The $2M$-phase transition metal dichalcogenides have recently attracted intensive research interest due to their rich topological and superconducting phase diagrams. Apart from the topological surface states of $2M\text{−}\mathrm{W}{\mathrm{S}}_2$ near $\mathrm{\Gamma }$ that originated from the strong topological order, using angle-resolved photoemission spectroscopy, we discover additional Rashba-split states on the surfaces of both $2M\text{−}\mathrm{W}{\mathrm{S}}_2$ and $2M\text{−}{\mathrm{WSe}}_2$, which extend in large momentum-energy regions. First-principles calculations well reproduce these states and attribute them to the weak topological orders. The calculations further indicate that the surface state connecting configurations are tunable under moderate pressure, suggesting that $2M\text{−}\mathrm{W}{\mathrm{S}}_2$ and ${\mathrm{WSe}}_2$ are promising platforms to study topological phase transition and explore topological superconductivity.

We have analyzed the optical spectra and the Goos-Hänchen shift (GHS) for a semiconductor slab having an embedded quantum well and being subjected to a sufficiently strong external dc magnetic field near magnetoexciton resonances. We consider two setup configurations, which satisfy the conditions for observing the breaking of total internal reflection. Specifically, in the first setup we have considered only two prisms covering the semiconductor slab, whereas in the second one there are two low-refractive-index dielectric layers separating two prisms from the semiconductor slab. The reflectivity and transmissivity spectra, as well as their corresponding GHS, were obtained to show that the electromagnetic eigenmodes localized on the semiconductor slab are coupled to the confined magnetoexcitons. The coupling strength is significantly different for two considered setups. Remarkably, in the second configuration, as a result of the strong coupling between magnetoexcitons and localized modes, an enhancement of the GHS, as huge as 100 times the wavelength, is predicted.

Magnetic Weyl semimetals (MWSM) that break time-reversal symmetry provide a unique platform for exploring the interplay between magnetism, electron correlations, and topological orders. However, unambiguously identifying a material as a MWSM remains challenging. In this study, we propose a nonreciprocal thermoelectric transport method for the detection of MWSM. By manipulating Fermi arc surface states, we induce asymmetry between the bands on opposite surfaces, leading to a nonlinear response and the emergence of the unidirectional Seebeck effect. Our theoretical calculations of the thermoelectric coefficients for a minimal MWSM model reveal that the nonreciprocal signals are directly influenced by the Fermi arc discrepancy, making them promising transport measurements for identifying MWSM and probing the Fermi arc structures after surface state manipulation.

We study the electronic band structure and optical response of a hybrid model, a $\alpha \text{−}{\mathcal{T}}_3$ model featuring a $\sqrt{3}\times \sqrt{3}$ Kekulé pattern modulation. Such a hybrid system may result from the depositing of adatoms in a hexagonal lattice, where the two sublattices are displaced in the perpendicular direction, like in germanene and silicene. We derive analytical expressions for the energy dispersion and the eigenfunctions using a tight-binding approximation of nearest-neighbor hopping electrons. The energy spectrum consists of a double-cone structure with Dirac points at zero momentum caused by Brillouin zone folding and a doubly degenerate flat band resulting from destructive quantum interference effects. Furthermore, we study the spectrum of intraband and interband transitions through the joint density of states, the optical conductivity, and the Drude spectral weight. We find that the optical conductivity exhibits steplike features, reminiscent of a 5/2-pseudospin Dirac semimetal, which include a characteristic and tunable absorption window arising from the intervalley response. These results might serve as a viable signature for detecting Kekulé periodicity in two-dimensional materials.

The paper presents a method for the simulation of acoustic and electromagnetic scattering from arbitrary-shaped particles. The method is based on an analytical scale separation of the scatterer and far-field regions for the Green's functions. The scattered field in the far zone is found through a specific volume integral of the scattered field inside the scatterer volume only. The main idea of the developed numerical procedure lies in the approach to the near-to-far-field transformation, while a required field inside a scatterer volume can be evaluated by any available method, e.g., the finite-element method, which is used in the work. A comparison of the numerical results obtained with the developed method and with the Mie and $T$-matrix solutions shows that a good and controllable accuracy can be achieved for both resonant metallic and dielectric scatterers.

