We investigate the effect of surface disorder on the chiral surface states of a three-dimensional quantum Hall system. Utilizing a transfer-matrix method, we find that the localization length of the surface state along the magnetic field decreases with the surface disorder strength in the weak disorder regime but increases anomalously in the strong disorder regime. In the strong disorder regime, the surface states mainly locate at the first inward layer to avoid the strong disorder in the outmost layer. The anomalous increase of the localization length can be explained by an effective model, which maps the strong disorder on the surface layer to the weak disorder on the first inward layer. Our work demonstrates that surface disorder can be an effective way to control the transport behavior of the surface states along the magnetic field. We also investigate the effect of surface disorder on the full distribution of conductances $P\left(g\right)$ of the surface states in the quasi-one-dimensional (1D) regime for various surface disorder strengths. In particular, we find that $P\left(g\right)$ is Gaussian in the quasi-1D metal regime and log-normal in the quasi-1D insulator regime. In the crossover regime, $P\left(g\right)$ exhibits highly nontrivial forms, whose shapes coincide with the results obtained from the Dorokhov-Mello-Pereyra-Kumar equation of a weakly disordered quasi-1D wire in the absence of time-reversal symmetry. Our results suggest that $P\left(g\right)$ is fully determined by the average conductance, independent of the details of the system, in agreement with the single-parameter scaling hypothesis.

In the present paper, we investigate the exciton effect on the shift current in single-walled boron-nitride nanotubes using effective mass theory combined with the Bethe-Salpeter equation to include the exciton effect. We found that the shift current is enhanced by the exciton effect compared to that of free electron-hole pairs and that the direction of the shift current is determined by the chiral index of the nanotube even in the presence of excitons. We provide a simple explanation of why the chiral index determines the direction of the shift current from a bond structure viewpoint via the Rice-Mele model.

We construct the first-principles theory for the scattering of the Coulomb field of a fast electron traveling along an array of nano- or microparticles. The electron's trajectory is arbitrarily oriented in the plane parallel to the surface. We show that Smith-Purcell radiation accompanying this process results in very rich diffraction patterns, which differ dramatically from the ones for conventional diffraction gratings; a numerical analysis was made for THz frequencies. The generalized Smith-Purcell dispersion relation has been derived; it describes the link between frequency, two periods of the structure, the electron's velocity, and the angle of radiation with maximal intensity. The unique spatial distribution of the generated light can form the basis for the generation of structured-light electron-driven photon sources.

Fracton phases of matter feature a variety of exciting phenomena stemming from the restricted mobility of their quasiparticles. Here we consider a model of interacting electrons in one dimension that describes hopping of spin-singlet pairs and obeys both charge and dipole conservation laws. The model contains Bethe ansatz integrable sectors which allow us to solve the ground state and calculate the exact spin and single-electron excitation gaps. We observe hallmarks of fractonic behavior, including localization of single-electron excitations and propensity to clustering. Our results demonstrate the important role of the dipole moment conservation law in a simple model of spin-1/2 fermions.

Intrinsic magnetic van der Waals (vdW) materials have attracted much attention, especially ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$ (FGT), which exhibits highly tunable properties. However, despite vast efforts, there are still several challenging issues to be resolved. Here, using a first-principles linear-response approach, we carry out a comprehensive investigation of both bulk and monolayer FGT. We find that although the magnetic exchange interactions in FGT are frustrated, our Monte Carlo simulations agree with the total energy calculations, confirming that the ground state of bulk FGT is indeed ferromagnetic (FM). We confirm that the Curie temperature raising under electron doping is an intrinsic effect. A tiny electron doping reduces the magnetic frustration, resulting in a significant increase of the Curie temperature. We also calculate the spin-wave dispersion, and find a small spin gap as well as a nearly flat band in the magnon spectra. These features can be compared with the future neutron scattering measurement to finally clarify the microscopic magnetic mechanism in these two-dimensional family materials.

