphorene nanostructures have a big potential application in a large area of actuals research in physics. However, their methods of synthesis still do not allow the production of perfect materials with an intact molecular structure. In this paper, the occurrence of atomic vacancies was considered in the edge structure of the zigzag phosphorene and graphene nanoribbons. For different concentrations of these edge vacancies, their influence on the metallic properties was investigated. The calculations were performed for different sizes of the unit cell. Furthermore, for a smaller size, the influence of a uniform magnetic field was added.

[37]  arXiv:1802.07993 [pdf, other]
Title: Tip-induced Superconductivity Coexisting with Preserved Topological Properties in Line-nodal Semimetal ZrSiS
Comments: with supplementary materials
Subjects: Superconductivity (cond-mat.supr-con)

ZrSiS was recently shown to be a new material with topologically non-trivial band structure which exhibits multiple Dirac nodes and a robust linear band dispersion up to an unusually high energy of 2\,eV. Such a robust linear dispersion makes the topological properties of ZrSiS insensitive to perturbations like carrier doping or lattice distortion. Here we show that a novel superconducting phase with a remarkably high $T_c$ of 7.5\,K can be induced in single crystals of ZrSiS by a non-superconducting metallic tip of Ag. From first-principles calculations we show that the observed superconducting phase might originate from dramatic enhancement of density of states due to the presence of a metallic tip on ZrSiS. Our calculations also show that the emerging tip-induced superconducting phase co-exists with the well preserved topological properties of ZrSiS.

[38]  arXiv:1802.08002 [pdf, other]
Title: Dependence of Tunneling magnetoresistance and magnetostatic coupling on MgO barrier thickness and post-annealing temperature of perpendicular CoFeB-based junctions with exchange bias
Subjects: Materials Science (cond-mat.mtrl-sci)

We report the fabrication of perpendicular magnetic tunnel junctions with large perpendicular exchange bias around 1000 Oe and high tunneling magnetoresistance of $(65.5\pm 0.5)\%$. In the stacks Ta/Pd/IrMn/CoFe/Ta$(\textit{x})$/CoFeB/MgO$(\textit{y})$/CoFeB/Ta/Pd with $x=(0.3-0.55)\,$nm and $y=(0.6-3)\,$nm, the exchange bias field and the tunneling magnetoresistance ratio can be modulated by varying the above thicknesses. For stacks with $x=0.4\,$nm and $y=2\,$nm, the tunneling magnetoresistance ratio primarily increases monotonically with increasing the post-annealing temperature, reaching a peak of $65.5\,\%$ after annealing at $T_\textrm{ann}=300\,^{\circ}$C. Further increase of $T_\textrm{ann}$ leads to a decrease of the tunneling magnetoresistance ratio, attributed to the magnetic collapse of the pinned electrode. Simultaneously, the barrier thickness and $T_\textrm{ann}$ dependence of the interlayer exchange coupling are investigated. A change in the character of the coupling is identified from antiferromagnetic for thick barriers to ferromagnetic for the thin ones. Additionally, the interlayer exchange coupling decreases with the increase of post-annealing temperature, with a simultaneous decrease of saturation magnetization per unit area and exchange bias, making clear the prominent contribution of $T_\textrm{ann}$ to the magnetic properties of the investigated stacks.

[39]  arXiv:1802.08003 [pdf, other]
Title: Strong Valley Zeeman Effect of Dark Excitons in Monolayer Transition Metal Dichalcogenides in a Tilted Magnetic Field
Comments: 6 pages, 5 figures, 1 table
Journal-ref: Phys. Rev. B 97, 081109(R) (2018)
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

The dependence of the excitonic photoluminescence (PL) spectrum of monolayer transition metal dichalcogenides (TMDs) on the tilt angle of an applied magnetic field is studied. Starting from a four-band Hamiltonian we construct a theory which quantitatively reproduces the available experimental PL spectra for perpendicular and in-plane magnetic fields. In the presence of a tilted magnetic field, we demonstrate that the dark exciton PL peaks brighten due to the in-plane component of the magnetic field and split for light with different circular polarization as a consequence of the perpendicular component of the magnetic field. This splitting is more than twice as large as the splitting of the bright exciton peaks in tungsten-based TMDs. We propose an experimental setup that will allow to access the predicted splitting of the dark exciton peaks in the PL spectrum.

[40]  arXiv:1802.08006 [pdf, other]
Title: Heat transport in insulators from ab initio Green-Kubo theory
Comments: 35 pages, 14 figures
Subjects: Statistical Mechanics (cond-mat.stat-mech); Computational Physics (physics.comp-ph)

The Green-Kubo theory of thermal transport has long be considered incompatible with modern simulation methods based on electronic-structure theory, because it is based on such concepts as energy density and current, which are ill-defined at the quantum-mechanical level. Besides, experience with classical simulations indicate that the estimate of heat-transport coefficients requires analysing molecular trajectories that are more than one order of magnitude longer than deemed feasible using ab inito molecular dynamics. In this paper we report on recent theoretical advances that are allowing to overcome these two obstacles. First, a general gauge invariance principle has been established, showing that thermal conductivity is insensitive to many details of the microscopic expression for the energy density and current from which it is derived, thus permitting to establish a rigorous expression for the energy flux from Density-Functional Theory, from which the conductivity can be computed in practice. Second, a novel data analysis method based on the statistical theory of time series has been proposed, allowing to considerably reduce the simulation time required to achieve a target accuracy on the computed conductivity. These concepts are illustrated in detail, starting from a pedagogical introduction to the Green-Kubo theory of linear response and transport, and illustrated with a few applications done with both classical and quantum-mechanical simulation methods.

[41]  arXiv:1802.08007 [pdf, other]
Title: Dependence of $T_c$ on the $q-ω$ structure of the spin-fluctuation spectrum
Comments: 9 pages, 4 figures
Subjects: Superconductivity (cond-mat.supr-con)

A phenomenological spin-fluctuation analysis (Ref. 1), based upon inelastic neutron scattering (INS) and angular resolved photoemission spectroscopy (ARPES) data for ${\rm YBCO}_{6.6}(T_c=61K)$, is used to calculate the functional derivative of the d-wave eigenvalue $\lambda_d$ of the linearized gap equation with respect to the imaginary part of the spin susceptibility $\chi''(q,\omega)$ at 70K. For temperatures near $T_c$, the variation of $T_c$ with respect to $\chi''(q,\omega)$ is proportional to this functional derivative. Based on this, we discuss how different parts of the $q$ and $\omega$ dependent spin-fluctuation spectrum of YBCO$_{6.6}$ contribute to $T_c$.