When the coupling of a quantum system to its environment is nonnegligible, its steady state is known to deviate from the textbook Gibbs state. The Bloch-Redfield quantum master equation, one of the most widely adopted equations for solving the open quantum dynamics, cannot predict all the deviations of the steady state of a quantum system from the Gibbs state. In this paper, for a generic spin-boson model, we use a higher-order quantum master equation (in system-environment coupling strength) to analytically calculate all the deviations of the steady state of the quantum system up to second order in coupling strength. We also show that this steady state is exactly identical to the corresponding generalized Gibbs state, the so-called quantum mean force Gibbs state, at arbitrary temperature. All these calculations are highly general, making them immediately applicable to a wide class of systems well modeled by the spin-Boson model, spanning a diverse range of topics, from nanomaterials to various condensed-phase processes, and quantum computing (e.g., environment-induced corrections to the steady state of a superconducting qubit). As an example, we use our results to study the dynamics and the steady state of a solid-state double-quantum-dot system under physically relevant choices of parameters.

In this paper, we study a non-Hermitian (NH) ladder subjected to a variety of driving protocols. The driven system looses chiral symmetry (CS), whose presence is indispensable for its topological characterization. Further, the bulk boundary correspondence (BBC) gets adversely affected due to the presence of the non-Hermitian skin effect (NHSE). Here, we present a formalism that retrieves the lost CS and subsequently restores the BBC via the construction of a generalized Brillouin zone (GBZ). Specifically, we employ delta and step drives to compare and contrast between them with regard to their impact on NHSE. Further, a widely studied harmonic drive is invoked in this context, not only for the sake of completeness, but its distinct computational framework offers valuable insights on the properties of out-of-equilibrium systems. While the delta and harmonic drives exhibit unidirectional skin effect in the system, the step drive may show a bidirectional skin effect. Also, there are specific points in the parameter space that are devoid of skin effect. These act as critical points that distinguish the skin modes to be localized at one boundary or the other. Moreover, for the computation of the non-Bloch invariants, we employ GBZ via a pair of symmetric time frames corresponding to the delta and step drives, while a high-frequency expansion was carried out to deal with the harmonic drive. Finally, we present phase boundary diagrams that demarcate distinct NH phases obtained via tracking the trajectories of the exceptional points. These diagrams demonstrate a coexistence of the zero and $\pi$ energy modes in the strong NH limit and thus may be relevant for studies of Floquet time crystals.

The discovery of high-$T_{\text{c}}$ superconductivity at FeSe/${\mathrm{SrTiO}}_3$ interface under ambient pressure has inspired great interest in exploring interfacial superconductivity in Fe-based systems. While most high-$T_{\text{c}}$ FeSe superconducting systems are electron-doped, the hole-doped counterparts are much more rare, let alone the effective tuning of the hole doping level. Herein, we have theoretically designed a tunable hole-doped FeSe interfacial system by epitaxially growing FeSe monolayer on a polar substrate made of NaCl bilayer on ${\mathrm{Cu}}_2\mathrm{N}$-terminated Cu bulk, in which the polar substrate has already been realized in recent experiment. Through first-principles electronic structure calculations, we find that the external electric field can effectively modulate the magnetic properties of FeSe monolayer, converting it from a stripe antiferromagnetic (AFM) state to a dimer AFM state when the applied field is larger than $\sim$ 0.24 V/Å. This conversion indicates that the magnetic fluctuations in FeSe on polar NaCl substrate can be controlled by the gate electric field. Furthermore, due to the work function difference, the polar NaCl substrate could dope holes to the FeSe monolayer while the doping concentration of 0.02 $\sim$ 0.04 holes/Fe can be tuned when the electric field ranges from -0.4 V/Å to 0.4 V/Å. Our findings suggest that the FeSe monolayer on the polar substrate made of NaCl bilayer on ${\mathrm{Cu}}_2\mathrm{N}$/Cu(001) is an ideal platform to manipulate the magnetism and doping effect of FeSe via the gate electric field, therefore possibly tuning the FeSe-derived interfacial superconductivity.

Kagome metals present a fascinating platform of quantum phases thanks to the interplay between the geometric frustration and strong electron correlation. Here, we propose the emergence of the electric odd-parity bond order (BO) that originates from the intra-unit-cell odd-parity configuration in recently discovered kagome metal ${\mathrm{CsTi}}_3{\mathrm{Bi}}_5$. The predicted $E_{1u}$ BO is induced by the beyond-mean-field mechanism, that is, the quantum interference among different sublattice spin fluctuations. Importantly, the accompanied nematic deformation of the Fermi surface is just $\sim 1$%, while the intensity of the quasiparticle interference signal exhibits drastic nematic anisotropy, consistent with the scanning tunneling microscope measurements in ${\mathrm{CsTi}}_3{\mathrm{Bi}}_5$. The present odd-parity BO triggers interesting phenomena, such as the nonlinear Hall effect and emergent electromagnetism.