Based on the multiband semiconductor Bloch equations a microscopic approach to high-harmonic generation in crystalline solids which is able to properly describe degenerate bands and band crossings is presented and analyzed. It is well known that numerical band structure calculations typically provide electronic wave functions with an undetermined $k$-dependent phase which results in matrix elements which contain arbitrary $k$-dependent phases. In addition, such approaches usually mix degenerate bands and bands with an energy difference smaller than the numerical precision in an arbitrary way for each point in $k$ space. These ambiguities are problematic if one considers the dynamics induced by electric fields since the matrix elements of the position operator involve a derivative of the wave functions with respect to $k$. When the light-matter interaction is described in the length gauge, the problem of arbitrary phases and degenerate subspace mixing of Bloch states is solved by adopting a smooth gauge transformation along the field direction. The results obtained within this method are validated by comparing with calculations in the velocity gauge. Although we obtain in both gauges the same overall result, the length gauge is advantageous since it converges with a smaller number of bands and thus requires significantly less numerical effort than the velocity gauge. Also a unique distinction between inter- and intraband contributions and thus an instructive physical interpretation is possible in the length gauge whereas in the velocity gauge this is unclear. The computed polarization-direction-dependent high-harmonic spectra agree well with experimental data reported for GaAs. Furthermore, it is demonstrated that, under proper conditions, the Berry curvature is largely responsible for the even-order harmonics which are polarized perpendicular to the driving field.

Linking the fundamental physics of band structure and scattering theory with macroscopic features such as measured temperature dependencies of electronic and thermal transport is indispensable to a thorough understanding of thermoelectric phenomena and ensures more targeted and efficient experimental research. Regarding ${\mathrm{Fe}}_2\mathrm{VAl}$-based compounds, experimental work has seen mostly qualitative and often speculative interpretations, preventing this class of materials from tapping their full potential when it comes to applications. In this paper, the temperature-dependent Seebeck coefficient and electrical resistivity of a set of $p$-type and $n$-type samples with the composition ${\mathrm{Fe}}_2{\mathrm{V}}_{1-x}{\mathrm{Ta}}_x{\mathrm{Al}}_{1-y}{\mathrm{Si}}_y$ are presented from 4 K up to 800 K as well as the Hall mobility and carrier concentration from 4 K to 520 K. We attempt a quantitative analysis of our data using a parabolic two- and three-band model and compare the model results with those from density functional theory calculations. Our findings show an increase of the band gap $E_{\text{g}}$ from almost zero in undoped ${\mathrm{Fe}}_2\mathrm{VAl}$ toward $E_{\text{g}}\approx 0.1\mathrm{eV}$ with increasing Ta substitution, consistent with results from first-principles calculations. Due to the resulting enhancement of the Seebeck coefficient, the maximum power factor is boosted up to 10.3 mW/${\mathrm{mK}}^2$, which is, to the best of our knowledge, the highest value among $n$-type bulk semiconductor systems reported near room temperature up until now. We further show that for the $p$-type ${\mathrm{Fe}}_2{\mathrm{V}}_{1-x}{\mathrm{Ta}}_x\mathrm{Al}$ compounds, the dominant scattering mechanism of electrons is intrinsically different compared to the $n$-type samples, for which acoustic phonon scattering can well describe the temperature-dependent Hall mobility in a broad range of temperatures.

Schottky barrier heights (SBHs) measured at polycrystalline metal-semiconductor (MS) interfaces have displayed an insensitivity to the work function (WF) of the metal, known as the “Fermi level pinning” (FLP) phenomenon. The obstacle presented by FLP in thwarting technological efforts to tune the SBH has been difficult to overcome because of a lack of understanding of the origin of the FLP effect. Presently, SBH explanation still largely relies on empirical models, and FLP remains a mystery. Here, the phenomenon of FLP for zinc-blende/diamond (ZBD) semiconductors is explicitly demonstrated to originate from interface metal-cation bonds, which are metallic in nature. Based on the analysis of two representative and electrically distinct types of interface, it is shown that screening by metallic bonds weakens the dependence of SBH on the metal and results in a SBH close to that found between the semiconductor and its own cation in elemental metal form. The latter SBH is shown to agree well with experimentally measured SBH from polycrystalline interface, i.e., the apparent pinning levels. A fundamental, self-consistent explanation of the FLP phenomenon thus emerges, and with it, strategies to avoid FLP for technological applications may also be suggested.

Edelstein effect of spin polarization means an electric field could induce a magnetization in nonmagnetic materials. This effect can be well understood by a Zeeman-like term due to an effective magnetic field in spin-orbit coupling systems. We propose a mechanism of current-induced spin polarization, where the real spin is not directly related to the effective magnetic field induced by an electric field. This effective field only results in the polarization of a general spin. Then the real spin polarization is generated through the coupling between the real spin and the general spin in pseudospin Dirac systems with low symmetry. The values of our spin polarization and the conventional one may have the same order of magnitude though ours involves two processes, which has been further verified by the numerical calculation in the $1T^{\prime }\text{−}{\mathrm{WTe}}_2$ distorted monolayer. It will be helpful for the deeper understanding of the current-induced spin polarization in pseudospin Dirac materials.