[42]  arXiv:1802.08045 [pdf, other]
Title: Spatially Resolved Thermodynamic Integration: An Efficient Method to Compute Chemical Potentials of Dense Fluids
Subjects: Soft Condensed Matter (cond-mat.soft)

Many popular methods for the calculation of chemical potentials rely on the insertion of test particles into the target system. In the case of liquids and liquid mixtures, this procedure increases in difficulty upon increasing density or concentration, and the use of sophisticated enhanced sampling techniques becomes inevitable. In this work we propose an alternative strategy, spatially resolved thermodynamic integration, or SPARTIAN for short. Here, molecules are described with atomistic resolution in a simulation subregion, and as ideal gas particles in a larger reservoir. All molecules are free to diffuse between subdomains adapting their resolution on the fly. To enforce a uniform density profile across the simulation box, a single-molecule external potential is computed, applied, and identified with the difference in chemical potential between the two resolutions. Since the reservoir is represented as an ideal gas bath, this difference exactly amounts to the excess chemical potential of the target system. The present approach surpasses the high density/concentration limitation of particle insertion methods because the ideal gas molecules entering the target system region spontaneously adapt to the local environment. The ideal gas representation contributes negligibly to the computational cost of the simulation, thus allowing one to make use of large reservoirs at minimal expenses. The method has been validated by computing excess chemical potentials for pure Lennard-Jones liquids and mixtures, SPC and SPC/E liquid water, and aqueous solutions of sodium chloride. The reported results well reproduce literature data for these systems.

[43]  arXiv:1802.08059 [pdf, other]
Title: Cooper-pair splitting in two parallel InAs nanowires
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

We report on the fabrication and electrical characterization of an InAs double - nanowire (NW) device consisting of two closely placed parallel NWs coupled to a common superconducting electrode on one side and individual normal metal leads on the other. In this new type of device we detect Cooper-pair splitting (CPS) with a sizeable efficiency of correlated currents in both NWs. In contrast to earlier experiments, where CPS was realized in a single NW, demonstrating an intrawire electron pairing mediated by the superconductor (SC), our experiment demonstrates an inter- wire interaction mediated by the common SC. The latter is the key for the realization of zero-magnetic field Majorana bound states, or Parafermions; in NWs and therefore constitutes a milestone towards topological superconductivity. In addition, we observe transport resonances that occur only in the superconducting state, which we tentatively attribute to Andreev Bound states and/or Yu-Shiba resonances that form in the proximitized section of one NW.

[44]  arXiv:1802.08069 [pdf, other]
Title: Quasi-two-dimensional thermoelectricity in SnSe
Comments: 10 pages, 5 figures, plus supporting information
Journal-ref: Phys. Rev. B 97, 045424 (2018)
Subjects: Materials Science (cond-mat.mtrl-sci); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

Stannous selenide is a layered semiconductor that is a polar analogue of black phosphorus, and of great interest as a thermoelectric material. Unusually, hole doped SnSe supports a large Seebeck coefficient at high conductivity, which has not been explained to date. Angle resolved photo-emission spectroscopy, optical reflection spectroscopy and magnetotransport measurements reveal a multiple-valley valence band structure and a quasi two-dimensional dispersion, realizing a Hicks-Dresselhaus thermoelectric contributing to the high Seebeck coefficient at high carrier density. We further demonstrate that the hole accumulation layer in exfoliated SnSe transistors exhibits a field effect mobility of up to $250~\mathrm{cm^2/Vs}$ at $T=1.3~\mathrm{K}$. SnSe is thus found to be a high quality, quasi two-dimensional semiconductor ideal for thermoelectric applications.

[45]  arXiv:1802.08084 [pdf, ps, other]
Title: Jarzynski Equality and its Special Trajectory Ensemble Average Demystified
Authors: P.D. Gujrati
Comments: 5 pages, no figure
Subjects: Statistical Mechanics (cond-mat.stat-mech)

The special trajectory ensemble average (TEA), denoted by a subscript 0, in the Jarzynski Equality (JE) results in the Jensen inequality <R>_0 GT-EQ delta(F) for the work R done on the system, and not the thermodynamic work inequality <R> GT-EQ delta(F) since we find <R> NEQ <R>_0. Therefore, contrary to the common belief, the Jensen inequality does not directly support the JE as a nonequilibrium result. Jarzynski's microscopic treatment of the inclusive energy considers only the external work d_eE_k but neglects the ubiquitous change d_iE_k due to external-internal force imbalance, though d_iE_k's are present even in a reversible process as we show. Because of this neglect, no thermodynamic force necessary for dissipation is allowed. Thus the JE has no built-in irreversibility, despite a time-dependent work protocol. We support our claim by an explicit calculation, which shows that <R>_0 > delta(F) even for a reversible process for which <R> = delta(F). This also confirms that <R> and <R>_0 are different averages.

[46]  arXiv:1802.08099 [pdf]
Title: Zak phase and band inversion in dimerized one-dimensional locally resonant metamaterials
Comments: 6 figures, under review
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Applied Physics (physics.app-ph)

Zak phase, which refers to the Berry's phase picked up by a particle moving across the Brillouin zone, characterizes the topological properties of Bloch bands in one-dimensional periodic system. Here the Zak phase in dimerized one-dimensional locally resonant metamaterials is investigated. It is found that there are some singular points in the bulk band across which the Bloch states contribute {\pi} to the Zak phase, whereas while in the rest of the band the contribution is nearly zero. These singular points associated with zero reflection are caused by two different mechanisms: the dimerization-independent anti-resonating of each branch, and the dimerization-dependent destructive interference in multiple backscattering. The structure undergoes a topological transition point in the band structure where the band inverts and the Zak phase, which is determined by the numbers of singular points in the bulk band, changes following a shift in dimerization parameter. Finally, the interface state between two dimerized metamaterial structures with different topological property in the first band gap is demonstrated experimentally. The quasi-one-dimensional configuration of the system allows one to explore topology-inspired new methods and applications in the sub-wavelength scale.

[47]  arXiv:1802.08100 [pdf, other]
Title: Negative differential resistance in Van der Waals heterostructures due to moiré-induced spectral reconstruction
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

Formation of moir\'{e} superlattices is common in Van der Waals heterostructures as a result of the mismatch between lattice constants and misalignment of crystallographic directions of the constituent two-dimensional crystals. We discuss theoretically electron transport in a Van der Waals tunnelling transistor in which one of the electrodes is made of two crystals forming a moir\'{e} superlattice at their interface. By investigating structures containing either the aligned graphene/hexagonal boron nitride heterostructure or twisted bilayer graphene, we show that negative differential resistance is possible in such transistors as a consequence of the superlattice-induced changes in the electronic density of states and without the need of momentum conserving tunnelling present in high-quality exfoliated devices.

[48]  arXiv:1802.08103 [pdf, other]
Title: Global entanglement and quantum phase transitions in the transverse XY Heisenberg chain
Comments: 9 pages, 9 figures
Journal-ref: Physical Review B 97, 024434 (2018)
Subjects: Statistical Mechanics (cond-mat.stat-mech); Quantum Physics (quant-ph)

We provide a study of various quantum phase transitions occurring in the XY Heisenberg chain in a transverse magnetic field using the Meyer-Wallach (MW) measure of (global) entanglement. Such a measure, while being readily evaluated, is a multipartite measure of entanglement as opposed to more commonly used bipartite measures. Consequently, we obtain analytic expression of the measure for finite-size systems and show that it can be used to obtain critical exponents via finite-size scaling with great accuracy for the Ising universality class. We also calculate an analytic expression for the isotropic (XX) model and show that global entanglement can precisely identify the level-crossing points. The critical exponent for the isotropic transition is obtained exactly from an analytic expression for global entanglement in the thermodynamic limit. Next, the general behavior of the measure is calculated in the thermodynamic limit considering the important role of symmetries for this limit. The so-called oscillatory transition in the ferromagnetic regime can only be characterized by the thermodynamic limit where global entanglement is shown to be zero on the transition curve. Finally, the anisotropic transition is explored where it is shown that global entanglement exhibits an interesting behavior in the finite-size limit. In the thermodynamic limit, we show that global entanglement shows a cusp singularity across the Ising and anisotropic transition, while showing non-analytic behavior at the XX multicritical point. It is concluded that global entanglement, despite its relative simplicity, can be used to identify all the rich structure of the ground-state Heisenberg chain.

