Two-dimensional superconductors have been realized in various atomically thin films such as the twisted bilayer graphene, some of which are anticipated to involve an unconventional pairing mechanism. Due to their low dimensionality, experimental probes of the exact nature of superconductivity in these systems have been limited. We propose, by applying a vertical supercurrent to a bilayer superconductor where the mirror symmetry is naturally broken by the twisting, there will be an anomalous thermal Hall effect induced by the supercurrent that can serve as a sharp probe for the in-plane anisotropy of the superconducting gap function. This effect occurs in the absence of an external magnetic field and spontaneous breaking of the time-reversal symmetry in the ground state. We derive explicit formulas for the induced thermal Hall conductivity and show them to be significant in the examples of twisted cuprates and twisted FeSe where monolayer superconductivity has already been observed. Though technical challenges still exist, we propose this to be a generic probe of the gap anisotropy in a twisted bilayer superconductor.

We study the universal dynamical relaxation behaviors of a quantum XY chain following a quench, paying special attention to the case in which the initial state is a critical ground state, or the postquenched Hamiltonian is at a critical point of equilibrium quantum phase transition, or both of them are critical. In such a “critical quench,” we find very interesting real-time dynamical scaling behaviors and we find crossover phenomena between them. For a quench from a noncritical point to a critical point, we find that, compared to the noncritical quench, the universal power-law scaling behavior does not change; however, there may be a crossover between the exponential decaying behavior and the power-law scaling. For a quench from a critical point to a noncritical point, the power-law scaling behaviors $t^{-3/2}$ and $t^{-3/4}$ in the noncritical quenches may be changed to $t^{-1}$ and $t^{-1/2}$, respectively. If the prequenched Hamiltonian is set to be a point that is close to but not exactly at a critical point, we find interesting crossover phenomena between different power-law scaling behaviors. We also study the quench from the vicinity of a multicritical point, we find crossover behaviors that are induced by a different mechanism, and we find another crossover exponent. All the results are related to the gap-closing properties of the energy spectrum of the critical points.

We report the strain dependence of magnetic anisotropy in $n$-type ferromagnetic semiconductor (FMS) (In,Fe)Sb thin films grown on different buffer layers; ranging from an InSb buffer layer that induces in-plane tensile strain, to AlSb, GaSb, and InAs buffer layers that induce an increasing order of in-plane compressive strain. Using ferromagnetic resonance (FMR) measurements and theoretical fittings, we show that the magnetocrystalline anisotropy constant $\left(K_{\mathrm{i}}\right)$ changes its sign, corresponding to a change in its preference for an in-plane magnetization easy axis to a perpendicular magnetization easy axis, when the epitaxial strain is changed from tensile to compressive. Meanwhile, the shape anisotropy constant $\left(K_{\mathrm{sh}}\right)$, which favors an in-plane magnetization easy axis, has larger contribution over $K_{\mathrm{i}}$. Thus, the effective magnetic anisotropy $\left(K_{\mathrm{eff}}=K_{\mathrm{i}}+K_{\mathrm{sh}}\right)$ results in in-plane magnetic anisotropy in all our (In,Fe)Sb thin films. Our study presents the observation of FMR in the $n$-type FMS (In,Fe)Sb at different temperatures and under various strain conditions. We discuss the origin of the strain-dependent magnetization anisotropy of (In,Fe)Sb with the help of a band-structure model while taking $p-d$ hybridization into account.

We have investigated the spin-orbit torques (SOTs) driven interfacial magnetization switching in the $\alpha \text{−}\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3/\mathrm{Pt}$ bilayers through pulsed current, DC current and second-harmonic transport measurements. The interfacial magnetic state stems from the combination of the strong spin-orbit coupling of the Pt layer and the magnetic-field-dependent bulk magnetic order of the $\alpha \text{−}\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3$ layer. The fieldlike SOT is demonstrated to be the dominated role and has shown a nonlinear relationship with the current amplitude and the magnetic field, indicating the involvement of the interfacial magnetic order. The vast richness of magnetic textures based on the antiferromagnetic insulator/heavy metal bilayers offers a fascinating playground for the investigation of spin transport. Our paper could provide the basis for a deeper understanding of the underlying mechanisms.

Enumerating polygons on regular lattices is a classic problem in rigorous statistical mechanics. The goal of enumerating polygons on the square lattice via fermionic path integration was achieved using a free-fermion quadratic action in the late 1970s. Given that polygon edges only link two vertices, it is considered plausible, if not natural, that an action of degree 2 in the Grassmann variables might suffice to enumerate lattice polygons in any dimension. Nevertheless, on nonplanar lattices the problem has remained open for more than four decades. Here, we derive the Grassmann action for exact enumeration of polygons on the simple cubic lattice. Moreover, we prove that this action is not quadratic but quartic, corresponding to a model of interacting fermions.

