In non-Hermitian systems, the phenomenon that the bulk-band eigenstates are accumulated at the boundaries of the systems under open boundary conditions is called the non-Hermitian skin effect (NHSE), which is one of the most iconic and important features of a non-Hermitian system. In this work, we investigate the fate of the NHSE in the presence of electric fields by analytically calculating the dynamical evolution of an initial bulk state and numerically computing the spectral winding number and the distributions of eigenstates, as well as the dynamical evolutions. We show abundant manipulation effects of dc and ac fields on the NHSE and that the physical mechanism behind these effects is the interplay between the Stark localization, dynamic localization, and the NHSE. In addition, the finite size analysis of the non-Hermitian system with a pure dc field shows the phenomenon of size-dependent NHSE. We further propose a scheme to realize the discussed model based on an electronic circuit. The results will help to deepen the understanding of the NHSE and its manipulation.

We consider the two-dimensional quantum Ising model, in absence of disorder, subject to periodic imperfect global spin flips. We show by a combination of exact diagonalization and tensor-network methods that the system can sustain a spontaneously broken discrete time-translation symmetry. Employing careful scaling analysis, we show the feasibility of a two-dimensional discrete time-crystal (DTC) prethermal phase. Despite an unbounded energy pumped into the system, in the high-frequency limit, a well-defined effective Hamiltonian controls a finite-temperature intermediate regime, wherein local time averages are described by thermal averages. As a consequence, the long-lived stability of the DTC relies on the existence of a long-range ordered phase at finite temperature. Interestingly, even for large deviations from the perfect spin flip, we observe a nonperturbative change in the decay rate of the order parameter, which is related to the long-lived stability of the magnetic domains in 2D.

The crystal structure of solid-state materials with unique lattices is considered one of the main factors determining physical properties, including superconductivity. Materials with honeycomb lattices have inspired intense research interest for their novel properties. A previously unknown compound, ${\mathrm{Ta}}_4\mathrm{CoSi}$, is discussed herein, along with its crystal structure and superconducting properties. It crystallizes in a $\mathrm{Cu}{\mathrm{Al}}_2\text{−}\mathrm{type}$ structure (space group P4/mcc, No. 124), with the lattice parameters $a=b=6.17755\left(7\right)\AA$ and $c=4.98443\left(8\right)\AA$, featuring honeycomb networks of Ta-Ta in the (110) plane. Superconductivity is observed below the critical temperature of ∼2.45 K, while lower and upper critical magnetic fields are ∼9.86 mT and 0.84 T, respectively. Experiments and theoretical calculations show that the honeycomb lattices significantly influence the superconducting properties. This material may thus provide a new platform for investigating the exotic superconductivity of honeycomb networks.

We provide a methodological approach to the estimate of the change of the quantum vacuum electromagnetic energy density in a high critical temperature superconducting metal bulk sample, when it undergoes the transition in temperature, from the superconducting to the normal phase. The various contributions to the Casimir energy in the two phases are highlighted and compared. While the transverse magnetic polarization of the vacuum mode allows for a macroscopic description of the superconducting transition, the changes in the transverse electric vacuum mode induced by the superconductive correlations are introduced within a microscopic model, which does not explicitly take into account the anisotropic structure of the material.

Dynamics of a particle in a resonantly driven quantum well can be interpreted as that of a particle in a crystal-like structure, with the time playing the role of the coordinate. By introducing an adiabatically varied phase in the driving protocol, we demonstrate a realization of the Thouless pumping in such a time crystalline structure. Next, we extend the analysis beyond a single quantum well by considering a driven one-dimensional optical lattice, thereby engineering a 2D time-space crystalline structure. Such a setup allows us to explore adiabatic pumping in the spatial and the temporal dimensions separately, as well as to simulate simultaneous time-space pumping.