We investigate the influence of a weakly nonlinear Josephson bath consisting of a chain of Josephson junctions on the dynamics of a small quantum system (LC oscillator). Focusing on the regime where the charging energy is the largest energy scale, we perturbatively calculate the correlation function of the Josephson bath to the leading order in the Josephson energy divided by the charging energy while keeping the cosine potential exactly. When the variation of the charging energy along the chain ensures fast decay of the bath correlation function, the dynamics of the LC oscillator that is weakly and capacitively coupled to the Josephson bath can be solved through the Markovian master equation. We establish a duality relation for the Josephson bath between the regimes of large charging and Josephson energies, respectively. The results can be applied to cases where the charging energy either is nonuniformly engineered or disordered in the chain. Furthermore, we find that the Josephson bath may become non-Markovian when the temperature is increased beyond the zero-temperature limit in that the bath correlation function gets shifted by a constant and does not decay with time.

We introduce a self-consistent theory of mobility edges in nearest-neighbor tight-binding chains with quasiperiodic potentials. Demarcating boundaries between localized and extended states in the space of system parameters and energy, mobility edges are quite typical in quasiperiodic systems which lack the energy-independent self-duality of the commonly studied Aubry-André-Harper model. The potentials in such systems are strongly and infinite-range correlated, reflecting their deterministic nature and rendering the problem distinct from that of disordered systems. Importantly, the underlying theoretical framework introduced is model independent, thus allowing analytical extraction of mobility edge trajectories for arbitrary quasiperiodic systems. We exemplify the theory using two families of models and show the results to be in very good agreement with the exactly known mobility edges as well as numerical results obtained from exact diagonalization.

We study the ground state of a system with an interface between $\nu =4$ and $\nu =3$ in the quantum Hall regime. Far from the interface, for a range of interaction strengths, the $\nu =3$ region is fully polarized but $\nu =4$ region is unpolarized. Upon varying the strength of the interactions and the width of the interface, the system chooses one of two distinct edge/interface phases. In phase $A$, stabilized for wide interfaces, spin is a good quantum number, and there are no gapless long-wavelength spin fluctuations. In phase $B$, stabilized for narrow interfaces, spin symmetry is spontaneously broken at the Hartree-Fock level. Going beyond Hartree-Fock, we argue that phase $B$ is distinguished by the emergence of gapless long-wavelength spin excitations bound to the interface, which can be detected by a measurement of the relaxation time $T_2$ in nuclear magnetic resonance.

Hyperuniform states of matter are characterized by anomalous suppression of long-wavelength density fluctuations. While most of the interesting cases of disordered hyperuniformity are provided by complex many-body systems such as liquids or amorphous solids, classical spin chains with certain long-range interactions have been shown to demonstrate the same phenomenon. Such spin-chain systems are ideal models for exploring the effects of quantum mechanics on hyperuniformity. It is well-known that the transverse field Ising model shows a quantum phase transition (QPT) at zero temperature. Under the quantum effects of a transverse magnetic field, classical hyperuniform spin chains are expected to lose their hyperuniformity. High-precision simulations of these cases are complicated because of the presence of highly nontrivial long-range interactions. We perform an extensive analysis of these systems using density matrix renormalization group simulations to study the possibilities of phase transitions and the mechanism by which they lose hyperuniformity. Even for a spin chain of length 30, we see discontinuous changes in properties like the “$\tau$ order metric” of the ground state, the measure of hyperuniformity, and the second cumulant of the total magnetization along the $x$-direction, all suggestive of first-order QPTs. An interesting feature of the phase transitions in these disordered hyperuniform spin chains is that, depending on the parameter values, the presence of a transverse magnetic field may lead remarkably to an increase in the order of the ground state as measured by the “$\tau$ order metric,” even if hyperuniformity is lost. Therefore, it would be possible to design materials to target specific novel quantum behaviors in the presence of a transverse magnetic field. Our numerical investigations suggest that these spin chains can show no more than two QPTs. We further analyze the long-range interacting spin chains via the Jordan-Wigner mapping onto a system of spinless fermions, showing that under the pairwise-interaction approximation and a mean-field treatment, there can be at most two quantum phase transitions. Based on these numerical and theoretical explorations, we conjecture that for spin chains with long-range pair interactions that have convergent cosine transforms, there can be a maximum of two quantum phase transitions at zero temperature.

