We explore the temporal dynamics of a surface plasmon on a graphene sheet after an abrupt increase of graphene's carrier density. The plasmon is transformed into two frequency-upshifted surface plasmons propagating in the forward and backward directions, transient free-space radiation, and two-stream dc motion of carriers. The two-stream carrier motion carries zero net electric current and therefore does not generate any magnetic field. Nevertheless, it can consume a substantial fraction, up to a half, of the initial plasmon energy in the form of the kinetic energy of the carriers. We revisit the recent claim in the literature that graphene's nonstationarity can amplify plasmons.

Zigzag-edge graphene sidewall ribbons grown on $6H$-SiC $\left\{11\overline{2}n\right\}$ facet walls are ballistic conductors. It is assumed that graphene sidewall ribbons grown on $4H$-SiC $\left\{11\overline{2}n\right\}$ facets would also be ballistic. In this work, we show that SiC polytype indeed matters: ballistic sidewall graphene ribbons only grow on $6H$-SiC facets. $4H$ and $4H$-passivated sidewall graphene ribbons are diffusive conductors. Detailed photoemission and microscopy studies show that $6H$-SiC sidewall zigzag ribbons are metallic with a pair of $n$-doped edge states associated with asymmetric edge terminations. In contrast, $4H$-SiC zigzag ribbons are strongly bonded to the SiC, severely distorting the ribbon's $\pi$ bands. ${\text{H}}_2$ passivation of the $4H$ ribbons returns them to a metallic state but they show no evidence of edge states in their photoemission-derived band structure.

We study theoretically the nonequilibrium exciton transport in monolayer transition metal dichalcogenides. We consider the situation where excitons interact with nonequilibrium phonons, e.g., under the conditions of localized excitation where a “hot spot” is formed. We develop the theory of the exciton drag by the phonons and analyze in detail the regimes of diffusive propagation of phonons and ballistic propagation of phonons where the phonon wind is generated. We demonstrate that a halolike spatial distribution of excitons akin observed in [Phys. Rev. Lett. 120, 207401 (2018)] can result from the exciton drag by nonequilibrium phonons.

We report a type of structural phase transitions in dielectric materials: the triggered incommensurate (IC) transition. We demonstrate evidence for such a transition in the perovskite antiferroelectric ${\mathrm{PbHfO}}_3$ by means of single-crystal x-ray diffraction, diffuse, and inelastic-scattering experiments, which we interpret using the Landau theory of phase transitions. This transition is not driven by an IC soft mode, as is the case for the majority of IC dielectrics, but by the soft mode associated with a different, coexisting order parameter related to antiferrodistortive (AFD) tilts of the oxygen octahedra. When cooling from the high-temperature cubic phase these two order parameters are established simultaneously and discontinuously at $T_{\text{IC}}\approx 468$ K. Two lattice instabilities are present in the cubic phase: AFD instability with critical temperature $T_0\approx 441$ K and ferroelectric instability with $T_0\approx 408$ K. The IC instability is absent. The analysis of the transition mechanism within the Landau theory of coupled order parameters indicates that the transition is of triggered character, conditioned upon the attractive biquadratic coupling between the IC modulation and the AFD octahedral tilts, and is driven by the AFD soft mode.

The protons in water ice are subject to so-called ice rules resulting in an extensive ground-state degeneracy. We study how an external electric field reduces this ground-state degeneracy in hexagonal water ice $I_h$ within a minimal model. We observe polarization plateaus when the field is aligned along the [001] and [010] directions. In each case, one plateau occurs at intermediate polarization with reduced but still extensive degeneracy. The remaining ground states can be mapped to dimer models on the honeycomb and the square lattice, respectively. Upon tilting the external field, we observe an order-disorder transition of Kasteleyn type into a plateau at saturated polarization and vanishing entropy. This transition is investigated analytically using the Kasteleyn matrix and numerically using a modified directed-loop Monte Carlo simulation. The protons in both cases exhibit algebraically decaying correlations. Moreover, the features of the static structure factor are discussed.

