We present theoretical methods for the analysis of acoustic phonon modes in superlattice structures, and terahertz-frequency quantum-cascade lasers (THz QCLs). Our generalized numerical solution of the acoustic-wave equation provides good agreement with experimental pump-probe measurements of the acoustic resonances in a THz QCL. We predict that the detailed layer structure in THz QCLs imprints up to $\sim 2\mathrm{GHz}$ detuning of the acoustic mode spacing, which cannot be seen in analytical models. This effect is strongest in devices with large and abrupt acoustic mismatch between layers. We use an acoustic deformation potential within a density-matrix approach to analyze electron transport induced in a range of the most common THz QCL active-region design schemes. We conclude that acoustic modes up to $\sim 200\mathrm{GHz}$ are capable of significantly perturbing QCL transport, highlighting their potential for ultrafast modulation of laser emission.

The optical properties of excitons confined in initially unstrained GaAs/AlGaAs quantum dots are studied as a function of a variable quasiuniaxial stress. To allow the validation of state-of-the-art computational tools for describing the optical properties of nanostructures, we determine the quantum dot morphology and the in-plane components of externally induced strain tensor at the quantum dot positions. Based on these experimental parameters, we calculate the strain-dependent excitonic emission energy, degree of linear polarization, and fine-structure splitting using a combination of eight-band $\mathbf{k}·\mathbf{p}$ formalism with multiparticle corrections using the configuration interaction method. The experimental observations are quantitatively well reproduced by our calculations and deviations are discussed.

Results are presented for a Floquet Ising chain with duality twisted boundary conditions, taking into account the role of weak integrability breaking in the form of four-fermion interactions. In the integrable case, a single isolated Majorana zero mode exists which is a symmetry in the sense that it commutes both with the Floquet unitary and the $Z_2$ symmetry of the Floquet unitary. When integrability is weakly broken, both in a manner so as to preserve or break the $Z_2$ symmetry, the Majorana zero mode is still found to be conserved for small system sizes. This is reflected in the dynamics of an infinite-temperature autocorrelation function which, after an initial transient that is controlled by the strength of the integrability breaking term, approaches a plateau that does not decay with time. The height of the plateau agrees with a numerically constructed conserved quantity and is found to decrease with increasing system sizes. It is argued that the existence of the plateau and its vanishing for larger system sizes is closely related to a localization-delocalization transition in Fock space triggered by the integrability-breaking interactions.

Zigzag graphene nanoribbons (GNRs), demonstrating edge magnetism, are fascinating materials for electronic and spintronic applications. In this paper, we investigate with explicit consideration of electron-phonon scattering the Seebeck effect and thermal conductivity at various carrier densities in zigzag GNRs, which undergo antiferromagnetic, ferromagnetic, and paramagnetic transitions. Seebeck coefficients are spin dependent in ferromagnetic zigzag GNRs and can have opposite signs for the majority and the minority spin carriers, which enables the separation of spin carriers spatially under a thermal gradient. The electronic thermal conductivity is small compared to the lattice thermal conductivity, except for GNRs in the ferromagnetic state. A 100% increase in thermal conductivity is expected at antiferromagnetic to ferromagnetic transition. These results demonstrate that zigzag GNRs are also materials of high interest for spin caloritronics.

We report magnetotransport measurements on ${\mathrm{Bi}}_1{\mathrm{Sb}}_1{\mathrm{Te}}_{1.5}{\mathrm{Se}}_{1.5}/{\mathrm{RuCl}}_3$ heterostructure nanodevices. ${\mathrm{Bi}}_1{\mathrm{Sb}}_1{\mathrm{Te}}_{1.5}{\mathrm{Se}}_{1.5}$ (BSTS) is a strong three-dimensional topological insulator (3D-TI) that hosts conducting topological surface states (TSS) enclosing an insulating bulk. $\alpha \text{−}{\mathrm{RuCl}}_3$ (namely, ${\mathrm{RuCl}}_3$) is an antiferromagnet that is predicted to behave as a Kitaev-like quantum spin liquid carrying chargeless Majorana excitations. Temperature ($T$)-dependent resistivity measurements on BSTS show the interplay between parallel bulk and surface transport channels. For $T<150$ K, surface state transport dominates over bulk transport. Multichannel weak antilocalization (WAL) is observed as a sharp cusp in the low-field magnetoconductivity, indicating strong spin-orbit coupling. The presence of top and bottom topological surface states (TSS) including a pair of electrically coupled Rashba surface states (RSS) is indicated. The nonlinear Hall effect, explained by a two-band model, further supports this interpretation. We observed that ${\mathrm{RuCl}}_3$ has an electrostatic effect on the TSS of BSTS, that is, it changes bandbending at the interface as a result of charge transfer. In this regard, the change in low-$T$ logarithmic resistivity upturn in the presence of out-of-plane magnetic field is analyzed employing the Lu-Shen model, supporting the presence of gapless surface states (TSS and RSS) with a $\pi$ Berry phase.