The recent report of an intermediate incommensurately modulated orthorhombic phase in $\mathrm{LaTa}{\mathrm{O}}_4$ has prompted a re-examination of the phase transition sequence in $\mathrm{LaTa}{\mathrm{O}}_4$ as a function of temperature. With falling temperature, the sequence of phases examined is (orthorhombic) $Cmc{2}_1\left(\mathrm{C}\right)\leftrightarrow Cmc{2}_1\left(\mathrm{IC}\right)\leftrightarrow \left(\mathrm{monoclinic}\right)P{2}_1/c$, with C and IC denoting commensurate and incommensurate phases, respectively. The orthorhombic to monoclinic transition, $T_{\mathrm{m}-\mathrm{o}}$, is a first order reconstructive transition occurring at 440 K and $T_{\mathrm{IC}-\mathrm{C}}$ is a first-order displacive transition occurring at 500–530 K. Strain and elasticity data confirm a first-order transition between the basic and modulated $Cmc{2}_1$ phases, with similarities to the isostructural fluoride $\mathrm{BaMn}{\mathrm{F}}_4$. A Raman spectroscopic study of the $\mathrm{LaTa}{\mathrm{O}}_4$ phase transition indicates that the IC-C phase transition is driven by a soft zone-boundary phonon (unstable) of the commensurate orthorhombic ($Cmc{2}_1$) phase. The soft phonon is found to appear (underdamped) above 443 K and vanishes (overdamped) around 528 K. A large supercell of the monoclinic phase below $T_{\mathrm{m}-\mathrm{o}}$ is proposed based on the Raman spectroscopic results.

In this paper, we study the magnetic properties of orthorhombic (Pbnm) perovskites $\mathrm{Lu}{\mathrm{Fe}}_{1-x}{\mathrm{Cr}}_x{\mathrm{O}}_3$ with $x=0.25$, 0.45, 0.55, and 0.75 by magnetization vs temperature and neutron powder diffraction measurements at room temperature. The magnetic moments are oriented along the $x$ direction with a G-type antiferromagnetic (AFM) arrangement that corresponds to irreducible representation ${\mathrm{\Gamma }}_4$ ($G_x{A}_y{F}_z$). Magnetization reversal (MR) is observed for $0.45\le x\le 0.75$. The MR phenomenon was modeled using Monte Carlo simulations, modeling both homogeneous and inhomogeneous cation distributions. For $\mathrm{Lu}{\mathrm{Fe}}_{0.25}{\mathrm{Cr}}_{0.75}{\mathrm{O}}_3$, the presence of MR could only be explained using an inhomogeneous distribution with well-defined magnetic clusters. Looking at the staggered magnetization of each cluster, we propose a superparamagnetic regime of ${\mathrm{Fe}}^{3+}$ clusters in a ${\mathrm{Cr}}^{3+}$-rich matrix for $\mathrm{Lu}{\mathrm{Fe}}_{0.25}{\mathrm{Cr}}_{0.75}{\mathrm{O}}_3$. By calculating the specific heat, we found that, in $\mathrm{Lu}{\mathrm{Fe}}_{0.45}{\mathrm{Cr}}_{0.55}{\mathrm{O}}_3$ and $\mathrm{Lu}{\mathrm{Fe}}_{0.25}{\mathrm{Cr}}_{0.75}{\mathrm{O}}_3$ the clusters order at nearly the same temperature $T_N^{\mathrm{Clus}}\sim 535$ and 520 K, respectively, while the matrix orders at $T_N^{\mathrm{Mat}}\sim 115\mathrm{K}$. There is considerable reduction in the cluster ordering temperature as compared with the bulk, as $T_N\sim 628\mathrm{K}$ for $\mathrm{LuFe}{\mathrm{O}}_3$, while the matrix orders at the expected temperature as $T_N\sim 115\mathrm{K}$ for $\mathrm{LuCr}{\mathrm{O}}_3$.