[49]  arXiv:1802.08104 [pdf, other]
Title: Extended Bose-Hubbard Model with dipolar and contact interactions
Comments: 9 pages, 10 figures
Subjects: Quantum Gases (cond-mat.quant-gas)

We study the phase diagram of the one-dimensional boson gas trapped inside an optical lattice with contact and dipolar interaction taking into account next-nearest terms for both tunneling and interaction. Using the density matrix renormalization group, we calculate how the locations of phase transitions change with increasing dipolar interaction strength for average density $\rho = 1$. Furthermore, we show an emergence of pair-correlated phases for a large dipolar interaction strength and $\rho \geq 2$, including a supersolid phase with an incommensurate density wave ordering manifesting the corresponding spontaneous breaking of the translational symmetry.

[50]  arXiv:1802.08120 [pdf, other]
Title: Microscopic dynamics of superfluid $^4$He: a comprehensive study
Subjects: Other Condensed Matter (cond-mat.other)

The dynamic structure factor of superfluid $^4$He has been investigated at very low temperatures by inelastic neutron scattering. The measurements combine different incoming energies resulting in an unprecedentedly large dynamic range with excellent energy resolution, covering wave vectors $Q$ up to 5 \AA$^{-1}$ and energies $\omega$ up to 15 meV. A detailed description of the dynamics of superfluid $^4$He is obtained from saturated vapor pressure up to solidification. The single-excitation spectrum is substantially modified at high pressures, as the maxon energy exceeds the roton-roton decay threshold. A highly structured multi-excitation spectrum is observed at low energies, where clear thresholds and branches have been identified. Strong phonon emission branches are observed when the phonon or roton group velocities exceed the sound velocity. The spectrum is found to display strong multi-excitations whenever the single-excitations face disintegration following Pitaevskii's type a or b criteria. At intermediate energies, an interesting pattern in the dynamic structure factor is observed in the vicinity of the recoil energy. All these features, which evolve significantly with pressure, are in very good agreement with the Dynamic Many-body calculations, even at the highest densities, where the correlations are strongest.

[51]  arXiv:1802.08123 [pdf, ps, other]
Title: Dual Shapiro steps of a phase-slip junction in the presence of a parasitic capacitance
Comments: 5+3 pages; 3 figures
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Quantum Physics (quant-ph)

Bloch oscillations in a single Josephson junction in the phase-slip regime relate current to frequency. They can be measured by applying a periodic drive to a DC-biased, small Josephson junction. Phase-locking between the periodic drive and the Bloch oscillations then gives rise to steps at constant current in the I-V curves, also known as dual Shapiro steps. Unlike conventional Shapiro steps, a measurement of these dual Shapiro steps is impeded by the presence of a parasitic capacitance. This capacitance shunts the junction resulting in a suppression of the amplitude of the Bloch oscillations. This detrimental effect of the parasitic capacitance can be remedied by an on-chip superinductance. Additionally, we introduce a large off-chip resistance to provide the necessary dissipation. We investigate the resulting system by a set of analytical and numerical methods. In particular, we obtain an explicit analytical expression for the height of dual Shapiro steps as a function of the ratio of the parasitic capacitance to the superinductance. Using this result, we provide a quantitative estimate of the dual Shapiro step height. Our calculations reveal that even in the presence of a parasitic capacitance, it should be possible to observe Bloch oscillations with realistic experimental parameters.

[52]  arXiv:1802.08136 [pdf]
Title: High Seebeck coefficient and ultra-low lattice thermal conductivity in Cs2InAgCl6
Comments: 19 pages. arXiv admin note: text overlap with arXiv:1801.03703
Subjects: Materials Science (cond-mat.mtrl-sci)

The elastic, electronic and thermoelectric properties of indium-based double-perovskite halide, Cs2InAgCl6 have been studied by first principles study. The Cs2InAgCl6 is found to be elastically stable, ductile, anisotropic and relatively low hard material. The calculated direct bandgap 3.67 eV by TB-mBJ functional fairly agrees with the experimentally measured value 3.3 eV but PBE functional underestimates the bandgap by 1.483 eV. The relaxation time and lattice thermal conductivity have been calculated by using relaxation time approximation (RTA) within the supercell approach. The lattice thermal conductivity (\k{appa}l) is quite low (0.2 Wm-1K-1). The quite low phonon group velocity in the large weighted phase space, and high anharmonicity (large phonon scattering) are responsible for small \k{appa}l. The room temperature Seebeck coefficient is 199 {\mu}VK-1. Such high Seebeck coefficient arises from the combination of the flat conduction band and large bandgap. We obtain power factors at 300K by using PBE and TB-mBJ potentials are ~29 and ~31 mWm-1K-2, respectively and the corresponding thermoelectric figure of merit of Cs2BiAgCl6 are 0.71 and 0.72. However, the maximum ZT value obtained at 700K is ~0.74 by TB-mBJ potential. The obtained results implies that Cs2InAgCl6 is a promising material for thermoelectric device applications.

[53]  arXiv:1802.08152 [pdf]
Title: Room Temperature Intrinsic Ferromagnetism in Epitaxial Manganese Selenide Films in the Monolayer Limit
Subjects: Materials Science (cond-mat.mtrl-sci); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

Monolayer van der Waals (vdW) magnets provide an exciting opportunity for exploring two-dimensional (2D) magnetism for scientific and technological advances, but the intrinsic ferromagnetism has only been observed at low temperatures. Here, we report the observation of room temperature ferromagnetism in manganese selenide (MnSe$_x$) films grown by molecular beam epitaxy (MBE). Magnetic and structural characterization provides strong evidence that in the monolayer limit, the ferromagnetism originates from a vdW manganese diselenide (MnSe$_2$) monolayer, while for thicker films it could originate from a combination of vdW MnSe$_2$ and/or interfacial magnetism of $\alpha$-MnSe(111). Magnetization measurements of monolayer MnSe$_x$ films on GaSe and SnSe$_2$ epilayers show ferromagnetic ordering with large saturation magnetization of ~ 4 Bohr magnetons per Mn, which is consistent with density functional theory calculations predicting ferromagnetism in monolayer 1T-MnSe$_2$. Growing MnSe$_x$ films on GaSe up to high thickness (~ 40 nm) produces $\alpha$-MnSe(111), and an enhanced magnetic moment (~ 2x) compared to the monolayer MnSe$_x$ samples. Detailed structural characterization by scanning transmission electron microscopy (STEM), scanning tunneling microscopy (STM), and reflection high energy electron diffraction (RHEED) reveal an abrupt and clean interface between GaSe(0001) and $\alpha$-MnSe(111). In particular, the structure measured by STEM is consistent with the presence of a MnSe$_2$ monolayer at the interface. These results hold promise for potential applications in energy efficient information storage and processing.