The stretched diamond lattice material ${\mathrm{LiYbO}}_2$ has recently been reported to exhibit two magnetic transitions ($T_{\mathrm{N}1}=1.1\mathrm{K}, T_{\mathrm{N}2}=0.45\mathrm{K}$) via specific heat, magnetization, and neutron scattering measurements [Bordelon et al., Phys. Rev. B 103, 014420 (2021)]. Here we report complementary magnetic measurements down to $T=0.28\mathrm{K}$ via the local-probe technique of muon spin relaxation. While we observe a rapid increase in the zero-field muon depolarization rate at $T_{\mathrm{N}1}$, for $T<T_{\mathrm{N}1}$ we do not observe the spontaneous muon precession which is typically associated with long-range magnetic ordering. The depolarization rate in the ordered state shows a surprising sensitivity to magnetic fields applied along the initial spin polarization direction. Using a simple one-dimensional model, we show that these results are consistent with the unusual random-phase bipartite incommensurate magnetic structure proposed by Bordelon et al. for the intermediate temperature range $T_{\mathrm{N}2}<T<T_{\mathrm{N}1}$. We also find evidence for magnetic fluctuations persisting to our lowest temperatures, but no obvious signature of the transition or spontaneous muon precession at and below $T_{\mathrm{N}2}$, respectively.

We analyze the thermodynamical limit of the quantum metric along critical submanifolds of theory space. Building upon various results previously known in the literature, we relate its singular behavior to normal directions, which are naturally associated with relevant operators in the renormalization group sense. We formulate these results in the language of information theory and differential geometry. We exemplify our theory through the paradigmatic examples of the XY and Haldane models, where the normal directions to the critical submanifolds are seen to be precisely those along which the metric has singular behavior, while for the tangent ones it vanishes—these directions lie in the kernel of the metric.

We present two developments for the numerical integration of a function over the Brillouin zone. First, we introduce a nonuniform grid, which we refer to as the Farey grid, that generalizes regular grids. Second, we introduce symmetry-adapted Voronoi tessellation, a general technique to assign weights to the points in an arbitrary grid. Combining these two developments, we propose a strategy to perform Brillouin zone integration and interpolation that provides a significant computational advantage compared to the usual approach based on regular uniform grids. We demonstrate our methodology in the context of first-principles calculations with the study of Kohn anomalies in the phonon dispersions of graphene and ${\mathrm{MgB}}_2$, and in the evaluation of the electron-phonon driven renormalization of the band gaps of diamond and bismuthene. In the phonon calculations, we find speedups by a factor of 3 to 4 when using density-functional perturbation theory, and by a factor of 6 to 7 when using finite differences in conjunction with supercells. As a result, the computational expense between density functional perturbation theory and finite differences becomes comparable. For electron-phonon coupling calculations, we find even larger speedups. Finally, we also demonstrate that the Farey grid can be expressed as a combination of the widely used regular grids, which should facilitate the adoption of this methodology.

We calculate the Haldane gap of the $\mathrm{SU}\left(3\right)$ spin $\left[300\right]$ Heisenberg model using variational uniform fully symmetric $\mathrm{SU}\left(3\right)$ matrix product states, and find that the minimal gap $\mathrm{\Delta }/J=0.0263$ is obtained in the $\left[210\right]$ sector at momentum $2\pi /3$. We also discuss the symmetry protected topological order of the ground state, and determine the full dispersion relation of the elementary excitations and the correlation lengths of the system.

The asymptotic dynamical correlation functions in one-dimensional spin chains are described by power laws. The corresponding exponents characterize different bulk and boundary critical behavior. We present results for the logarithmic contribution to the boundary correlations of an isotropic Heisenberg chain. The exponent of the logarithm, $\lambda =1$, is derived using a renormalization group technique. We confirm our analytical results by comparing with numerical quantum Monte Carlo data.

We derive Ward identities for fermionic systems exhibiting a gauge symmetry that gets globally broken. In particular, we focus on the relation that connects the gauge field response functions to the transverse susceptibilities of the order parameter. We find that the long-wavelength and zero-energy limit of the former are related to the coefficients of a low-energy expansion of the latter. We examine three physical cases: the superconductor, the Néel antiferromagnet, and the spiral magnet. In the case of a metallic spiral magnet that completely breaks the SU(2) spin symmetry, we explicitly show that the Ward identities are fulfilled within the random phase approximation. We subsequently derive microscopic expressions for the spin stiffnesses and spin-wave velocities, which can be plugged into low-energy models to study the effect of long-wavelength bosonic fluctuations on top of mean-field solutions.