Multiple two-dimensional materials are being naïvely termed stable on the grounds of displaying phonon dispersions with no negative frequencies and of not collapsing on molecular dynamics calculations at fixed volume. But, if these phases do not possess the smallest possible structural energy, how does one understand and establish their actual meta stability? To answer this question, twelve two-dimensional group-IV monochalcogenide monolayers (SiS, SiSe, SiTe, GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbS, PbSe, and PbTe) with a buckled honeycomb atomistic structure—belonging to symmetry group $P3m1$—displaying an out-of-plane intrinsic electric polarization are shown to be metastable by three independent methods. First, we uncover a coordination-preserving structural transformation from the low-buckled honeycomb structure onto the lower-energy $Pnm{2}_1$ (or $Pmmn$ for PbS, PbSe, and PbTe) phase to estimate energy barriers $E_B$ that must be overcome during such structural transformation. Using the curvature of the local minima and $E_B$ as inputs to Kramers escape formula, large escape times are found, implying the structural metastability of the buckled honeycomb phase (with the exception of PbS and PbSe, these phases display escape times ranging from 700 years to multiple times the age of the universe and can be considered “stable” for practical purposes in that relative sense). The second demonstration is provided by phonon dispersion relations that include the effect of long-range Coulomb forces and display no negative vibrational modes. The third and final demonstration of structural metastability is furnished by room-temperature ab initio molecular dynamics for selected compounds. The magnitude of the electronic band gap evolves with chemical composition. Different from other binary two-dimensional compounds such as transition metal dichalcogenide monolayers and hexagonal boron nitride monolayers which only develop an in-plane piezoelectric response, the twelve group-IV monochalcogenide monolayers with a buckled honeycomb structure also display out-of-plane piezoelectric properties.

The discovery of ferroelectricity in ${\mathrm{HfO}}_2$-based thin films opens up new opportunities for using this silicon-compatible ferroelectric to realize low-power logic circuits and high-density nonvolatile memories. The functional performances of ferroelectrics are intimately related to their dynamic responses to external stimuli such as electric fields at finite temperatures. Molecular dynamics is an ideal technique for investigating dynamical processes on large length and time scales, though its applications to new materials are often hindered by the limited availability and accuracy of classical force fields. Here we present a deep neural network–based interatomic force field of ${\mathrm{HfO}}_2$ learned from ab initio data using a concurrent learning procedure. The model potential is able to predict structural properties such as elastic constants, equation of states, phonon dispersion relationships, and phase transition barriers of various hafnia polymorphs with accuracy comparable with density functional theory calculations. The validity of this model potential is further confirmed by the reproduction of experimental sequences of temperature-driven ferroelectric-paraelectric phase transitions of ${\mathrm{HfO}}_2$ with isobaric-isothermal ensemble molecular dynamics simulations. We suggest a general approach to extend the model potential of ${\mathrm{HfO}}_2$ to related material systems including dopants and defects.

The Néel order of an antiferromagnet subject to a spin torque can undergo precession in a circular orbit about any chosen axis. To orient and stabilize the motion against the effects of magnetic anisotropy, the spin polarization should have components in-plane and normal to the plane of the orbit, where the latter must exceed a threshold. For biaxial antiferromagnets, the precessional motion is described by the equation for a damped-driven pendulum, which has hysteresis as a function of the spin current with a critical value where the period diverges. The fundamental frequency of the motion varies inversely with the damping and as ${(x^p-1)}^{1/p}$, with the drive-to-criticality ratio $x$ and the parameter $p>2$. An approximate closed-form result for the threshold spin current is presented, which depends on the minimum cutoff frequency the orbit can support. Precession about the hard axis has zero cutoff frequency and the lowest threshold, while the easy axis has the highest cutoff. A device setup is proposed for electrical control and detection of the dynamics, which is promising to demonstrate a tunable terahertz nano-oscillator.

The magnetic and thermodynamic properties of layered single-crystal ${\mathrm{K}}_2\mathrm{Ni}{\left({\mathrm{MoO}}_4\right)}_2$ having both structural and magnetic dimers have been investigated. The crystal structure of ${\mathrm{K}}_2\mathrm{Ni}{\left({\mathrm{MoO}}_4\right)}_2$ is composed of edge-sharing ${\mathrm{NiO}}_6$-octahedral pairs bridged by the ${\mathrm{MoO}}_4^{2-}$ polyatomic ion groups in a plane, and the ${\mathrm{K}}^{+}$ ions sit in the van der Waals gap between the layers. The temperature dependence of magnetic susceptibility shows a spin-singlet ground state with an activation gap of $\mathrm{\Delta }/k_B\approx 38$ K. A high-field magnetization study at $T=1.5$ K exhibits a half-magnetization plateau at ${\mu }_{0}H\sim 25$ T, corresponding to a level crossing of the singlet ground state with the lowest triplet state. Further, we have performed density functional theory calculations to determine magnetic exchange interactions. The nearest-neighbor coupling constant $J_1\sim 10$ K between the Ni spins turns out to be an order of magnitude larger than all interdimer couplings. Our experimental and theoretical results suggest that ${\mathrm{K}}_2\mathrm{Ni}{\left({\mathrm{MoO}}_4\right)}_2$ constitutes a nearly isolated two-dimensional $S=1$ dimer model.