Recently discovered intrinsic antiferromagnetic topological insulator $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ presents an exciting platform for realization of the quantum anomalous Hall effect and a number of related phenomena at elevated temperatures. An important characteristic making this material attractive for applications is its predicted large magnetic gap at the Dirac point (DP). However, while the early experimental measurements reported on large DP gaps, a number of recent studies claimed to observe a gapless dispersion of the $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ Dirac cone. Here, using micro($\mu$)-laser angle-resolved photoemission spectroscopy, we study the electronic structure of 15 different $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ samples, grown by two different chemists groups. Based on the careful energy distribution curves analysis, the DP gaps between 15 and 65 meV are observed, as measured below the Néel temperature at about 10–16 K. At that, roughly half of the studied samples show the DP gap of about 30 meV, while for a quarter of the samples the gaps are in the 50 to 60 meV range. Summarizing the results of both our and other groups, in the currently available $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ samples the DP gap can acquire an arbitrary value between a few and several tens of meV. Furthermore, based on the density functional theory, we discuss a possible factor that might contribute to the reduction of the DP gap size, which is the excess surface charge that can appear due to various defects in surface region. We demonstrate that the DP gap is influenced by the applied surface charge and even can be closed, which can be taken advantage of to tune the $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ DP gap size.

A significant number of Kondo-lattice ferromagnets order perpendicular to the easy magnetization axis dictated by the crystalline electric field. The nature of this phenomenon has attracted considerable attention, but remains poorly understood. In the present paper we use inelastic neutron scattering supported by magnetization and specific heat measurements to study the spin dynamics in the hard-axis ferromagnet $\mathrm{Ce}\mathrm{Ag}{\mathrm{Sb}}_2$. In the zero-field state we observed two sharp magnon modes, which are associated with Ce ordering and extended up to $\approx 3$ meV with a considerable spin gap of 0.6 meV. Application of a magnetic field perpendicular to the moment direction reduces the spectral intensity and suppresses the gap and significantly enhances the low-temperature specific heat at a critical field of $B_{\mathrm{c}}\approx 2.8$ T via a mean-field-like transition. Above the transition, in the field-polarized state, the gap eventually reopens due to the Zeeman effect. We modeled the observed dispersion using linear spin-wave theory taking into account the ground-state ${\mathrm{\Gamma }}_6$ doublet and exchange anisotropy. Our model correctly captures the essential features of the spin dynamics including magnetic dispersion, distribution of the spectral intensity, as well as the field-induced behavior, although several minor features remain obscure. The observed spectra do not show significant broadening due to the finite lifetime of the quasiparticles. Along with a moderate electronic specific heat coefficient $\gamma =46$ mJ/mol ${\mathrm{K}}^2$ this indicates that the Kondo coupling is relatively weak and the Ce moments are well localized. Altogether, our results provide profound insight into the spin dynamics of the hard-axis ferromagnet $\mathrm{Ce}\mathrm{Ag}{\mathrm{Sb}}_2$ and can be used as solid ground for studying magnetic interactions in isostructural compounds including $\mathrm{Ce}\mathrm{Au}{\mathrm{Sb}}_2$, which exhibits nematicity and unusual mesoscale magnetic textures.

Carrier induced defect creation at the semiconductor-oxide interface has been known as the origin of electronic device degradation for a long time, but how exactly the interface lattice can be damaged by carriers (especially low-energy ones) remains unclear. Here we carry out real-time time-dependent density functional theory simulations on concrete $\mathrm{Si}/\mathrm{Si}{\mathrm{O}}_2$ interfaces to study the interaction between excited electrons and interface bonds. We show that the normal interface Si-H bonds are generally resistant to electrons due to the delocalized nature and high energy level of the Si-H antibonding states, and due to the high-energy barrier to break the Si-H bond. However, if an additional hydrogen atom exists by attaching to a nearby oxygen atom (forming a “Si-H···H-O” complex), the Si-H bond will be greatly weakened, including the reduction of energy barrier for bond breaking, and the lowering of the antibonding state energy level which favors electron injection. Together with the multiple vibrational excitation process, the corresponding Si-H bond can be broken much more easily. Thus we propose that the Si-H···H-O complex will be the center for defect creation and device degradation. We also explain why such a center might be relatively easy to form during the hydrogen annealing process.

