Finite-length armchair graphene nanoribbons can behave as one-dimensional topological materials, that may show edge states in their zigzag-terminated edges, depending on their width and termination. We show here a full solution of tight-binding graphene rectangles of any length and width that can be seen as either finite-length armchair or zigzag ribbons. We find exact analytical expressions for both bulk and edge eigenstates and eigenenergies. We write down exact expressions for the Coulomb interactions among edge states and introduce a Hubbard-dimer model to analyze the emergence and features of different magnetic states at the edges, whose existence depends on the ribbon length. We find ample room for experimental testing of our predictions in $N=5$ armchair ribbons. We compare the analytical results with ab initio simulations to benchmark the quality of the dimer model and to set its parameters. A further detailed analysis of the ab initio Hamiltonian allows us to identify those variations of the tight-binding parameters that affect the topological properties of the ribbons.

We unravel the influence of quasiparticle screening in the nonequilibrium exciton dynamics of monolayer ${\mathrm{WS}}_2$. We report pump photon energy-dependent exciton blue- and redshifts from time-resolved-reflectance contrast measurements. Based on a phenomenological model, we isolate the effective impact of excitons and free carriers on the renormalization of the quasifree particle band gap, exciton binding energy, and linewidth broadening. By this, our work not only provides a comprehensive picture of the competing phenomena governing the exciton dynamics in ${\mathrm{WS}}_2$ upon photoexcitation, but also demonstrates that exciton and carrier contributions to dynamic screening of the Coulomb interaction differ significantly.

The intersection of topology, symmetry, and magnetism yields a rich structure of possible phases. In this work, we study theoretically the consequences of magnetism on ${\mathrm{IrF}}_4$, which was recently identified as a possible candidate topological nodal chain semimetal in the absence of magnetic order. We show that the spin-orbital nature of the Ir moments gives rise to strongly anisotropic magnetic couplings resembling a tetrahedral compass model on a diamond lattice. The predicted magnetic ground state preserves key symmetries protecting the nodal lines, such that they persist into the ordered phase at the mean-field level. The consequences for other symmetry reductions are also discussed.

We propose a highly efficient and accurate numerical scheme named the truncated atomic plane wave method to determine the subband structure of twisted bilayer graphene (TBG) inspired by the Bistritzer-MacDonald model. Our method utilizes real-space information of carbon atoms in the moiré unit cell and projects the full tight-binding Hamiltonian into a much smaller subspace using atomic plane waves. Using our method, we are able to present accurate electronic band structures of TBG in a wide range of twist angles together with the detailed moiré potential and screened Coulomb interaction at the first magic angle. Furthermore, we generalize our formalism to solve the problem of low-frequency moiré phonons in TBG.

We use general symmetry-based arguments to construct an effective model suitable for studying optical properties of lead halide perovskites. To build the model, we identify an atomic-level interaction between electromagnetic fields and the spin degree of freedom that should be added to a minimally coupled $\mathbf{k}·\mathbf{p}$ Hamiltonian. As a first application, we study two basic optical characteristics of the material: the Verdet constant and the refractive index. Beyond these linear characteristics of the material, the model is suitable for calculating nonlinear effects such as the third-order optical susceptibility. Analysis of this quantity shows that the geometrical properties of the spin-electric term imply isotropic optical response of the system, and that optical anisotropy of lead halide perovskites is a manifestation of hopping of charge carriers. To illustrate this, we discuss third-harmonic generation.

We theoretically investigate three-dimensional (3D) layered crystals of alternating graphene and $h\mathrm{BN}$ layers with different symmetries. Depending on the hopping parameters between the graphene layers, we find that these synthetic 3D materials can feature semimetallic, gapped, or Weyl semimetal phases. Using first-principles calculations to parametrize the low-energy Hamiltonians we establish the most likely electronic phases. Our results demonstrate that 3D crystals stacked from individual 2D materials represent a synthetic materials class with emergent properties different from their constituents.

Relativistic electrons experience very slight wave packet distortion and negligible momentum recoil when interacting with nanometer-sized samples, as a consequence of the ultrashort interaction time. Accordingly, modeling fast electrons as classical point charges provides extremely accurate theoretical predictions of energy loss spectra. Here we investigate the aloof interaction of nanometer-sized electron beams of a few keV with micron-sized samples, a regime where the classical description generally fails due to significant wave function broadening and momentum recoil. To cope with these effects, we use macroscopic quantum electrodynamics to analytically derive a generalized expression for the electron energy loss probability which accounts for recoil. Quantum features of the interaction are shown to get dramatically strong as the interaction length is increased and/or the electron kinetic energy is decreased. Moreover, relatively large values of the energy loss probability are found at higher energy losses and larger impact parameters, a marked quantum effect which is classically forbidden by the evanescent profile of the field produced by a moving point charge.