A detailed account of magnetolattice coupling, magnetic frustration, and magnetoelectric effects in ${\mathrm{Fe}}_{1-x}{\mathrm{Cr}}_x{\mathrm{VO}}_4$ ($x=0-1.0$) studied by temperature-dependent synchrotron x ray diffraction (SXRD), temperature- and magnetic-field-dependent dielectric permittivity $\varepsilon$, and magnetization measurements are presented in this paper where progressive Cr doping leads to structural transitions from triclinic (T)—monoclinic (M)—orthorhombic (O) symmetries. SXRD data shows an intricate relationship between magnetic, ferroelectric, and lattice degrees of freedom in these systems. ${\mathrm{FeVO}}_4$ reaches a magnetically ordered state with two successive antiferromagnetic orderings at ${\mathrm{T}}_{N1}$ (21.85 K) and ${\mathrm{T}}_{N2}$ (15.65 K), having collinear and noncollinear natures, respectively, as evidenced in DC magnetization measurements. Progressive ${\mathrm{Cr}}^{3+}$ incorporation at the ${\mathrm{Fe}}^{3+}$ site in ${\mathrm{Fe}}_{1-x}{\mathrm{Cr}}_x{\mathrm{VO}}_4$ shifts these transitions to higher temperatures in T phase ($x=0.0$ and 0.10). At $x=0.175$ [in (T $+$ M) phase], these transitions become closer to each other. Beyond this concentration, a single broad antriferromagnetic transition is observed in M $(x=0.20-0.30$) and O ($x=0.90-1.0$) phases. A nonlinear behavior in isotherm M-H curves below ${\mathrm{T}}_{N2}$ indicates field-induced spin-reorientation transitions at higher magnetic field. In dielectric permittivity $\varepsilon$ a sharp peak at ${\mathrm{T}}_{N2}$ in T and near M regions with a minimal suppression because of applied magnetic field is found and no such peak is observed in far M phase. A discontinuity evidenced in electromagnetic susceptibility indicates magnetoelectric effect at the polar to nonpolar transition regions. The structural incongruence in progressive transformation from T to M to O symmetries plays a vital role in controlling the nature of magnetic interactions. Our results indicate a strong correlation between structural transitions, magnetolattice coupling, magnetic frustrations, and magnetoelectric effect in ${\mathrm{Fe}}_{1-x}{\mathrm{Cr}}_x{\mathrm{VO}}_4$.

Neutron diffraction and magnetic susceptibility studies of orthorhombic single crystal $\left({\mathrm{Sr}}_{0.97}{\mathrm{K}}_{0.03}\right){\mathrm{MnSb}}_2$ confirm the three-dimensional (3D) C-type antiferromagnetic (AFM) ordering of the ${\mathrm{Mn}}^{2+}$ moments at $T_{\mathrm{N}}=305\pm 3$ K, which is slightly higher than that of the parent ${\mathrm{SrMnSb}}_2$ with $T_{\mathrm{N}}=297\pm 3$ K. Susceptibility measurements of the K-doped and parent crystals above $T_{\mathrm{N}}$ are characteristic of 2D AFM systems. This is consistent with high-temperature neutron diffraction of the parent compound that displays persisting 2D AFM correlations well above $T_{\mathrm{N}}$ to at least $\sim 560$ K with no evidence of a ferromagnetic phase. Analysis of the de Haas–van Alphen magnetic oscillations of the K-doped crystal is consistent with hole doping.

Symmetry arguments based on the point group of a system and thermodynamic measurements are often combined to identify the order parameter in unconventional superconductors. However, lattice translations, which can induce additional momenta with vanishing order parameter in the Brillouin zone, are neglected, especially in gap functions otherwise expected to be constant, such as in chiral superconductors. After a general analysis of the symmetry conditions for vanishing gap functions, we study the case of chiral $p$- and chiral $f$-wave pairing on a square lattice, a situation relevant for ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$. Specifically, we calculate the impurity-induced density of states, specific heat, superfluid density, and thermal conductivity employing a self-consistent $T$-matrix calculation and compare our results to the case of a nodal ($d$-wave) order parameter. While there is a clear distinction between a fully gapped chiral state and a nodal state, the strongly anisotropic case is almost indistinguishable from the nodal case. Our findings illustrate the difficulty of interpreting thermodynamic measurements. In particular, we find that the available measurements are consistent with a chiral ($f$-wave) order parameter. Our results help to reconcile the thermodynamic measurements with the overall picture of chiral spin-triplet superconductivity in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$.

The quantum behavior of superconducting nanowires may essentially depend on the employed experimental setup. Here we investigate a setup that enables passing equilibrium supercurrent across an arbitrary segment of the wire without restricting fluctuations of its superconducting phase. The low-temperature physics of the system is determined by a combined effect of collective soundlike plasma excitations and quantum phase slips. At $T=0$ the wire exhibits two quantum phase transitions, both being controlled by the dimensionless wire impedance $g$. While thicker wires with $g>16$ stay superconducting, in the thinnest wires with $g<2$ the supercurrent is totally destroyed by quantum fluctuations. The intermediate phase with $2<g<16$ is characterized by two different correlation lengths demonstrating superconductinglike behavior at shorter scales combined with vanishing superconducting response in the long scale limit.