In this work, we investigated magnetotransport properties in epitaxial heterostructure of $\mathrm{Sn}\mathrm{Te}\text{/}{\mathrm{Sn}}_{1-x}{\mathrm{Eu}}_x\mathrm{Te}$ under dark and infrared light illumination. The longitudinal resistance has indicated the presence of Shubnikov–de Haas oscillations in the temperature range of $1.9–30\mathrm{K}$. The fast Fourier transform analysis, performed under light and dark conditions, enabled us to understand the magnetotransport mechanism through the individual layers, leading to the proper address of each frequency that compound the Shubnikov–de Haas oscillations.

The phonon-assisted optical transitions of excitons in intrinsic diamond have been investigated theoretically and experimentally. Excellent agreement is indicated between absorption spectra measured and calculated based on the phonon coupling strengths from first-principles calculations using just a single adjustable parameter. Temperature shift and broadening are incorporated via a pseudo-Voigt function analysis developed for the derivative of the experimental absorption spectra. The absorption coefficients of excitons in discrete and continuum states are separated and contributions from different phonon modes are discussed, which enable theoretical derivation of the exceptionally high phonon-assisted radiative rate of excitons in diamond among indirect-gap semiconductors. This study expands the applicability of the second-order perturbation theory involving electron-phonon interactions in materials important for future photonics and power electronics.

High-frequency pulsed electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) were used to determine electron nuclear interactions on remote ligand shells of silicon and carbon in spin-3/2 color centers with an optically induced high-temperature spin alignment in hexagonal $4H$-, $6H$-, and rhombic $15R$-silicon carbide (SiC) polytypes. The EPR and ENDOR experimental data relate unambiguously to spin-3/2 centers in which the optically induced alignment of the spin-level populations occurs. The identification is based on resolved ligand hyperfine interactions with carbon and silicon nearest, next-nearest, and the more distant neighbors and on the determination of the unpaired electron spin densities. The hyperfine interactions with ${}^{29}\mathrm{Si}$ and ${}^{13}\mathrm{C}$ nuclei are unambiguously separated due to the selective population of the fine-structure levels with certain values of ${\mathrm{M}}_{\mathrm{S}}$. The signs of these interactions and, as a result, the signs of oscillating spin density at ${}^{29}\mathrm{Si}$ and ${}^{13}\mathrm{C}$ nuclei, are determined. On the basis of the EPR and optically induced ENDOR measurements, signs of the fine-structure splitting for all the centers were demonstrated, which made it possible to establish the character of optically induced spin alignment, including the inverse populations of the spin levels for these centers. The values of hyperfine interaction with ${}^{29}\mathrm{Si}$ and ${}^{13}\mathrm{C}$ nuclei, including those remote from the localization of the spin-3/2 center, are tabulated, which can be used by a number of algorithms in quantum information processing as long-term memory.

At a discrete set of magic angles, twisted bilayer graphene has been shown to host extraordinarily flat bands, correlated insulating states, unconventional superconductivity, and distinct Landau level degeneracies. In this paper, we design a highly efficient quantum Otto engine using a twisted bilayer graphene sample. Flat electronic bands at magic angles make the prospect of extracting work from our Otto engine lucrative because exploiting correlated phenomena may lead to nanoscale devices operating at considerably larger efficiencies. We use an eight-band continuum model of twisted bilayer graphene to compute efficiencies and work outputs for magic and nonmagic angle twists and compare the results with an $AB$ stacked bilayer and a monolayer. It is observed that the efficiency varies smoothly with the twist angle, and the maximum is attained at the magic angle.