We study charge transport in a single-electron transistor implemented as an interferometer such that the Coulomb-blockaded middle island contains a circular chiral Majorana or Dirac edge mode. We concentrate on the regime of small conductance and provide an asymptotic solution in the limit of high transport voltage exceeding the charging energy. The solution is achieved using an instantonlike technique. The distinctions between Majorana and Dirac cases appear when the tunnel junctions are unequal. The main difference is in the offset current at high voltages which can be higher, up to 50% in the Majorana case. It is caused by an additional particle-hole symmetry of the distribution function of Majorana fermions. There is also an eye-catching distinction between the oscillation patterns of the current as a function of the gate charge. We conjecture this distinction survives at lower transport voltages as well.

One of the most fascinating and puzzling aspects of non-Hermitian systems is their spectral degeneracies, i.e., exceptional points (EPs), at which both eigenvalues and eigenvectors coalesce to form a defective state space. While coupled magnetic systems are natural hosts of EPs, the relation between the linear and nonlinear spin dynamics in the proximity of EPs remains relatively unexplored. Here we theoretically investigate the spin dynamics of easy-plane magnetic bilayers in the proximity of exceptional points. We show that the interplay between the intrinsically dissipative spin dynamics and external drives can yield a rich dynamical phase diagram. In particular, we find that, in antiferromagnetically coupled bilayers, a periodic oscillating dynamical phase emerges in the region enclosed by EPs. Our results not only offer a pathway for probing magnetic EPs and engineering magnetic nano-oscillators with large-amplitude oscillations, but also uncover the relation between exceptional points and dynamical phase transitions in systems displaying nonlinearities.

We discuss a Rydberg atom chain with a tunable blockade radius from the gauge theoretic perspective. When the blockade radius is one lattice spacing, this system can be formulated in terms of the PXP model, and there is a ${\mathbb{Z}}_2$ Ising phase transition known to be equivalent to a confinement–deconfinement transition in a gauge theory, the lattice Schwinger model. Further increasing the blockade radius, one can add a next-nearest neighbor (NNN) interaction into the PXP model. We discuss the interpretation of the NNN interaction in terms of the gauge theory and how a finite NNN interaction alters the deconfinement behavior and propose a corresponding experimental protocol. When the blockade radius reaches two lattice spacing, the model reduces to the PPXPP model. A novel gauge theory equivalent to the PPXPP model is formulated, and the phases in the two formulations are delineated. These results are readily explored experimentally in Rydberg quantum simulators.

Magnetization dynamics triggered by ultrashort laser pulses has been attracting significant attention, with a strong focus on the dynamics excited by visible/near-infrared pulses. Only recently has a strong magnetic response in solid materials induced by intense x-ray pulses from free-electron lasers been observed. The exact mechanisms that trigger the x-ray-induced demagnetization are not yet fully understood. They are the subject of ongoing experimental and theoretical investigations. Here, we present a theoretical analysis of electronic processes occurring during demagnetization of a Co multilayer system irradiated by x-ray pulses tuned to the $L_3$ absorption edge of cobalt. We show that, like in the case of x-ray-induced demagnetization at the $M$ edge of Co, electronic processes play a predominant role in the demagnetization until the pulse fluence does not exceed the structural damage threshold. The impact of electronic processes can explain reasonably well the available experimental data, without a need to introduce the mechanism of stimulated elastic forward scattering.

We investigate the critical behavior, both in space and time, of the wetting interface within the coexistence region around the first-order phase transition of a fully connected quantum Ising model in slab geometry. For that, we employ the Lindblad master equation formalism in which temperature is inherited by the coupling to a dissipative bath, rather than being a functional parameter as in the conventional Cahn's free energy. Lindblad's approach gives not only access to the dissipative dynamics and steady-state configuration of the quantum wetting interface throughout the whole phase diagram but also shows that the wetting critical behavior can be successfully exploited to characterize the phase diagram as an alternative to the direct evaluation of the free energies of the competing phases.