We uncover the very rich graph topology of generic bounded non-Hermitian spectra, distinct from the topology of conventional band invariants and complex spectral winding. The graph configuration of complex spectra are characterized by the algebraic structures of their corresponding energy dispersions, drawing new intimate links between combinatorial graph theory, algebraic geometry, and non-Hermitian band topology. Spectral graphs that are conformally related belong to the same equivalence class, and are characterized by emergent symmetries not necessarily present in the physical Hamiltonian. The simplest class encompasses well-known examples such as the Hatano-Nelson and non-Hermitian SSH models, while more sophisticated classes represent novel multicomponent models with interesting spectral graphs resembling stars, flowers, and insects. With recent rapid advancements in metamaterials, ultracold atomic lattices, and quantum circuits, it is now feasible to not only experimentally realize such esoteric spectra, but also investigate the non-Hermitian flat bands and anomalous responses straddling transitions between different spectral graph topologies.

In this letter, we investigate the stable and commensurate van der Waals heterostructures of metallic and semiconducting $1H$ transition-metal dichalcogenides, ${\mathrm{NbS}}_2$ and ${\mathrm{MoSe}}_2$ $\left({\mathrm{WSe}}_2\right)$, which possess almost the same lattice constant of the pristine honeycomb structure. In the most stable structure, the metallic and semiconducting layers are stacked in a similar manner to $3R$ stacking but the period is a pair consisting of a metallic layer and a semiconducting layer. The heterostructure aligns the spin polarization in each valley among all layers and induces spin-selective charge transfer between the metallic and semiconducting layers. Especially in heterotrilayers, the electronic spin is conserved due to mirror symmetry along the out-of-plane axis in contrast to the $3R$ stacking structure. A drastic enhancement of spin Hall effect is numerically shown as an example of electronic spin transport phenomena in the heterotrilayers.

Time-resolved x-ray emission/absorption spectroscopy (Tr-XES/XAS) is an informative experimental tool sensitive to electronic dynamics in materials, widely exploited in diverse research fields. Typically, Tr-XES/XAS requires x-ray pulses with both a narrow bandwidth and subpicosecond pulse duration, a combination that in principle finds its optimum with Fourier transform-limited pulses. In this work, we explore an alternative experimental approach, capable of simultaneously retrieving information about unoccupied (XAS) and occupied (XES) states from the stochastic fluctuations of broadband extreme ultraviolet pulses of a free electron laser. We used this method, in combination with singular-value decomposition and Tikhonov regularization procedures, to determine the XAS/XES response from a crystalline silicon sample at the $L_{2,3}$ edge, with an energy resolution of a few tens of meV. Finally, we combined this spectroscopic method with a pump-probe approach to measure structural and electronic dynamics of a silicon membrane. Tr-XAS/XES data obtained after photoexcitation with an optical laser pulse at 390 nm allowed us to observe perturbations of the band structure, which are compatible with the formation of the predicted precursor state of a nonthermal solid-liquid phase transition associated with a bond softening phenomenon.

At the 2D limit, hexagonal systems such as monolayer transition metal dichalcogenides (TMDs) and graphene exhibit unique coupled spin and momentum-valley physics (valley pseudospin) owing to broken spatial inversion symmetry and strong spin-orbit coupling. Circularly polarized light provides the means for pseudospin-selective excitation of excitons (or electrons and holes) and can yield momentum-valley polarized populations of carriers that are the subject of proposed valleytronic applications. The chirality of these excited carriers has important consequences for the available relaxation/scattering pathways, which must conserve (pseudo)angular momentum as well as energy. One available relaxation channel that satisfies these constraints is coupling to chiral phonons. Here, we show that chiral carrier-phonon coupling following valley-polarized photoexcitation is expected to lead to a strongly valley-polarized chiral phonon distribution when this relaxation mechanism is dominant. This momentum valley phonon polarization is directly measurable using ultrafast phonon diffuse scattering techniques. Using ab initio calculations, we show how the dynamic phonon occupations and valley anisotropy determined by nonequilibrium observations can provide a new window on the physical processes that drive carrier valley depolarization in monolayer TMDs.