We investigate the interaction between excitons in a two-dimensional atomic crystal and quasibound states in the continuum (q-BIC) in a two-dimensional all-dielectric asymmetric metasurface. By introducing coherent and incoherent coupling terms in a coupled oscillator model, we demonstrate the coexistence of coherent and incoherent coupling processes in the strongly coupled system and the resultant sub/superradiant polariton states. Based on the multipole decomposition method and near-field analysis of full wave simulations, we study the microscopic excitation of multipole components in q-BIC and their coupling to excitons. We reveal that not only the magnetic dipole but also the interference between electric dipole and its toroidal counterpart dominate in the exciton-BIC strong coupling regime. The fractions of the magnetic and electric dipole ingredients are modulated by the topology of the two-dimensional all-dielectric metasurface, exhibiting distinctly different features in the high- and low-energy hybrid modes in the strong coupling system. Our findings are expected to be of importance for both fundamental research in TMD-based light-matter interactions and practical applications in the design of novel, tunable exciton-polariton devices with high compactibility.

The boundary charge that accumulates at the edge of a one-dimensional single-channel insulator is known to possess the universal property that its change under a lattice shift towards the edge by one site is given by the sum of the average bulk electronic density and a topologically invariant contribution, restricted to the values 0 and $-1$ [Pletyukhov et al., Phys. Rev. B 101, 165304 (2020)]. This quantized contribution is associated with particle-hole duality, ensures charge conservation, and fixes the mod(1) ambiguity appearing in the modern theory of polarization. In the present paper we generalize the above-mentioned single-channel results to the multichannel case by employing the technique of boundary Green's functions. We show that the topological invariant associated with the change in boundary charge under a lattice shift in multichannel models can be expressed as a winding number of a certain combination of components of bulk Green's functions as a function of the complex frequency, as it encircles the section of the energy axis that corresponds to the occupied part of the spectrum. We observe that this winding number is restricted to values ranging from $-N_c$ to zero, where $N_c$ is the number of channels (orbitals) per site. Furthermore, we consider translationally invariant one-dimensional multichannel models with an impurity and introduce topological indices which correspond to the quantized charge that accumulates around said impurity. These invariants are again given in terms of winding numbers of combinations of components of bulk Green's functions. Through this construction we provide a rigorous mathematical proof of the so-called nearsightedness principle formulated by Kohn [Kohn, Phys. Rev. Lett. 76, 3168 (1996)] for noninteracting multichannel lattice models.

Recent studies have revealed the significance of electron-phonon interaction (EPI) in phonon transport at intermediate temperatures. In some metals, the EPI can even dominate over the anharmonic phonon-phonon (ph-ph) scattering, leading to an anomalous phonon transport regime in which the lattice thermal conductivity ${\kappa }_L$ becomes nearly temperature ($T$) independent in contrast to the usual $1/T$ dependence. However, the experimental verification of this anomalous transport regime is very challenging due to the difficulty in separating the phonon contributions from the dominating electron ones to the measured total thermal conductivity in metals. In this work, using first-principles calculations, we predict that in bilayer graphene, the phonon transport can be driven to the anomalous regime by tuning the doping level. At high doping levels close to the Van Hove singularity, the EPI can result in a fivefold reduction of ${\kappa }_L$ at room temperature, and ${\kappa }_L$ becomes $T$ independent. This anomalous behavior is found to have its origin in three aspects: (i) mirror symmetry breaking enables direct coupling between flexural phonons, the dominant carriers of ${\kappa }_L$, and electrons; (ii) dominance of normal processes in the anharmonic ph-ph scattering facilitates the EPI to be more prominent; (iii) dominance of these normal ph-ph processes induces the indirect effect of EPI on ${\kappa }_L$. This is distinct from monolayer graphene, where the mirror symmetry prohibits the direct scattering of the flexural phonons by electrons and only the indirect EPI affects ${\kappa }_L$. This work gives insight into the manipulation of heat conduction via externally induced EPI in two-dimensional materials in which mirror symmetry breaks and normal processes dominate the ph-ph scattering.