[54]  arXiv:1802.08172 [pdf, ps, other]
Title: Shear strength of wet granular materials: macroscopic cohesion and effective stress
Comments: 16 pages, 22 figures (25 images or plots). Submitted for publication in European Physical Journal E
Subjects: Materials Science (cond-mat.mtrl-sci)

Rheometric measurements on assemblies of wet polystyrene bead assemblies, in steady uniform quasistatic shear flow, for varying liquid content within the small saturation (pendular) range of isolated liquid bridges, are supplemented with a systematic study by discrete numerical simulations. Numerical results and experimental ones agree quantitatively is the intergranular friction coefficient is set to 0.09, suitable for the dry material. Shear resistance and solid fraction are recorded as functions of the reduced pressure p, comparing normal stress to capillary bridge tensile strength. The Mohr-Coulomb relation with p-independent cohesion c applies for p above 2. The assumption that contact force contributions to stress act as effective stresses predicts shear strength quite well throughout the numerically investigated range of parameters.. A generalized Mohr-Coulomb cohesion c is defined, which relates to the dry material internal friction, coordination numbers and capillary force network anisotropy. The Rumpf formula approximation, ignoring capillary shear stress is correct for the larger saturation range within the pendular regime, but fails to describe its decrease for small liquid contents.

[55]  arXiv:1802.08181 [pdf, ps, other]
Title: Infrared study of the quasi-two-dimensional electron system at the interface between SrTiO$_{3}$ and crystalline or amorphous LaAlO$_3$
Comments: 21 pages, 7 figures
Subjects: Materials Science (cond-mat.mtrl-sci)

We have used grazing-angle infrared spectroscopy to detect the Berreman effect (BE) in the quasi-two-dimensional electron system (q-2DES) which forms spontaneously at the interface between SrTiO$_{3}$ (STO) and a thin film of LaAlO$_3$ (LAO). From the BE, which allows one to study longitudinal optical excitations in ultrathin films like the q-2DES, we have extracted at different temperatures its thickness, the charge density and mobility of the carriers under crystalline LAO (sample A), and the charge density under amorphous LAO (sample B). This quantity turns out to be higher than in sample A, but a comparison with Hall measurements shows that under amorphous LAO the charges are partly localized at low $T$ with a low activation energy (about 190 K in $k_B$ units), and are thermally activated according to a model for large polarons. The thickness of the q-2DES extracted from our spectra turns out to be 4 $\pm 1$ nm for crystalline LAO, 7 $\pm 2$ nm for amorphous LAO.

[56]  arXiv:1802.08190 [pdf, ps, other]
Title: Large and realistic models of Amorphous Silicon
Subjects: Disordered Systems and Neural Networks (cond-mat.dis-nn)

Amorphous silicon (a-Si) models are analyzed for structural, electronic and vibrational characteristics. Several models of various sizes have been computationally fabricated for this analysis. It is shown that a recently developed structural modeling algorithm known as force-enhanced atomic refinement (FEAR) provides results in agreement with experimental neutron and x-ray diffraction data while producing a total energy below conventional schemes. We also show that a large model (500 atoms) and a complete basis is necessary to properly describe vibrational and thermal properties. We compute the density for a-Si, and compare with experimental results.

[57]  arXiv:1802.08196 [pdf, other]
Title: The role of canting and depleted-triplet minima in superconducting spin valve structures
Comments: 12 pages, 11 figures
Subjects: Superconductivity (cond-mat.supr-con)

The trilayer and pentalayer spin valve structures are revisited to determine the behavior of pair correlations and Josephson current when the magnetic layers are canted at arbitrary angle. The two systems display markedly different behaviors in the center magnetic layer. While the trilayer generates a triplet component that is weakly affected by canting, the pentalayer tunes in singlet pair correlations depending heavily on canting. We also show that a minimum with depleted $m=\pm1$ triplet components, rather than a $0-\pi$ transition, may be observed in the current profile $I_c(d_F)$ of a trilayer spin valve. The depleted-triplet minimum (DTM) is directly attributable to a decrease of $m=\pm1$ triplet correlations with increased thickness of the central ferromagnet, accompanied by a hidden, simultaneous sign change of the Gor'kov functions contributed from the left and right superconductors. We introduce a toy model for superconducting-magnetic proximity systems to better illuminate the behavior of individual components of the Gor'kov function and compare with a full numerical calculation.

[58]  arXiv:1802.08199 [pdf]
Title: Effects of Grain Boundary Disorder on Yield Strength
Subjects: Materials Science (cond-mat.mtrl-sci)

It was recently reported that segregation of Zr to grain boundaries (GB) in nanocrystalline Cu can lead to the formation of disordered intergranular films [1,2]. In this study we employ atomistic computer simulations to study how the formation of these films affects the dislocation nucleation from the GBs. We found that full disorder of the grain boundary structure leads to the suppression of dislocation emission and significant increase of the yield stress. Depending on the solute concentration and heat-treatment, however, a partial disorder may also occur and this aids dislocation nucleation rather than suppressing it, resulting in elimination of the strengthening effect.

[59]  arXiv:1802.08208 [pdf, other]
Title: Thermally driven anomalous Hall effect transitions in FeRh
Subjects: Materials Science (cond-mat.mtrl-sci)

Materials exhibiting controllable magnetic phase transitions are currently in demand for many spintronics applications. Here we investigate from first principles the electronic structure and intrinsic anomalous Hall, spin Hall and anomalous Nernst response properties of the FeRh metallic alloy which undergoes a thermally driven antiferromagnetic-to-ferromagnetic phase transition. We show that the energy band structures and underlying Berry curvatures have important signatures in the various Hall effects. Specifically, the suppression of the anomalous Hall and Nernst effects in the AFM state and a sign change in the spin Hall conductivity across the transition are found. It is suggested that the FeRh can be used a spin current detector capable of differentiating the spin Hall effect from other anomalous transverse effects. The implications of this material and its thermally driven phases as a spin current detection scheme are also discussed.

[60]  arXiv:1802.08211 [pdf, other]
Title: Resonant torsion magnetometry in anisotropic quantum materials
Comments: 7 pages including 6 figures and methods section
Subjects: Strongly Correlated Electrons (cond-mat.str-el)

Unusual behavior of quantum materials commonly arises from their effective low-dimensional physics, which reflects the underlying anisotropy in the spin and charge degrees of freedom. Torque magnetometry is a highly sensitive technique to directly quantify the anisotropy in quantum materials, such as the layered high-T$_c$ superconductors, anisotropic quantum spin-liquids, and the surface states of topological insulators. Here we introduce the magnetotropic coefficient $k=\partial^2 F/\partial \theta^2$, the second derivative of the free energy F with respect to the angle $\theta$ between the sample and the applied magnetic field, and report a simple and effective method to experimentally detect it. A sub-$\mu$g crystallite is placed at the tip of a commercially available atomic force microscopy cantilever, and we show that $k$ can be quantitatively inferred from a shift in the resonant frequency under magnetic field. While related to the magnetic torque $\tau=\partial F/\partial \theta$, $k$ takes the role of torque susceptibility, and thus provides distinct insights into anisotropic materials akin to the difference between magnetization and magnetic susceptibility. The thermodynamic coefficient $k$ is discontinuous at second-order phase transitions and subject to Ehrenfest relations with the specific heat and magnetic susceptibility. We apply this simple yet quantitative method on the exemplary cases of the Weyl-semimetal NbP and the spin-liquid candidate RuCl$_3$, yet it is broadly applicable in quantum materials research.