We study the ground-state phase diagram of a spin-$\frac{1}{2}$ frustrated XXZ ladder, in which two antiferromagnetic chains are coupled by competing rung and diagonal interactions, $J_{\perp }$ and $J_{\times }$. Previous studies on the isotropic model have revealed that a fluctuation-induced effective dimer attraction between the legs stabilizes the columnar dimer (CD) phase in the highly frustrated regime $J_{\perp }\approx 2J_{\times }$, especially for ferromagnetic $J_{\perp ,\times }<0$. By means of effective field theory and numerical analyses, we extend this analysis to the XXZ model, and we obtain a rich phase diagram. The diagram includes four gapped featureless phases with no symmetry breaking: the rung singlet (RS) and Haldane phases as well as their twisted variants, the RS* and Haldane* phases, which are all distinct in the presence of certain symmetries. Significantly, the Haldane-CD transition point in the isotropic model turns out to be a crossing point of two transition lines in the XXZ model, and the stripe Néel and RS* phases appear between these lines. This indicates a nontrivial interplay between the effective dimer attraction and the exchange anisotropy. In the easy-plane regime, the four featureless phases and two critical phases are found to compete in a complex manner depending on the signs of $J_{\perp ,\times }$.

We present a simple, first-principles scheme for calculating mechanical properties of nonequilibrium bulk systems assuming an ideal ballistic distribution function for the electronic states described by the external voltage bias. This allows for fast calculations of estimates of the current-induced stresses inside bulk systems carrying a ballistic current. The stress is calculated using the Hellmann-Feynman theorem, and is in agreement with the derivative of the nonequilibrium free energy. We illustrate the theory and present results for one-dimensional (1D) metal chains of Au, Cu, Al, Pt, Pb, and Ir. We compare the dependence of ultimate tensile strength on the applied voltage which shows a remarkable difference. In particular, for these model systems, gold is seen to be the most stable under strong current, while aluminum is the least stable. Interestingly, this agrees with the ordering of break voltages among the metals found in experiments, suggesting that a current-induced “embrittlement” effect could play a role.

We present an implementation of the adaptively compressed exchange (ACE) operator in the linearized-augmented plane waves formalism. ACE is a low-rank representation of the Fock exchange that avoids any loss of precision for the total energy. Our study shows that this property remains in the all-electron case, as we apply this method in nonrelativistic total-energy calculations with a hybrid exchange-correlation functional PBE0. The obtained data for light atoms and molecules are within a few $\mu \mathrm{Ha}$ of the precise multiresolution-analysis calculations. Aside from ACE, another key ingredient to achieve such a high precision with Fock exchange was the use of high-energy local orbitals. Finally, we use this implementation to calculate PBE0 gaps in solids and compare the results to other all-electron results.

Transition metal monosilicides CoSi, CoGe, RhSi, and RhGe in the chiral cubic B20 structure (the CoSi family) have recently been found to host unconventional chiral fermions beyond spin-1/2 Weyl fermions, and also to exhibit exotic physical phenomena such as long Fermi arc surface states, gyrotropic magnetic effect, and quantized circular photogalvanic effect. Thus, exploring novel spin-related transports in these unconventional chiral fermion semimetals may open a new door for spintronics and spin caloritronics. In this paper, we study the intrinsic spin Hall effect (SHE) and spin Nernst effect (SNE) in the CoSi family based on ab initio relativistic band structure calculations. First, we find that unlike nonchiral cubic metals, the CoSi family have two independent nonzero spin Hall (Nernst) conductivity tensor elements, namely, ${\sigma }_{xy}^z$ and ${\sigma }_{xz}^y$ (${\alpha }_{xy}^z$ and ${\alpha }_{xz}^y$) instead of one element. Furthermore, the SHC (${\sigma }_{xy}^z$ and ${\sigma }_{xz}^y$) and helicity of the chiral structure are found to be correlated, thus enabling SHE detection of structural helicity and also chiral fermion chirality. Second, the intrinsic SHE and SNE in some of the CoSi family are large. In particular, the calculated spin Hall conductivity (SHC) of RhGe is as large as $-140 (\hslash /\mathrm{e})$(S/cm). The calculated spin Nernst conductivity (SNC) of CoGe is also large, being $-1.3 (\hslash /\mathrm{e})$(A/m K) at room temperature. Due to their semimetallic nature with low electrical conductivity, these topological semimetals may have large spin Hall and spin Nernst angles, being comparable to that of platinum metal. The SHC and SNC of these compounds can also be increased by raising or lowering the chemical potential to, e.g., the topological nodes, via either chemical doping or electrical gating. Our findings thus indicate that transition metal monosilicides of the CoSi family not only would provide a material platform for exploring novel spin transports and exotic phenomena in unconventional chiral fermion semimetals, but also could be promising materials for developing better spintronic and spin caloritronic devices.