We analyze the Andreev spectrum in a four-terminal Josephson junction between one-dimensional topological superconductors in class D. We find that a topologically protected crossing in the space of three superconducting phase differences can occur between the two lowest Andreev bound states. This crossing can be detected through the transconductance quantization, in units of $2e^2/h$, between two voltage-biased terminals. As the crossing occurs away from the Fermi level, our prediction of a quantized response is specific to the nonequilibrium conditions set by the voltage drive. Therefore, it provides an example of a topological signature that cannot be ascribed to a ground-state property. We discuss possible realizations of such junctions with semiconducting crossed nanowires and with quantum-spin Hall insulators.

The origin of diverse nematicity and their order parameters in Fe-based superconductors have been attracting increasing attention. Recently, a new type of nematic order has been discovered in heavily hole-doped $\left(n_d=5.5\right)$ compound $A{\mathrm{Fe}}_2{\mathrm{As}}_2(A=\text{Cs},\text{Rb})$. The discovered nematicity has $B_{2g}\left(=d_{xy}\right)$ symmetry, rotated by ${45}^{\circ }$ from the $B_{1g}\left(=d_{x^2-y^2}\right)$ nematicity in usual compounds with $n_d\approx 6$. We predict that the “nematic bond order,” which is the symmetry breaking of the correlated hopping, is responsible for the $B_{2g}$ nematic order in $A{\mathrm{Fe}}_2{\mathrm{As}}_2$. The Dirac pockets in $A{\mathrm{Fe}}_2{\mathrm{As}}_2$ is essential to stabilize the $B_{2g}$ bond order. Both $B_{1g}$ and $B_{2g}$ nematicity in $A_{1-x}{\mathrm{Ba}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ are naturally induced by the Aslamazov-Larkin many-body process, which describes the spin-fluctuation-driven charge instability. The present study gives a great hint to control the nature of charge nematicity by modifying the orbital character and the topology of the Fermi surface.

We introduce a simple criterion for lattice models to predict quantitatively the crossover between the classical and the quantum scaling of the Kibble-Zurek mechanism, as the one observed in a quantum ${\varphi }^4$ model on a one-dimensional lattice [Phys. Rev. Lett. 116, 225701 (2016)]. We corroborate that the crossover is a general feature of critical models on a lattice, by testing our paradigm on the quantum Ising model in transverse field for arbitrary spin $s\left(s\ge 1/2\right)$ in one spatial dimension. By means of tensor network methods, we fully characterize the equilibrium properties of this model, and locate the quantum critical regions via our dynamical Ginzburg criterion. We numerically simulate the Kibble-Zurek quench dynamics and show the validity of our picture, also according to finite-time scaling analysis.

The phase transitions and magnetization processes in the antiferromagnetic compounds $\mathrm{Fe}{}_{0.5}\mathrm{TiS}{}_{2-y}\mathrm{Se}{}_y[\mathrm{FeTi}{}_2$(S,$\mathrm{Se}){}_4]$ with an ordered layered crystal structure of the $\mathrm{Cr}{}_3{\mathrm{S}}_4$ type have been studied by using x-ray diffraction, measurements of the specific heat, electrical resistivity, magnetoresistance, and the magnetization in steady and pulsed magnetic fields together with calculations within the Ising model accounting the magnetoelastic interactions. The change from the spin-flip to spin-flop type phase transition and a monotonic growth of the critical transition field from $\sim 50$ kOe at $y=0$ up to 470 kOe at $y=2$ has been observed with the Se for S substitution in antiferromagnetic (AFM) compounds $\mathrm{Fe}{}_{0.5}\mathrm{TiS}{}_{2-y}\mathrm{Se}{}_y$. In the selenium-poor compounds $\left(y<0.5\right)$, the field-induced AFM-FM phase transition at low temperatures is accompanied by ultrasharp changes in the magnetization and magnetoresistance and by huge magnetic hysteresis. The presence of remnant magnetoresistance in these compounds after the application of a magnetic field indicates the formation of a metastable field-induced FM state. Despite an AFM ground state, the $\mathrm{Fe}{}_{0.5}\mathrm{TiS}{}_{2-y}\mathrm{Se}{}_y$ compounds with $y<0.5$ after application of a magnetic field behave at low temperatures as high-anisotropic Ising-type ferromagnets with the coercive field $H_c$ up to $\sim 60$ kOe. It has been shown, that magnetoelastic interactions may be responsible for the formation of the metastable field-induced high-coercive FM state in the Ising-type antiferromagnets.

We find extraordinary behavior of the local two-terminal spin accumulation signals in ferromagnet (FM)/semiconductor (SC) lateral spin-valve devices. With respect to the bias voltage applied between two FM/SC Schottky tunnel contacts, the local spin accumulation signal can show nonmonotonic variations, including a sign inversion. A part of the nonmonotonic features can be understood qualitatively by considering the rapid reduction in the spin polarization of the FM/SC interfaces with increasing bias voltage. In addition to the sign inversion of the FM/SC interface spin polarization, the influence of the spin-drift effect in the SC layer and the nonlinear electrical spin conversion at a biased FM/SC contact are discussed.