[61]  arXiv:1802.08220 [pdf]
Title: The optical properties of transferred graphene and the dielectrics grown on it obtained by ellipsometry
Comments: 8 pages, 8 figures, 2 tables, presented on "Solid State Surfaces and Interfaces", November 2016 Piestany, Slovakia
Journal-ref: Applied surface Science, 437 (2018), 410-417
Subjects: Materials Science (cond-mat.mtrl-sci)

Graphene layers grown by chemical vapour deposition (CVD) method and transferred from Cu-foils to the oxidized Si-substrates were investigated by spectroscopic ellipsometry (SE), Raman and X-Ray Photoelectron Spectroscopy (XPS) methods. The optical properties of transferred CVD graphene layers do not always correspond to the ones of the exfoliated graphene due to the contamination from the chemicals used in the transfer process. However, the real thickness and the mean properties of the transferred CVD graphene layers can be found using ellipsometry if a real thickness of the SiO2 layer is taken into account. The pulsed layer deposition (PLD) and atomic layer deposition (ALD) methods were used to grow dielectric layers on the transferred graphene and the obtained structures were characterized using optical methods. The approach demonstrated in this work could be useful for the characterization of various materials grown on graphene.

[62]  arXiv:1802.08221 [pdf, other]
Title: Uncovering Weyl Fermions in the Quantum Limit of NbP
Comments: 6 pages including 5 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el)

The Fermi surface topology of a Weyl semimetal (WSM) depends strongly on the position of the chemical potential. If it resides close to the band touching points (Weyl nodes), as it does in TaAs, separate Fermi surfaces of opposite chirality emerge, leading to novel phenomena such as the chiral magnetic effect. If the chemical potential lies too far from the nodes, however, the chiral Fermi surfaces merge into a single large Fermi surface with no net chirality. This is realized in the WSM NbP, where the Weyl nodes lie far below the Fermi energy and where the transport properties in low magnetic fields show no evidence of chiral Fermi surfaces. Here we show that the behavior of NbP in high magnetic fields is nonetheless dominated by the presence of the Weyl nodes. Torque magnetometry up to 60 tesla reveals a change in the slope of $\tau/B$ at the quantum limit B$^\star$ ($\approx 32\,\rm{T}$), where the chemical potential enters the $n=0$ Landau level. Numerical simulations show that this behaviour results from the magnetic field pulling the chemical potential to the chiral $n=0$ Landau level belonging to the Weyl nodes. These results show that high magnetic fields can uncover topological singularities in the underlying band structure of a potential WSM, and can recover topologically non-trivial experimental properties, even when the position of the chemical potential precludes their observation in zero magnetic field.

[63]  arXiv:1802.08222 [pdf, other]
Title: Spin-1 Topological Monopoles in Parameter Space of Ultracold Atoms
Comments: 8 pages, 6 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)

Magnetic monopole, a hypothetical elementary particle with isolated magnetic pole, is crucial for the quantization of electric charge. In recent years, analogues of magnetic monopoles, represented by topological defects in parameter spaces, have been studied in a wide range of physical systems. These works mainly focused on Abelian Dirac monopoles in spin-1/2 or non-Abelian Yang monopoles in spin-3/2 systems. Here we propose to realize three types of spin-1 topological monopoles and study their geometric properties using the parameter space formed by three hyperfine states of ultracold atoms coupled by radio-frequency fields. These spin-1 monopoles, characterized by different monopole charges, possess distinct Berry curvature fields and spin textures, which are directly measurable in experiments. The topological phase transitions between different monopoles are accompanied by the emergence of spin "vortex", and can be intuitively visualized using Majorana's stellar representation. We show how to determine the Berry curvature, hence the geometric phase and monopole charge from dynamical effects. Our scheme provides a simple and highly tunable platform for observing and manipulating spin-1 topological monopoles, paving the way for exploring new topological quantum matter.

[64]  arXiv:1802.08243 [pdf, other]
Title: Magnetic properties of metal-organic coordination networks based on 3d transition metal atoms
Comments: 17 pages, 7 figures, submitted to Molecules (MDPI) on February 21st 2018, Supplementary Material in submit/2172866
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Materials Science (cond-mat.mtrl-sci); Strongly Correlated Electrons (cond-mat.str-el)

The magnetic anisotropy and exchange coupling between spins localized at the positions of 3d transition metal atoms forming two-dimensional metal-organic coordination networks (MOCNs) grown on the Au(111) metal surface are studied. In particular, we consider MOCNs made of Ni or Mn metal centers linked by TCNQ (7,7,8,8-tetracyanoquinodimethane) organic ligands, which form rectangular networks with 1:1 stoichiometry. Based on the analysis of X-ray magnetic circular dichroism (XMCD) data taken at T= 2.5 K, we find that Ni atoms in the Ni-TCNQ MOCNs are coupled ferromagnetically and do not show any significant magnetic anisotropy, while Mn atoms in the Mn-TCNQ MOCNs are coupled antiferromagnetically and do show a weak magnetic anisotropy with in-planemagnetization. We explain these observations using both amodelHamiltonian based on mean-fieldWeiss theory and density functional theory calculations that include spin-orbit coupling. Our main conclusion is that the antiferromagnetic coupling between Mn spins and the in-plane magnetization of the Mn spins can be explained neglecting effects due to the presence of the Au(111) surface, while for Ni-TCNQ the metal surface plays a role in determining the absence of magnetic anisotropy in the system.

Cross-lists for Fri, 23 Feb 18

[65]  arXiv:1802.07747 (cross-list from hep-th) [pdf, other]
Title: Higher-form symmetries and spontaneous symmetry breaking
Authors: Ethan Lake
Comments: 37 pages
Subjects: High Energy Physics - Theory (hep-th); Strongly Correlated Electrons (cond-mat.str-el)

We study various aspects of spontaneous symmetry breaking in theories that possess higher-form symmetries, which are symmetries whose charged objects have a dimension $p>0$. We first sketch a proof of a higher version of Goldstone's theorem, and then discuss how boundary conditions and gauge-fixing issues are dealt with in theories with spontaneously broken higher symmetries, focusing in particular on $p$-form $U(1)$ gauge theories. We then elaborate on a generalization of the Coleman-Mermin-Wagner theorem for higher-form symmetries, namely that in spacetime dimension $D$, continuous $p$-form symmetries can never be spontaneously broken if $p\geq D-2$. We also make a few comments on relations between higher symmetries and asymptotic symmetries in Abelian gauge theory.