Stoichiometric ${\mathrm{SrTiO}}_3$ (STO) is a wide band-gap insulator in which oxygen deficiency generates a low-temperature metallic ferromagnetic state. Here, we report weak ferromagnetic-like and metallic transport in oxygen-deficient bicrystal of $\mathrm{STO}$ with a low-temperature electron mobility and surface carrier density of $2800\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ and $3.500\times {10}^{16}\mathrm{c}{\mathrm{m}}^{-2}$, respectively. Moreover, we show that magnetotransport behavior in the bulk and across the grain boundary (GB) in $\mathrm{STO}$ single crystals are significantly different. In the bulk, our magnetotransport results agree with those reported in the literature. Importantly, low-temperature magnetotransport across the GB shows a disorder-induced metal-insulator transition, and it is modeled as a quasi–two-dimensional system below 10 K that accounts for weak localization (WL), supported by electron-electron, electron-phonon, and Hikami-Larkin-Nagaoka (HLN) quantum interference models. The HLN model accounts for realistic length scales: the dephasing length $L_{\varphi }$, and the spin-orbit (SO) scattering length $L_{\mathrm{SO}}$, which are comparable to the electron mean-free path, resembling the surface-conducting nature across the GB. Our experimental work shows the importance of structural defects in the interpretation of magnetotransport in ${\mathrm{SrTiO}}_3$ and shows that WL is concurrent with a double Schottky barrier at the GB interface.

β-Mn-type ${\mathrm{Co}}_x{\mathrm{Zn}}_y{\mathrm{Mn}}_z$ $\left(x+y+z=20\right)$ alloys have recently attracted increasing attention as a class of chiral magnets with skyrmions at and above room temperature. However, experimental studies on the transport properties of this material are scarce. In this work, we report the successful growth of the β-Mn-type ${\mathrm{Co}}_{9.24}{\mathrm{Zn}}_{9.25}{\mathrm{Mn}}_{1.51}$ and ${\mathrm{Co}}_{9.02}{\mathrm{Zn}}_{9.18}{\mathrm{Mn}}_{1.80}$ single crystals and a systematic study on their magnetic and transport properties. The skyrmion phase was found in a small temperature range just below the Curie temperature. The isothermal ac susceptibility and dc magnetization as a function of magnetic field confirm the existence of the skyrmion phase. A negative linear magnetoresistance over a wide temperature range 2–380 K is observed and attributed to the suppression of the magnetic ordering fluctuation under high fields. Both the magnetization and electrical resistivity are almost isotropic. The quantitative analysis of the Hall resistance suggests that the anomalous Hall effect of ${\mathrm{Co}}_{9.24}{\mathrm{Zn}}_{9.25}{\mathrm{Mn}}_{1.51}$ and ${\mathrm{Co}}_{9.02}{\mathrm{Zn}}_{9.18}{\mathrm{Mn}}_{1.80}$ single crystals is dominated by the intrinsic mechanism. Our findings contribute to a deeper understanding of the transport properties of ${\mathrm{Co}}_x{\mathrm{Zn}}_y{\mathrm{Mn}}_z$ $\left(x+y+z=20\right)$ alloys material and advance their application in spintronic devices.

Density functional theory calculations are carried out to study the electronic and topological properties of $M\mathrm{P}{X}_3$ $(M=\mathrm{Mn},\mathrm{Fe},\mathrm{Co},\mathrm{Ni}; X=\mathrm{S},\mathrm{Se})$ monolayers in the ferromagnetic (FM) metastable magnetic state. We find that FM ${\mathrm{MnPSe}}_3$ monolayers host topological semimetal signatures that are gapped out when spin-orbit coupling is included. These findings are supported by explicit calculations of the Berry curvature and the Chern number. The choice of the Hubbard-$U$ parameter to describe the $d$-electrons is thoroughly discussed, as well as the influence of using a hybrid-functional approach. The presence of band inversions and the associated topological features are found to be formalism-dependent. Nevertheless, routes to achieve the topological phase via the application of external biaxial strain are demonstrated. Within the hybrid-functional picture, topological band structures are recovered under a pressure of 15% (17 GPa). The present work provides a potential avenue for uncovering new topological phases in metastable ferromagnetic phases.