Thermal fluctuation in magnets causes temperature-dependent self-energy corrections in magnons; however, its effects on the topological orders of magnons is not well explored. Here we demonstrate that such corrections can induce a Chern insulating phase in two-dimensional collinear antiferromagnets with sublattice asymmetries by increasing temperature. We present the phase diagram of the system and show that the trivial magnon bands at zero temperature exhibit Chern insulating phase above a critical temperature before the paramagnetic phase transition. The self-energy corrections close and reopen the band gap at $\mathrm{\Gamma }$ or $\mathbf{K}$ points, accompanied by a magnon chirality switch and nontrivial Berry curvature transition. The thermal Hall effect of magnons or detecting the magnon polarization can highlight the experimentally prominent signatures of the topological transitions. We include the numerical results based on the van der Waals magnet ${\mathrm{MnPS}}_3$, calling for experimental implementation. Our work presents a paradigm for constructing topological phases that is beyond the linear spin wave theory.

The interplay between spin wave (SW) and magnetic vortex is studied. We find three types of magnon scatterings: skew scattering, symmetric side deflection, and back reflection, which associate with, respectively, magnetic topology, energy density distribution, and linear momentum transfer torque within the vortex. The vortex core exhibits two translational modes: the intrinsic circular mode and a coercive elliptical mode, which can be excited by the net and oscillating magnon spin-transfer torque effects of SWs. Lastly, we propose a vortex-based SW valve in which we obtain excellent valving performance and access arbitrary control of the phase shift via inhomogeneity modulation. The phase shift arises from the enlarged wavelength of the SW in inhomogeneous magnetization.

The quantum anomalous Hall (QAH) effect is rarely predicted in antiferromagnetic (AFM) materials. Here, by first-principles calculations, we propose that the monolayer of MoO is AFM and can be tuned to be a QAH insulator with a band gap of 50 meV. The MoO monolayer is a tetragonal lattice and we have checked its stability by the phonon spectrum and molecular dynamical simulation. It has a collinear AFM order with magnetic moments larger than $2{\mu }_B$ on each Mo atom. In the absence of strain, it is an AFM metal with a direct gap if spin-orbital coupling is considered. Tensile strain results in a metal-insulator phase transition, but it is still topologically trivial protected by an effective time-reversal symmetry. Shear strain breaks this symmetry and leads to the expected nontrivial electronic bands with Chern number $C=-1$. In addition, its Néel temperature could be larger than room temperature, providing another platform for the application of AFM materials in spintronic devices.

The competition between antiferromagnetism and superconductivity is one of the central questions in research involving strongly correlated systems. In this paper, we utilize a double-layer model containing Hubbard interaction and interlayer Heisenberg interaction to reveal their competition. This model is free of the sign problem under certain conditions, and we perform projector quantum Monte Carlo simulations to extract the ground-state correlations of magnetism and superconductivity. Our results show that the superconductivity emerges when the antiferromagnetism is suppressed by tuning the filling or the anisotropy of the interlayer Heisenberg interaction. This model can be seen as an analog of unconventional superconductors and may help us to understand the transition from an antiferromagnetic insulator to a superconductor.

Recent nuclear magnetic resonance and calorimetric experiments have observed that ${\mathrm{UTe}}_2$ exhibits a transition between two distinct superconducting phases as a function of magnetic field strength for a field applied along the crystalline $b$ axis. To determine the nature of these phases, we employ a microscopic two-band minimal Hamiltonian with the essential crystal symmetries and structural details. We also adopt anisotropic ferromagnetic exchange terms. We study the resulting pairing symmetries and properties of these low- and high-field phases in mean-field theory.

We study an imaginary Stark ladder model, and we propose a realization of the model in a dissipative chain with linearly increasing site-dependent dissipation strength. Due to the existence of $K$ symmetry and passive $\mathcal{PT}$ symmetry, the model exhibits quite a different feature from its Hermitian counterpart. With the increase of dissipation strength, the system first undergoes a passive $\mathcal{PT}$-symmetry breaking transition, with the shifted eigenvalues changing from real to complex, and then a $K$-symmetry restoring transition, characterized by the emergence of a pure imaginary spectrum with equal spacing. Accordingly, the eigenstates change from $\mathcal{PT}$-unbroken extended states to $\mathcal{PT}$-broken states, and finally to Stark-localized states. In the framework of the quantum open system governed by the Lindblad equation with linearly increasing site-dependent dissipation, we unveil that the dynamical evolution of a single-particle correlation function is governed by the Hamiltonian of the imaginary Stark ladder model. By studying the dynamical evolution of the density distribution under various initial states, we demonstrate that the damping dynamics displays distinct behaviors in different regions. A localized damping is observed in the strong dissipation limit.