[66]  arXiv:1802.07841 (cross-list from physics.optics) [pdf]
Title: Topological insulator materials for advanced optoelectronic devices
Comments: 28 pages, 8 figures
Subjects: Optics (physics.optics); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

Topological insulators are quantum materials that have an insulating bulk state and a topologically protected metallic surface state with spin and momentum helical locking and a Dirac-like band structure. Unique and fascinating electronic properties, such as the quantum spin Hall effect, quantum anomalous Hall effect, and topological magnetoelectric effect, as well as magnetic monopole images and Majorana fermions, have been observed in the topological insulator materials. With these unique properties, topological insulator materials have great potential applications in spintronics and quantum information processing, as well as magnetoelectric devices with higher efficiency and lower energy consumption. On the other hand, topological insulator materials also exhibit a number of excellent optical properties, including Kerr and Faraday rotation, ultrahigh bulk refractive index, near-infrared frequency transparency, unusual electromagnetic scattering, and ultra-broadband surface plasmon resonances. Specifically, Dirac plasmon excitations have been observed in Bi2Se3 micro-ribbon arrays at THz frequencies. Ultraviolet and visible frequency plasmonics have been observed in nanoslit and nanocone arrays of Bi1.5Sb0.5Te1.8Se1.2 crystals. High transparency has been observed in Bi2Se3 nanoplates. An ultrahigh refractive index has been observed in bulk Bi1.5Sb0.5Te1.8Se1.2 crystals as well as in Sb2Te3 thin films. These excellent optical properties mean that topological insulator materials are suitable for various optoelectronic devices, including plasmonic solar cells, ultrathin holograms, plasmonic and Fresnel lens, broadband photodetectors, and nanoscale waveguides. In this chapter, we focus on the excellent electronic and optical properties of topological insulator materials and their wide applications in advanced optoelectronic devices.

[67]  arXiv:1802.07873 (cross-list from physics.chem-ph) [pdf]
Title: Charge Transfer Database for Bio-molecule Tight Binding Model Derived from Thousands of Proteins
Comments: 19 pages, 5 figures
Subjects: Chemical Physics (physics.chem-ph); Soft Condensed Matter (cond-mat.soft)

The anisotropic feature of charge transfer reactions in realistic proteins cannot be ignored, due to the highly complex chemical structure of bio-molecules. In this work, we have performed the first large-scale quantitative assessment of charge transfer preference in protein complexes by calculating the charge transfer couplings in all 20*20 possible amino acid side chain combinations, which are extracted from available high-quality structures of thousands of protein complexes. The charge transfer database quantitatively shows distinct features of charge transfer couplings among millions of amino acid side-chains combinations. The knowledge graph of charge transfer couplings reveals that only one average or representative structure cannot be regarded as the typical charge transfer preference in realistic proteins. This data driven model provides us an alternative route to comprehensively understand the pairwise charge transfer coupling parameters based structural similarity, without any require of the knowledge of chemical intuition about the chemical interactions.

[68]  arXiv:1802.08160 (cross-list from quant-ph) [pdf, other]
Title: Realization of a quantum walk in momentum space with a Bose-Einstein condensate
Comments: 16 pages, 4 figures
Subjects: Quantum Physics (quant-ph); Quantum Gases (cond-mat.quant-gas); Atomic Physics (physics.atom-ph)

Randomness is the essence of many processes in nature and human society. It can provide important insights into phenomena as diverse as disease transmission, financial markets, and signal processing [1, 2]. Quantum randomness is intrinsically different from classical stochasticity since it is affected by interference and entanglement. This entanglement makes quantum walks promising candidates for the implementation of quantum computational algorithms [3-5] and as a detector of quantum behavior [6-8]. We present a discrete-time quantum walk that uses the momentum of ultra-cold rubidium atoms as the walk space and two internal atomic states as the "coin" degree of freedom. We demonstrate the principle features of a quantum walk, contrasting them to the behavior of a classical walk. By manipulating either the walk or coin operator we show how the walk dynamics can be biased or reversed. Our walk offers distinct advantages arising from the robustness of its dynamics in momentum space [9-11], and extendability to higher dimensions [12-14] and many-body regimes [5, 15-19].

[69]  arXiv:1802.08197 (cross-list from quant-ph) [pdf, other]
Title: Chaos in Dirac electron optics: Emergence of a relativistic quantum chimera
Comments: 6 pages, 4 figures
Subjects: Quantum Physics (quant-ph); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Chaotic Dynamics (nlin.CD)

We uncover a remarkable quantum scattering phenomenon in two-dimensional Dirac material systems where the manifestations of both classically integrable and chaotic dynamics emerge simultaneously and are electrically controllable. The distinct relativistic quantum fingerprints associated with different electron spin states are due to a physical mechanism analogous to chiroptical effect in the presence of degeneracy breaking. The phenomenon mimics a chimera state in classical complex dynamical systems but here in a relativistic quantum setting - henceforth the term "Dirac quantum chimera," associated with which are physical phenomena with potentially significant applications such as enhancement of spin polarization, unusual coexisting quasibound states for distinct spin configurations, and spin selective caustics. Experimental observations of these phenomena are possible through, e.g., optical realizations of ballistic Dirac fermion systems.

[70]  arXiv:1802.08210 (cross-list from math.PR) [pdf, ps, other]
Title: Half-space Macdonald processes
Comments: 106 pages, 17 figures
Subjects: Probability (math.PR); Statistical Mechanics (cond-mat.stat-mech); Mathematical Physics (math-ph); Combinatorics (math.CO)

Macdonald processes are measures on sequences of integer partitions built using the Cauchy summation identity for Macdonald symmetric functions. These measures are a useful tool to uncover the integrability of many probabilistic systems, including the Kardar-Parisi-Zhang (KPZ) equation and a number of other models in its universality class. In this paper we develop the structural theory behind half-space variants of these models and the corresponding half-space Macdonald processes. These processes are built using a Littlewood summation identity instead of the Cauchy identity, and their analysis is considerably harder than their full-space counterparts.
We compute moments and Laplace transforms of observables for general half-space Macdonald measures. Introducing new dynamics preserving this class of measures, we relate them to various stochastic processes, in particular the log-gamma polymer in a half-quadrant (they are also related to the stochastic six-vertex model in a half-quadrant and the half-space ASEP). For the polymer model, we provide explicit integral formulas for the Laplace transform of the partition function. Non-rigorous saddle point asymptotics yield convergence of the directed polymer free energy to either the Tracy-Widom GOE, GSE or the Gaussian distribution depending on the average size of weights on the boundary.

[71]  arXiv:1802.08214 (cross-list from quant-ph) [pdf, ps, other]
Title: Computational complexity of PEPS zero testing
Comments: 4+3 pages, 5 figures
Subjects: Quantum Physics (quant-ph); Strongly Correlated Electrons (cond-mat.str-el)

Projected entangled pair states aim at describing lattice systems in two spatial dimensions that obey an area law. They are specified by associating a tensor with each site, and they are generated by patching these tensors. We consider the problem of determining whether the state resulting from this patching is null, and prove it to be NP-hard; the PEPS used to prove this claim have a boundary and are homogeneous in their bulk. A variation of this problem is next shown to be undecidable. These results have various implications: they question the possibility of a 'fundamental theorem' for PEPS; there are PEPS for which the presence of a symmetry is undecidable; there exist parent hamiltonians of PEPS for which the existence of a gap above the ground state is undecidable. En passant, we identify a family of classical Hamiltonians, with nearest neighbour interactions, and translationally invariant in their bulk, for which the commuting 2-local Hamiltonian problem is NP-complete.