We propose a Monte Carlo simulation to understand electron transport in a nonequilibrium steady state (NESS) for the lattice Coulomb Glass model, created by continuous excitation of single electrons to high energies followed by relaxation of the system. Around the Fermi level, the NESS state approximately obeys the Fermi-Dirac statistics, with an effective temperature $\left(T_{\mathrm{eff}}\right)$ greater than the system's bath temperature $\left(T\right)$. $T_{\mathrm{eff}}$ is a function of $T$ and the rate of photon absorption by the system. Furthermore, we find that the change in conductivity is only a function of relaxation times and is almost independent of the bath temperature. Our results indicate that the conductivity of the NESS state can still be characterized by the Efros-Shklovskii law with an effective temperature $T_{\mathrm{eff}}>T$. Additionally, the dominance of phononless hopping over phonon-assisted hopping is used to explain the relevance of the hot electron model to the conductivity of the NESS state.

The switching dynamics of ferroelectric materials is a crucial intrinsic property which directly affects the operation and performance of ferroelectric devices. In conventional ferroelectric materials, the typical ferroelectric switching mechanism is governed by a universal process of domain-wall motion. However, recent experiments indicate that van der Waals ferroelectric $\mathrm{CuIn}{\mathrm{P}}_2{\mathrm{S}}_6$ possesses anomalous polarization switching dynamics under an electric field. It is important to understand the switching dynamics, but it remains theoretically unexplored in $\mathrm{CuIn}{\mathrm{P}}_2{\mathrm{S}}_6$ due to the lack of description of its order-disorder phase transition characteristics by density functional theory. Here we employ a machine-learning potential trained from the first-principles density functional theory dataset to conduct large-scale atomistic simulations of temperature-driven order-disorder ferroelectric phase transition in $\mathrm{CuIn}{\mathrm{P}}_2{\mathrm{S}}_6$. Most importantly, it is found that the electric-field-driven polarization switching in $\mathrm{CuIn}{\mathrm{P}}_2{\mathrm{S}}_6$ is mediated by a single Cu dipole flip rather than a conventional domain-wall-motion mechanism. This intrinsic unconventional switching behavior can be attributed to the competition between the energy barrier of domain-wall motion and single-dipole flip.

Materials with square lattices composed of group IV or V elements provide a promising platform for topological phases to emerge. We present the study on single crystals of ${\mathrm{LuPb}}_2$, which is a compound based on the Pb square net. The de Haas–van Alphen effect measurements reveal clear quantum oscillations, from which nonzero Berry phases are extracted. DFT calculation confirmed the presence of anisotropic Dirac dispersion near Fermi energy, suggesting that ${\mathrm{LuPb}}_2$ is a candidate Dirac semimetal. Additionally, ${\mathrm{LuPb}}_2$ shows a large quasilinear magnetoresistance. Through analysis of the magnetotransport data, we have determined that the quasilinear magnetoresistance is primarily governed by the average carrier mobility, as predicted by the classic Parish-Littlewood model. Furthermore, we observed the surface superconductivity, with a highly anisotropic upper critical field and enhancement of the resistivity critical field, which we attributed to the presence of a potential nanoscale lead layer on the surface of the sample. These findings are expected to provide a new platform for hosting anisotropic Dirac fermions and superconductivity.

Recently, charge density wave (CDW) order has been discovered in a magnetic kagome metal FeGe, providing a platform to explore the possible connection between magnetism and CDW in a kagome lattice. Based on density functional theory and symmetry analysis, we present a comprehensive investigation of electronic structure, magnetic properties, and possible structural distortion of FeGe. We estimate the magnetic parameters including Heisenberg and Dzyaloshinskii-Moriya (DM) interactions, and find that the ferromagnetic nearest-neighbor $J_1$ dominates over the others, while the magnetic interactions between nearest kagome layers favors antiferromagnetic. The Néel temperature $T_N$ and Curie-Weiss temperature ${\theta }_{\text{CW}}$ are successfully reproduced, and the calculated magnetic anisotropy energy is also consistent with the experimental results. However, these reasonable Heisenberg interactions and magnetic anisotropy cannot explain the double cone magnetic transition, and the DM interactions, which even exist in the centrosymmetric materials, can result in this small magnetic cone angle. Unfortunately, due to the crystal symmetry of the high-temperature structure, the net contribution of DM interactions to double cone magnetic structure is absent. Based on the experimental $2\times 2\times 2$ supercell, we thus explore the subgroups of the parent phase. Group theoretical analysis reveals that there are 68 different distortions, and only four of them (space group $P622$ or $P{6}_{3}22$) without inversion and mirror symmetry thus can explain the low-temperature magnetic structure. Furthermore, we suggest that these four proposed CDW phases can be identified by using Raman spectroscopy. Since DM interactions are very sensitive to small atomic displacements and symmetry restrictions, we believe that symmetry analysis is a useful method to reveal the interplay of delicate structural distortions and complex magnetic configurations.