In the framework of $\mathbf{k}.\mathbf{p}$ theory, we have calculated the expressions for $g$ factors for electrons and holes taking into account up to 16-band contributions for materials with tetragonal symmetry with $D_{4h}$ as the point group. Because the recent experimental results for Landé factors do not evidence their anisotropy in the plane perpendicular to the tetragonal crystal c axis, we use these experimental results and the present $\mathbf{k}.\mathbf{p}$ calculations to determine the Kane energies perpendicular to and along the c axis, as well as the Luttinger parameters ${\kappa }_1$ and ${\kappa }_2$ for ${\mathrm{MAPbI}}_3$ and ${\mathrm{CsPbBr}}_3$.

Films of the hexagonal InTe with thicknesses from one to three tetralayers (TLs) were synthesized on the bilayer graphene/SiC by molecular beam epitaxy. Valence bands of the one- and two-TL-thick films were found to be flat-like near $\overline{\mathrm{\Gamma }}$ point, but become parabolic for the three-TL-thick film and beyond. The band gap of the InTe was found to be equal to 2.1 eV for the single tetralayer and tends to reduce its size with thickness. The band structure calculations revealed a large spin splitting of the InTe single tetralayer lower conduction band with exclusive out-of-plane spin polarization. Bearing in mind inaccessibility of the hexagonal InTe in a bulk form, all above-mentioned findings open up a way for the further study of this perspective material.

We study the quantized charge pumping of higher-order topological insulators (HOTIs) with edge-corner correspondences based on the combination of the rotation of in-plane magnetic field and the quantum spin Hall effect. The picture of a specific charge pumping process is uncovered with the help of the nonequilibrium Green's function method. Significantly, we demonstrate that the quantized charge pumping current is achieved without the participation of bulk states, and the charges move along the boundary of the sample. Furthermore, the effects of external parameters on the pumping current is also studied. We find that the magnitude and direction of the pumping current can be manipulated by adjusting the coupling strength between the leads and sample. Our paper deepens the understanding of the charge pumping in HOTIs and extends the study of their transport properties.

Graphene nanoribbons provide an ideal platform for electronic interferometry in the integer quantum Hall regime. Here, we solve the time-dependent four-component Schrödinger equation for single carriers in graphene and expose several dynamical effects of the carrier localization on their transport characteristics in $pn$ junctions. We simulate two kinds of Mach-Zender interferometers (MZI). The first is based on quantum point contacts and is similar to traditional GaAs/AlGaAs interferometers. As expected, we observe Aharonov-Bohm oscillations and phase averaging. The second is based on valley beam splitters, where we observe unexpected phenomena due to the intersection of the edge channels that constitute the MZI. Our results provide further insights into the behavior of graphene interferometers. Additionally, they highlight the operative regime of such nanodevices for feasible single-particle implementations.

We describe the possibility for topologically robust edge states existing on interfaces of triangular lattices which are supported by rotational symmetries that are sensitive to boundary conditions. Such states are trivial from the perspective of Berry curvature, but result instead from an interplay between crystalline symmetries and finite boundary effects. We show such states comprise a distinct topological phase, provided the gauge-dependent symmetries are maintained. Such a model describes a number of recent bosonic experimental demonstrations on triangular lattices, the physics for which has thus far eluded explanation.