Replacements for Fri, 23 Feb 18

[72]  arXiv:1606.07990 (replaced) [pdf]
Title: Tailoring optical metamaterials to tune the atom-surface Casimir-Polder interaction
Comments: 21 pages, 9 figures
Journal-ref: E. A. Chan, S. A. Aljunid, G. Adamo, A. Laliotis, M. Ducloy, D. Wilkowski, Tailoring optical metamaterials to tune the atom-surface Casimir-Polder interaction. Sci. Adv. 4, eaao4223 (2018)
Subjects: Atomic Physics (physics.atom-ph); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Optics (physics.optics)
[73]  arXiv:1607.04507 (replaced) [pdf, other]
Title: Two impurities in a Bose-Einstein condensate: from Yukawa to Efimov attracted polarons
Authors: Pascal Naidon
Comments: 4 pages + Appendices (10 pages in total), 1 figure
Subjects: Quantum Gases (cond-mat.quant-gas)
[74]  arXiv:1608.01096 (replaced) [pdf, ps, other]
Title: Conductivity of disordered 2d binodal Dirac electron gas: Effect of the internode scattering
Subjects: Strongly Correlated Electrons (cond-mat.str-el); High Energy Physics - Theory (hep-th)
[75]  arXiv:1612.07633 (replaced) [pdf, other]
Title: Unravelling incommensurate magnetism and its emergence in iron-based superconductors
Comments: 18 pages, 10 figures + supplementary material
Subjects: Superconductivity (cond-mat.supr-con); Strongly Correlated Electrons (cond-mat.str-el)
[76]  arXiv:1703.05363 (replaced) [pdf, other]
Title: Orthogonality catastrophe and fractional exclusion statistics
Comments: 9 pages, 2 figures. Final version published in Phys. Rev. E. Some comments and references added respect to v2
Journal-ref: Phys. Rev. E 97, 022133 (2018)
Subjects: Statistical Mechanics (cond-mat.stat-mech); High Energy Physics - Theory (hep-th); Quantum Physics (quant-ph)
[77]  arXiv:1704.01021 (replaced) [pdf, other]
Title: Nonvolatile Solid-State Charged-Polymer Gating of Topological Insulators into the Topological Insulating Regime
Comments: Submitted, 5 pages of text, 6 figures
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Materials Science (cond-mat.mtrl-sci)
[78]  arXiv:1704.02663 (replaced) [pdf, ps, other]
Title: Entropic Dynamics: Mechanics without Mechanism
Authors: Ariel Caticha
Comments: Invited chapter to appear in "Recent Advances in Info-Metrics" edited by M. Chen and A. Golan. Version 2 includes an improved treatment of the prior and added remarks on the differences between Entropic Dynamics and hidden-variable theories. arXiv admin note: text overlap with arXiv:1509.03222
Subjects: Quantum Physics (quant-ph); Statistical Mechanics (cond-mat.stat-mech)
[79]  arXiv:1705.05694 (replaced) [pdf, other]
Title: Note: Derivative divide, a method for the analysis of broadband ferromagnetic resonance in the frequency domain
Subjects: Materials Science (cond-mat.mtrl-sci)
[80]  arXiv:1706.10213 (replaced) [pdf]
Title: Layer- and doping-tunable long range ferromagnetic order-ing in two-dimensional CrS2
Subjects: Materials Science (cond-mat.mtrl-sci); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)
[81]  arXiv:1707.05671 (replaced) [pdf, other]
Title: Time evolution of effective central charge and signatures of RG irreversibility after a quantum quench
Comments: Some discussion modified. Title slightly modified. References added. Scipost submission
Subjects: Statistical Mechanics (cond-mat.stat-mech); Strongly Correlated Electrons (cond-mat.str-el); High Energy Physics - Theory (hep-th)
[82]  arXiv:1708.00184 (replaced) [pdf, ps, other]
Title: The derivation of Markov processes that violate detailed balance
Authors: Julian Lee
Comments: 16 pages, 4 figures. Revised version submitted to a journal. Numerical computation for a model with smooth transition to equilibrium added. Requirement for the derivation of Schnakenberg entropy production formula is weakened. The relations with the quantities in previous works explained in the appendices. A figure showing the full equilibrium distribution of the extended three-state model added
Subjects: Statistical Mechanics (cond-mat.stat-mech)
[83]  arXiv:1708.00683 (replaced) [pdf, other]
Title: The dependence of the absolute value penetration depth on doping in $\mathrm{\mathbf{{(Ba_{1-x}K_x)Fe_2As_2}}}$
Comments: Replaced Fig. 3, improved discussion and abstract, minor changes to the rest of the text
Subjects: Superconductivity (cond-mat.supr-con); Materials Science (cond-mat.mtrl-sci); Strongly Correlated Electrons (cond-mat.str-el)
[84]  arXiv:1708.04282 (replaced) [pdf, ps, other]
Title: Inner Nonlinear Waves and Inelastic Light Scattering of Fractional Quantum Hall States as Evidence of the Gravitational Anomaly
Authors: P. Wiegmann
Comments: 6 pages, v.2 minor changes in introduction and display of equations
Journal-ref: Phys. Rev. Lett. 120, 086601, 20 February 2018
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
[85]  arXiv:1709.00325 (replaced) [pdf, other]
Title: Kondo Impurities Coupled to Helical Luttinger Liquid: RKKY-Kondo Physics Revisited
Comments: 6 pages, 2 figures; the Bibliography, Conclusions and discussions are substantially expanded in this version which has been accepted for publication in Phys. Rev. Lett
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)
[86]  arXiv:1709.05039 (replaced) [pdf, ps, other]
Title: Energy spectrum, the spin polarization, and the optical selection rules of the Kronig-Penney superlattice model with spin-orbit coupling
Authors: Rui Li
Comments: 8 pages, 5 figures
Journal-ref: Phys. Rev. B 97, 085430 (2018)
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Quantum Physics (quant-ph)
[87]  arXiv:1709.06208 (replaced) [pdf]
Title: Hydrogen as a Source of Flux Noise in SQUIDs
Comments: 12 pages, 4 figures
Subjects: Materials Science (cond-mat.mtrl-sci)
[88]  arXiv:1710.00380 (replaced) [pdf, other]
Title: Quantum critical singularities in two-dimensional metallic XY ferromagnets
Comments: Experimental results on dynamics near the quantum-critical point have been added with some additions in the text to point out their correspondence with theory. Some corrections have also been made
Journal-ref: Phys. Rev. B 97, 085134 (2018)
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
[89]  arXiv:1710.00650 (replaced) [pdf, ps, other]
Title: Functional Renormalization Group and Kohn-Sham scheme in Density Functional Theory
Comments: 6 pages, 3 figures, and 1 table
Journal-ref: Phys. Lett. B (2018)
Subjects: Strongly Correlated Electrons (cond-mat.str-el); High Energy Physics - Theory (hep-th); Nuclear Theory (nucl-th); Quantum Physics (quant-ph)
[90]  arXiv:1710.00921 (replaced) [pdf, other]
Title: Black hole scrambling from hydrodynamics
Comments: V2: 6 pages, 1 figure. Footnote 3 from V1 regarding the disappearance of the pole at the special point of chaos is rephrased in terms of a vanishing residue (of the retarded longitudinal stress-energy tensor two-point function) and inserted into the main text in order to make its meaning more transparent. Other small clarifications and references are also added