We present an unbiased mean-field analysis of magnetic and charge orders in the two-dimensional Hubbard model on a square lattice, at both zero and finite temperatures. Unrestricted Hartree-Fock calculations on large finite lattices are complemented by solutions restricted to Néel and circular spiral order in the thermodynamic limit. The magnetic states are classified by a systematic scheme based on the dominant Fourier components of the spin texture. On finite lattices a whole zoo of ordering patterns appears. We show that many of these states are finite-size artifacts related to the limited choice of ordering wave vectors on a finite lattice. In the thermodynamic limit only three classes of states with a relatively simple structure survive: Néel, circular spiral, and stripe states. Stripes involve also charge order and can be unidirectional or bidirectional, with horizontal and/or vertical orientation. We present complete phase diagrams in the plane spanned by electron density and temperature, for a moderate Hubbard interaction and various choices of the next-nearest-neighbor hopping amplitude.

The notion of higher-order topological phases can have interesting generalizations to systems with subsystem symmetries that exhibit fractonic dynamics for charged excitations. In this work, we systematically study the higher-order topological phases protected by a combination of subsystem symmetries and ordinary global symmetries in two and three-dimensional interacting boson systems, with some interacting fermionic examples. In particular, we discover some important results, including (i) an Abelian subsystem symmetry group that does not allow for a nontrivial higher-order symmetry-protected topological phase in 2D without global symmetry. (ii) inhomogeneous subsystem symmetries do not allow for that either.

Positron annihilation in solid-state matter can be utilized to detect and identify open-volume defects. The momentum distribution of the annihilation radiation is an important observable in positron-based measurements and can reveal information on the chemical surroundings of the defect sites. In this paper, we present a variational quantum Monte Carlo method for simulation of the momentum densities of annihilating electron-positron pairs in semiconductors and insulators. We study finite-size effects, effects of lattice vibrations, and different levels of trial wave functions. Small simulation cells and simple wave function forms are found to be sufficient for accurate calculations in the simulation of pristine lattices, enabling cheap accumulation of results. We compare calculated predictions of the Doppler broadening of the 511-keV $2\gamma$ annihilation line in aluminium nitride and silicon against experimental data measured from reference samples. Our results achieve better agreement with experiments in these materials than conventional state-of-the-art methods and prove that direct modeling of the electron-positron correlations is important for a supporting theory of positron annihilation spectroscopies.

In general, optical and acoustic phonons have different energy scaling and are separated by an energy gap. However, the two phonon branches can also interact and provide an inherently poor thermal conductivity in complex minerals with a large number of atoms per unit cell. For instance, the copper-chalcogenide based minerals with high crystalline anharmonicity are inherently poor thermal conductors. We have studied large unit cell ${\mathrm{Cu}}_{26}{\mathrm{Nb}}_2{\mathrm{Sn}}_6{\mathrm{Se}}_{32}, {\mathrm{Cu}}_{26}{\mathrm{Nb}}_2{\mathrm{Sn}}_6{\mathrm{Se}}_{31.5}$, and ${\mathrm{Cu}}_{26}{\mathrm{Nb}}_2{\mathrm{Sn}}_6{\mathrm{Se}}_{30}{\mathrm{Te}}_2$ synthetic minerals with a strategically tailored anionic disorder. These compounds have a $p$-type degenerate behavior with carrier concentration $\left(\sim 2.7–\sim 15.3\right)\times {10}^{20}\mathrm{c}{\mathrm{m}}^{-3}$, at 300 K and a very low lattice thermal conductivity, $\left(\sim 0.76–\sim 1.49\right)\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ at $\sim 625\mathrm{K}$. Here, the softening of Cu-Se bonds and hence the crystal framework play an important role for very poor thermal conductivity. The existence of two low frequency Raman active optical modes (at $\sim 55$ and $72{\mathrm{cm}}^{-1}$) associated with soft Cu and Se atoms, three localized Einstein modes in specific heat, suggest a high scattering of acoustic and optical branches with very short phonon lifetime $(\sim 0.3–0.6\mathrm{ps})$. The excess vibrational density of states at low energies with compressed and flat optical branches strongly hinders the heat transport. The involvement of the Te atom at Se sites results in a lowering of the acoustic phonon cutoff frequency and the softening of optical phonons, significantly. Overall, ${\mathrm{Cu}}_{26}{\mathrm{Nb}}_2{\mathrm{Sn}}_6{\mathrm{Se}}_{30}{\mathrm{Te}}_2$ has the lowest thermal conductivity at $\sim 625\mathrm{K}$ and is a promising thermoelectric material because of robust acoustic-optical phonon scattering, very low sound velocity, and high crystalline anharmonicity.