State tomography is an essential tool for analyzing physical states in quantum science. In magnets, elementary excitation called a magnon, or a spin wave, dominates magnetization dynamics and various magnetic properties. Here, we propose and demonstrate state tomography for magnetization dynamics, enabling us to obtain a density matrix and Wigner function of the magnetization dynamics. Using the technique, we found that parametrically excited magnons can form a mixed state composed of two coherent states. Magnetization state tomography will pave a way to explore a wide range of states of magnons, such as a squeezed state in condensed matter.

Triangular Heisenberg antiferromagnets are prototypes of geometric frustration, even if for nearest-neighbor interactions quantum fluctuations are not usually strong enough to destroy magnetic ordering: stronger frustration is required to stabilize a spin-liquid phase. On the basis of static magnetization and electron spin resonance measurements, we demonstrate the emergence of ${\tilde{j}}_{\text{eff}}=\frac{1}{2}$ moments in the triangular-lattice magnet ${\mathrm{Na}}_2\mathrm{Ba}\mathrm{Co}{\left(\mathrm{P}{\mathrm{O}}_4\right)}_2$. These moments are subject to an extra source of frustration that causes magnetic correlations to set in far above both the magnetic ordering and Weiss temperatures. Corroborating the ${\tilde{j}}_{\text{eff}}=\frac{1}{2}$ ground state, theory identifies ferromagnetic Kitaev exchange anisotropy as an additional frustrating agent, altogether putting forward ${\mathrm{Na}}_2\mathrm{Ba}\mathrm{Co}{\left(\mathrm{P}{\mathrm{O}}_4\right)}_2$ as a promising Kitaev spin-liquid material.

The cluster slave-spin method is employed to investigate systematically the ground state properties of the Hubbard model on a square lattice with doping $\delta$ and coupling strength $U$ being its parameters. At half-filling, a relation between the staggered magnetization $M$ and the antiferromagnetic (AFM) gap ${\mathrm{\Delta }}_{\text{AFM}}$ is established in the small $U$ limit to compare with that from the Hartree-Fock theory, and a first-order metal-insulator Mott transition in the paramagnetic state is substantiated, which is characterized by discontinuities and hystereses at $U_{\text{Mott}}=10t$. The interaction $U_c$ for the crossover in the AFM state, separating the weak- and strong-coupling regimes, is found to remain almost unchanged with large dopings, and smaller than $U_{\text{Mott}}$ at half-filling because of long range AFM correlations. Finally, an overall phase diagram in the $U-\delta$ plane is presented, which is composed of four regimes: the AFM insulator at half-filling, the AFM metal with the compressibility $\kappa >0$ or $\kappa <0$, and the paramagnetic metal, as well as three phase transitions: (i) From the AFM metal to the paramagnetic metal, (ii) between the AFM metal phases with positive and negative $\kappa$, and (iii) separating the AFM insulating phase at $\delta =0$ from the AFM metal phase for $\delta >0$.

In the “ballistic” regime, the transport across a normal metal (N)/superconductor (S) point contact is dominated by a quantum process called Andreev reflection. Andreev reflection causes an enhancement of the conductance below the superconducting energy gap, and the ratio of the zero-bias and the high-bias conductance cannot be greater than 2 when the superconductor is conventional in nature. In this regime, the features associated with Andreev reflection also provide energy and momentum-resolved spectroscopic information about the superconducting phase. Here, we theoretically consider various types of nonballistic N/S point contacts within a network resistor model and show that even when the superconductor under investigation is simple and conventional in nature, depending on the shape, size, and anatomy of the point contacts, a wide variety of spectral features may appear in the conductance spectra. We also show that in experiments involving simple, conventional superconductors, such as Nb, all such features do indeed appear. Such features may misleadingly mimic theoretically expected signatures of certain exotic physical phenomena including unconventional superconductivity and topological effects. We also present how the role of such nonballistic effects can be ruled out in experiments.