We investigate the possibility of ferromagnetic ordering in the nondegenerate Hubbard model on the face-centered cubic lattice within the functional renormalization group technique using temperature as a scale parameter. We assume the relations between nearest, next-nearest, and next-next-nearest hopping parameters providing higher-order (giant) van Hove singularity of the density of states. The ferromagnetic instability formation with lowering temperature is described consistently in the one-loop approximation for a one-particle irreducible vertex of two-particle electron interaction. The chemical potential versus temperature phase diagrams are calculated. We find ferromagnetic order only for sufficiently strong divergence of the density of states and fillings in the vicinity of van Hove singularity. The obtained Curie temperature is more than an order of magnitude smaller than the results of the random-phase approximation. The main origin of the suppression of ferromagnetism is the screening of interaction in the particle-particle channel. We also do not find the pronounced tendency towards incommensurate order when the Fermi level is moved away from a van Hove singularity, such that the first-order quantum phase transitions from the ferro- to paramagnetic phase are obtained.

The influence of phonons on the low-energy excitonic excitations at zero temperature in the extended Falicov-Kimball model has been investigated. In the framework of the unrestricted Hartree-Fock approximation, a set of self-consistent equations for the excitonic condensate order parameter and a lattice distortion is derived when both electron-phonon coupling and electron-hole Coulomb interaction are treated on an equal footing. The low-energy excitation properties of the excitonic condensate are addressed in signatures of the optical conductivity and the dynamical excitonic susceptibility function. The real part of the optical conductivity is evaluated by the Kubo linear response theory and the imaginary part of the dynamical excitonic susceptibility is found by adapting the random phase approximation. In the semimetal state, one always finds a sharp peak in the optical conductivity spectrum indicating the stability of the excitonic condensation in the BCS type if the correlation between electrons and phonons becomes significant. In contrast, the peak is smeared out on the semiconducting side indicating the stability of the BEC-type excitonic condensate. In this semiconducting side, the sharp peak signature appears and the system turns to the BCS-type excitonic condensation state by increasing the electron-phonon correlations. In either the semimetal or the semiconducting normal state, increasing the electron-phonon correlations always reinforces a low-energy sharp peak in the dynamical excitonic susceptibility spectrum, indicating the existence of the tightly bound excitonic excitations before the condensation state. Specifically, on the semiconducting side, the “halo” phase with the preformed excitons exiting outside of the BEC-excitonic condensation state has been specified. The halo phase becomes more recognizable by raising the electron-phonon correlations.

We show the phase transition between the trivial and the topological semimetal phases in a non-Hermitian two-leg ladder model. We find that, the topological properties of this special model can be captured by a equivalent model with chiral symmetry in which the equivalent model owns the pure imaginary nonreciprocal intercell and long-range hopping simultaneously. Based on the equivalent model, we reveal that the system possesses the gapless topological semimetal phase induced by four exceptional points, which is different from the general topological semimetal phase usually owning two exceptional points. Significantly, we demonstrate that the system has the distinct transmission behaviors within the trivial and topological semimetal phases, leading the phase transition between trivial and topological semimetal phases and the boundary between left and right non-Hermitian skin effects can both be characterized dynamically via the propagation speed of the evolved initial state. Our investigation further supplies the contents on a topological semismetal phase in the non-Hermitian two-leg ladder model and may provide a possible tool to visualize the phase transition of the topological semimetal phase.

Weyl semimetals combine topological and semimetallic effects, making them candidates for interesting and effective thermoelectric transport properties. Here, we present experimental results on polycrystalline NbP, demonstrating the simultaneous existence of a large Nernst effect and a large magneto-Seebeck effect, which is typically not observed in a single material at the same temperature. We compare transport results from two polycrystalline samples of NbP with previously published work, observing a shift in the temperature at which the maximum Nernst and magneto-Seebeck thermopowers occur, while still maintaining thermopowers of similar magnitude. Theoretical modeling shows how doping strongly alters both the Seebeck and Nernst magnetothermopowers by shifting the temperature-dependent chemical potential, and the corresponding calculations provide a consistent interpretation of our results. Thus, we offer doping as a tuning mechanism for shifting magnetothermoelectric effects to temperatures appropriate for device applications, improving $zT$ at desirable operating temperatures. Furthermore, the simultaneous presence of both a large Nernst and magneto-Seebeck thermopower is uncommon and offers unique device advantages if the thermopowers are used additively. Here, we also propose a unique thermoelectric device that would collectively harness the large Nernst and magneto-Seebeck thermopowers to greatly enhance the output and $zT$ of conventional thermoelectric devices.