A theoretical principle for explaining the peculiarity in “edge-free” wrinkled graphene has not been firmly established. Herein, we perform DFT calculations to verify the graphene nanowrinkle (GNW) feature on metal as a model system based on experimental observation. We unveil that the interfacial interaction between the graphene and the substrate plays a crucial role in leading to quantum confinement. The longitudinal direction and the effective confined length were investigated as key parameters to control the electronic structure of graphene by corrugation engineering. A series of standing waves corresponding to the “particle in a box” model was also confirmed by the charge densities of GNW.

We demonstrate the possibility of using time-space crystalline structures to simulate eight-dimensional systems based on only two physical dimensions. A suitable choice of system parameters allows us to obtain a gapped energy spectrum, making topological effects become relevant. The nontrivial topology of the system is evinced by considering the adiabatic state pumping along temporal and spatial crystalline directions. Analysis of the system is facilitated by rewriting the system Hamiltonian in a tight-binding form, thereby putting space, time, and the additional synthetic dimensions on an equal footing.

The broad and Stokes-shifted photoluminescence line in ultra-wide-band gap semiconductors has numerous potential applications starting from white light generation, nanoscale optical thermometers to quantum information readout. Here, using density functional theory with an effective U approach and hybrid functional study, we establish the correlation between self-trapped hole polaron relaxation dynamics and broadness in photoluminescence spectra of ultra-wide-band gap oxide ${\mathrm{LiGaO}}_{\text{2}}$. After electron capture, self-trapped hole undergoes large lattice relaxation and gives rise to a broad and strongly Stokes-shifted emission band peaking at 4.17 eV. The donated holes from single acceptor defects form tightly bound states and serve as deep defects. These defects also provide additional stability to bound hole polarons and produce broader luminescence bands, with peaks in the infrared to the visible range of 2.4–4.1 eV. Our findings shed light on the fundamental properties of ultra-wide-band gap semiconductor oxides and provide insights into strategies for tuning luminescence bands for future optoelectronic applications.

The triple phase transitions or simultaneous transitions of three different phases, namely, topological, parity-time $\left(\mathcal{P}\mathcal{T}\right)$ symmetry breaking, and metal-insulator transitions, are observed in an extension of the $\mathcal{P}\mathcal{T}$ symmetric non-Hermitian Aubry-André-Harper model. In this model, besides the non-Hermitian complex quasiperiodic on-site potential, non-Hermiticity is also included in the nearest-neighbor hopping terms. Moreover, the nearest-neighbor hopping terms are also quasiperiodic. The presence of two non-Hermitian parameters, one from the on-site potential and another one from the hopping part, ensures $\mathcal{P}\mathcal{T}$ symmetry transition in the system. In addition, tuning these two non-Hermitian parameters, we identify a parameters regime, where we observe the triple phase transition. Following some recent studies, an electrical circuit based experimental realization of this model is also discussed.

We demonstrate that superconductors with broken inversion symmetry support a family of stable, spatially localized configurations of the self-knotted magnetic field. These solutions, which we term “toroflux,” are the superconducting counterparts of the Chandrasekhar-Kendall states (spheromaks) that appear in highly conducting, force-free astrophysical and nuclear-fusion plasmas. The superconducting torofluxes are solutions of superconducting models, in the presence of a parity-breaking Lifshitz invariant associated with the $O$ point-group symmetry. These solutions are characterized by a nonvanishing helicity of the magnetic field, and also by a toroidal dipole moment of the magnetic field. We demonstrate that a magnetic dipole or a ferromagnetic inclusion in the bulk of a noncentrosymmetric superconductor sources finite-energy toroflux solutions.