We show in random matrix theory, microwave measurements, and computer simulations that the mean free path of a random medium and the strength and position of an embedded reflector can be determined from radiation scattered by the system. The mean free path and strength of the reflector are determined from the statistics of transmission. The statistics of transmission are independent of the position of the reflector. The reflector's position can be found, however, from the average dwell time for waves incident from one side of the sample.

We propose to measure band topology via quantized drift of Bloch oscillations in a two-dimensional Harper-Hofstadter lattice subjected to tilted fields in both directions. When the difference between the two tilted fields is large, Bloch oscillations uniformly sample all momenta, and hence the displacement in each direction tends to be quantized at multiples of the overall period, regardless of any momentum of initial state. The quantized displacement is related to a reduced Chern number defined as a line integral of Berry curvature in each direction, providing an almost perfect measurement of Chern number. Our scheme can apply to detecting Chern number and topological phase transitions not only for the energy-separable band, but also for energy-inseparable bands which cannot be achieved by conventional Thouless pumping or integer quantum Hall effect.

The topological effect in thermoelectric transport is an important research aspect in semimetals, but whether an anomalous Nernst effect exists in nonmagnetic topological semimetals in a finite external field is still under debate. To demonstrate how to discern the topological effect in nonmagnetic topological semimetals, we present a comprehensive study on the magnetothermoelectric properties for four Weyl semimetals: TaAs, TaP, NbAs, and NbP. We observe large magneto-Seebeck and Nernst signals at intermediate temperatures, which are attributed to a multiband ordinary contribution and an inelastic, phonon-drag effect, while the latter is ignored in previous studies. This phonon-drag effect also induces an unusual, prominent temperature and field dependence of quantum oscillations in thermoelectric transport signals. On the other hand, only signatures of a relatively small anomalous thermoelectric effect are found in TaAs compared with the ordinary effect.

We apply quantum continuum mechanics to the calculation of the excitation spectrum of a coupled electron-hole bilayer. The theory expresses excitation energies in terms of ground-state intra- and interlayer pair correlation functions, which are available from quantum Monte Carlo calculations. The final formulas for the collective modes deduced from this approach coincide with the formulas obtained in the “quasilocalized particle approximation” by Kalman et al. [G. Kalman, V. Valtchinov, and K. I. Golden, Phys. Rev. Lett. 82, 3124 (1999)], and likewise the theory predicts the existence of gapped excitations in the charged channel, with the gap arising from electron-hole correlation. An immediate consequence of the gap is that the static density-density response function of the charged channel vanishes as $q^2$ for wave vector $q\to 0$, rather than linearly in $q$, as commonly expected. In this sense, the system is incompressible. This feature, which has no analog in the classical electron-hole plasma, is consistent with the existence of an excitonic ground state and implies the existence of a discontinuity in the chemical potential of electrons and holes when the numbers of electrons and holes are equal. It should be experimentally observable by monitoring the densities of electrons and holes in response to potentials that attempt to change these densities in opposite directions.

We consider three-dimensional lattice $\mathrm{SU}\left(N_c\right)$ gauge theories with multiflavor ($N_f>1$) scalar fields in the adjoint representation. We investigate their phase diagram, identify the different Higgs phases with their gauge-symmetry pattern, and determine the nature of the transition lines. In particular, we study the role played by the quartic scalar potential and by the gauge-group representation in determining the Higgs phases and the global and gauge-symmetry-breaking patterns characterizing the different transitions. The general arguments are confirmed by numerical analyses of Monte Carlo results for two representative models that are expected to have qualitatively different phase diagrams and Higgs phases. We consider the model with $N_c=3, N_f=2$ and with $N_c=2, N_f=4$. This second case is interesting phenomenologically to describe some features of cuprate superconductors.