In this paper we conduct Monte Carlo simulations to investigate the thermodynamic properties of a geometry of artificial spin ice recently proposed in the literature that had been termed “rewritable” spin ice, for its experimental realization allows total control over the microstates of the system. Our results show that in the thermodynamic limit a single phase transition between a fully magnetized state and a paramagnetic state exists, whereas for finite systems an intermediate phase also emerges, engendering a low temperature pseudo phase transition. This intermediate phase is characterized by large magnetic domains separated by domain walls composed of monopolelike excitations, resulting in low net magnetization values. We also show that two types of low energy excitations that behave as magnetic monopoles emerge in the system, both of which are geometrically constrained to move along a predefined path.

In systems above one dimension, continuously degenerate minima of the single-particle dispersion are realized due to one or a combination of system parameters such as lattice structure, isotropic spin-orbit coupling, and interactions. A unit codimension of the dispersion minima leads to a divergent density of states which enhances the effects of interactions, and may lead to novel states of matter as exemplified by Luttinger liquids in one-dimensional bosonic systems. Here we show that in dilute, homogeneous bosonic systems above one dimension, weak, spin-independent, interparticle interactions stabilize a metallic state at zero temperature in the presence of a curved manifold of dispersion minima. In this metallic phase, the system possesses a quasi-long-range order with nonuniversal scaling exponents. At a fixed positive curvature of the manifold, increasing either the dilution or the interaction strength destabilizes the metallic state toward charge density wave states that break one or more symmetries. The magnitude of the wave vector of the dominant charge density wave state is controlled by the product of the mean density of bosons and the curvature of the manifold. We obtain the zero-temperature phase diagram, and identify the phase boundary.

$\mathrm{SU}\left(N\right)$ symmetric fermions on a lattice, which can be realized in ultracold-atom-based quantum simulators, have very promising prospects for realizing exotic states of matter. Here we present the ground-state phase diagram of the repulsive SU(3) Hubbard model on a honeycomb lattice at 1/3 filling obtained from infinite projected entangled pair states tensor network calculations. In the strongly interacting limit the ground state has plaquette order. Upon decreasing the interaction strength $U/t$ we find a first-order transition at $U/t=7.2\left(2\right)$ into a dimerized, color-ordered state, which extends down to $U/t=4.5\left(5\right)$, at which the Mott transition occurs and the ground state becomes uniform. Our results may serve as a prediction and benchmark for future quantum simulators of SU(3) fermions.

Can the mere crumpling of a paper produce electricity? An inhomogeneous strain can induce electrical response in all dielectrics and not just piezoelectric materials. This phenomenon of flexoelectricity is rather modest unless unusually large strain gradients are present. In this paper, we analyze the crumpling of thin elastic sheets and establish scaling laws for their electromechanical behavior to prove that an extremely strong flexoelectric response is achieved at submicron length scales. Connecting with recent experiments on crumpling of a polymer paper, we argue that crumpling is a viable energy-harvesting route with applications in wearable electronics and related contexts.

We present a benchmarking study of adsorption energies on transition metal surfaces computed with select functionals across different density functional theory codes. In addition to gradient corrected functionals, we evaluate the accuracies of representative metaGGAs, including MS2, SCAN, and $\mathrm{SCAN}+\text{rVV10}$, as well as a short-range screened hybrid functional, HSE06. The study shows that the challenge of finding a functional that can simultaneously capture both covalent and noncovalent molecule-surface interactions persists, with no single functional in the benchmarking study with average errors $<0.2$ eV. We find that HSE06 on average does not improve the accuracy compared to PBE for the surface chemistry of transition metals. The BEEF-vdW dispersion-corrected generalized gradient approximation (GGA) and the MS2 metaGGA yield the lowest errors in both chemisorption and dispersion energies, demonstrating that moving up the Jacob's ladder of functionals to screened hybrids does not necessarily improve the description of transition metal surface chemistry.

We study the electronic contribution to the nonlinear optical response of the $\alpha \text{−}{T}_3$ model. This model is an interpolation between a graphene $\left(\alpha =0\right)$ and dice $\left(\alpha =1\right)$ lattice. Using a second-quantized formalism, we calculate the first- and third-order responses for a range of $\alpha$ and chemical potential values as well as considering a band gap in the first-order case. Conductivity quantization is observed for the first-order, while higher-order harmonic generation is observed in the third-order response with the chemical potential determining which applied field frequencies both quantization and harmonic generation occur at. We observe a range of experimentally accessible critical fields between ${10}^2–{10}^6$ V/m with dynamics depending on $\alpha ,\mu$, and the applied field frequency. Our results suggest an $\alpha \text{−}{T}_3$-like lattice could be an ideal candidate for use in terahertz devices.