Subjects: High Energy Physics - Theory (hep-th); Strongly Correlated Electrons (cond-mat.str-el); General Relativity and Quantum Cosmology (gr-qc); Chaotic Dynamics (nlin.CD)
[91]  arXiv:1710.02077 (replaced) [pdf, other]
Title: Delocalization of charge and current in a chiral quasiparticle wave-packet
Authors: Subhajit Sarkar
Comments: 19 pages, 6 figures, Version accepted for publication in Phys. Rev. B
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Other Condensed Matter (cond-mat.other)
[92]  arXiv:1710.07916 (replaced) [pdf, other]
Title: Chern-Simons theory and atypical Hall conductivity in the Varma phase
Comments: 5 pages, 3 figures; New version with an improved discussion about our findings
Journal-ref: Phys. Rev. B 97, 075135 (2018)
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Superconductivity (cond-mat.supr-con); High Energy Physics - Theory (hep-th)
[93]  arXiv:1710.08278 (replaced) [pdf, other]
Title: Highly detailed computational study of a surface reaction model with diffusion: four algorithms analyzed via time-dependent and steady-state Monte Carlo simulations
Comments: 23 pages, 7 figures, 2 tables
Subjects: Statistical Mechanics (cond-mat.stat-mech)
[94]  arXiv:1710.08385 (replaced) [pdf, other]
Title: Entangled multi-component 4D quantum Hall states from photonic crystal defects
Comments: 14 pages, 11 figures
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Disordered Systems and Neural Networks (cond-mat.dis-nn); Materials Science (cond-mat.mtrl-sci)
[95]  arXiv:1710.11305 (replaced) [pdf, other]
Title: A probabilistic cellular automata model for the dynamics of a population driven by logistic growth and weak Allee effect
Comments: 19 pages, 6 figures, 55 references. Accepted for publication in J. Phys. A: Math. Theor. (IoP)
Subjects: Statistical Mechanics (cond-mat.stat-mech); Cellular Automata and Lattice Gases (nlin.CG); Populations and Evolution (q-bio.PE)
[96]  arXiv:1711.00011 (replaced) [pdf, other]
Title: Majorana quasiparticles in condensed matter
Authors: Ramón Aguado
Comments: 75 pages, 26 figures. Minor modifications and updated references
Journal-ref: Riv. Nuovo Cimento 40(11), 523-593 (2017)
Subjects: Superconductivity (cond-mat.supr-con); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)
[97]  arXiv:1711.02154 (replaced) [pdf, other]
Title: Multi-determinant generalized Hartree-Fock wave functions in Monte Carlo calculations
Comments: 7 pages, 5 figures, 1 table, revised version
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
[98]  arXiv:1711.06285 (replaced) [pdf, other]
Title: Fermi arc reconstruction at junctions between Weyl semimetals
Authors: Vatsal Dwivedi
Comments: 13 pages, 7 figures
Journal-ref: Phys. Rev. B 97, 064201 (2018)
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Other Condensed Matter (cond-mat.other)
[99]  arXiv:1711.08829 (replaced) [pdf, ps, other]
Title: On Efimov Nonlocal and Nonpolynomial Quantum Scalar Field Theory
Comments: 25 pages; v3: references added, minor corrections
Subjects: High Energy Physics - Theory (hep-th); Statistical Mechanics (cond-mat.stat-mech)
[100]  arXiv:1711.10616 (replaced) [pdf, other]
Title: Bulk diffusion in a kinetically constrained lattice gas
Comments: 29 pages, 12 figures. v2: minor changes
Journal-ref: J. Phys. A: Math. Theor. 51, 125002 (2018)
Subjects: Statistical Mechanics (cond-mat.stat-mech); Mathematical Physics (math-ph); Probability (math.PR); Cellular Automata and Lattice Gases (nlin.CG)
[101]  arXiv:1712.00070 (replaced) [pdf, other]
Title: When and how much the altruism impacts your privileged information? Proposing a new paradigm in game theory: The boxers game
Comments: 16 pages, 6 figures
Subjects: Statistical Mechanics (cond-mat.stat-mech); Computer Science and Game Theory (cs.GT)
[102]  arXiv:1712.02156 (replaced) [pdf, other]
Title: Interfacial Strain Effects on Lithium Diffusion Pathways in the Spinel Solid Electrolyte Li-Doped MgAl$_2$O$_4$
Comments: Data supporting this manuscript are openly available at doi: 10.15125/BATH-00438 and doi: 10.5281/zenodo.1069417
Subjects: Materials Science (cond-mat.mtrl-sci)
[103]  arXiv:1712.03537 (replaced) [pdf, other]
Title: Correlation Effects in the Quench-Induced Phase Separation Dynamics of a Two-Component Ultracold Quantum Gas
Comments: 16 pages, 9 Figures
Subjects: Quantum Gases (cond-mat.quant-gas); Atomic Physics (physics.atom-ph); Quantum Physics (quant-ph)
[104]  arXiv:1712.04840 (replaced) [pdf, ps, other]
Title: Breaking Symmetry with Light: Ultra-Fast Ferroelectricity and Magnetism from Three-Phonon Coupling
Comments: 10 pages
Journal-ref: Phys. Rev. B 97, 085145 (2018)
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
[105]  arXiv:1712.05069 (replaced) [pdf, other]
Title: Majorana Fermions and Orthogonal Complex Structures
Comments: 15 pages, 6 figures, typos corrected
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Mathematical Physics (math-ph); Quantum Physics (quant-ph)
[106]  arXiv:1712.05396 (replaced) [pdf, other]
Title: The t-t'-J model in one dimension using extremely correlated Fermi liquid theory and time dependent density matrix renormalization group
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
[107]  arXiv:1712.08529 (replaced) [pdf, ps, other]
Title: Tunable transmittance of strained graphene-like materials
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Quantum Gases (cond-mat.quant-gas)
[108]  arXiv:1801.00755 (replaced) [pdf, other]
Title: Graphene lattice-layer entanglement under non-Markovian phase noise
Comments: 21 pages, 4 figures
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); High Energy Physics - Theory (hep-th); Quantum Physics (quant-ph)
[109]  arXiv:1801.01823 (replaced) [pdf, other]
Title: Separating intrinsic interactions from extrinsic correlations in a network of sensory neurons
Subjects: Neurons and Cognition (q-bio.NC); Disordered Systems and Neural Networks (cond-mat.dis-nn)
[110]  arXiv:1802.01083 (replaced) [pdf, ps, other]
Title: Mesoscopic Description of the Equal Load Sharing Fiber Bundle Model
Comments: 5 pages
Subjects: Statistical Mechanics (cond-mat.stat-mech)
[111]  arXiv:1802.03483 (replaced) [pdf, other]
Title: Coherence properties of shallow donor qubits in ZnO
Subjects: Quantum Physics (quant-ph); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)
[112]  arXiv:1802.05430 (replaced) [src]
Title: Virtual walks in spin space: a study in a family of two-parameter models
Comments: Major revisions are required following some private communications
Subjects: Statistical Mechanics (cond-mat.stat-mech)
[113]  arXiv:1802.07627 (replaced) [pdf, other]
Title: Readout Of Singlet-Triplet Qubits At Large Magnetic Field Gradients
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall)
[ total of 113 entries: 1-113 ]
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