Low-dimensional systems based on black phosphorus (BP) have recently attracted much interest. Since the crystal structure of BP consists of 2D layers with a strong in-plane anisotropy, its electronic properties at a high magnetic field ($B$) are quite interesting. Here, we report a magneto-optical study of bulk BP at high $B$ to 12 T perpendicular to the 2D layers. In the obtained optical conductivity spectra, periodic peaks are clearly observed corresponding to Landau levels of up to $n=6$ quantum number. They exhibit almost linear shifts with $B$, and from their analysis an electron-hole reduced mass of 0.13 $m_0$ is obtained, where $m_0$ is the electron mass. Many of the peaks appear in pairs, which is interpreted in terms of Zeeman splitting and an electron-hole combined $g$-factor of 6.3 is obtained. In addition, a magnetoexciton peak is observed to shift quadratically with $B$, and the exciton binding energy at $B=0$ is estimated to be 9.7 meV.

Interest in the two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) continues to intensify, driven by their suitable band gaps to supplant silicon as next-generation semiconductor materials. Among various TMDs, tungsten diselenide ($\mathrm{WS}{\mathrm{e}}_2$) is renowned for its superior electrical properties in carrier density and mobility under ambient conditions. Despite its notable attributes, the behavior of monolayer $\mathrm{WS}{\mathrm{e}}_2$ in the electron-doped regime under cryogenic conditions remains largely uncharted, particularly concerning its magnetotransport properties. In this study, we reveal the transport mechanisms of monolayer $\mathrm{WS}{\mathrm{e}}_2$ from high temperatures down to the cryogenic regime. As evident by Efros–Shklovskii variable-range hopping (E-S VRH) in the cryogenic regime, strong Coulomb interactions arise between electrons. Above 8 K, an uncommon nonsaturated quadratic large magnetoresistance (MR) can be explained by the wave-function shrinkage model, which is consistent with the E-S VRH transport mechanism. Notably, the nonsaturated quadratic large MR shows a magnitude up to 1740% at 13 T. These findings underscore the potential applications for monolayer $\mathrm{WS}{\mathrm{e}}_2$ in cryogenic field-effect devices, magnetic sensors, and memory devices and mark a significant advance in magnetotransport research.

The development of van der Waals magnetic semiconductors has created a fascinating platform for exploring spin-dependent optoelectronic properties in the two-dimensional (2D) limit. However, the light absorbance in most 2D ferromagnetic (FM) semiconductors is extremely low, which strictly restricts the generation efficiency of spin-polarized currents. Using first-principles calculations and group theory analysis, we demonstrate that the light absorbance of FM monolayer MnNF exhibits pronounced anisotropy in light absorbance for light polarized along the $x$ and $y$ directions. Specifically, the lowest-energy optical absorption peaks are significantly higher than those of well-studied FM semiconductors such as monolayer ${\mathrm{CrI}}_3$ and CrSBr. The high absorbance primarily originates from transitions between the N-$p$ and Mn-$d$ orbitals, which can be explained using the group theory analysis. Notably, the photogenerated electrons and holes are fully spin polarized in the visible range, with absorbed photon flux comparable to that of the reported high-performance ultrathin solar cells using transition metal dichalcogenides. Furthermore, the direct absorption edge is dipole forbidden, indicating a long carrier recombination lifetime. Additionally, we show that the light absorbance can be effectively tuned by both strain and thickness. These findings highlight MnNF as a promising candidate for next-generation optospintronic devices.

${\mathrm{NbAl}}_3$ is a novel semimetal with a type-II Dirac node $\sim 230$ meV above the Fermi energy. We have performed both out-of-plane ($B\parallel c$) and in-plane magnetotransport measurements ($B\perp c$) on single-crystalline ${\mathrm{NbAl}}_3$. In our out-of-plane data, we observe an interesting linear component in the transverse magnetoresistance, and the mobility spectrum analysis of the out-of-plane data reveals an emergence of high-mobility electrons at low temperatures. Near $B\parallel c$, Shubnikov–de Haas oscillations are discerned in the magnetoresistance. The oscillation frequencies agree with the density functional theory calculation, the same theory that shows that the Dirac node is far above the Fermi energy. Therefore, the out-of-plane results cannot be attributed to the type-II Dirac node but suggest ${\mathrm{NbAl}}_3$ has additional Dirac or Weyl nodes close to the Fermi energy. To support this, we examine the in-plane data obtained with the magnetic field perpendicular to the tilting direction of the type-II Dirac cone. Such field direction excludes the possibility of chiral anomaly from the predicted type-II Dirac node. Remarkably, we observe the planar Hall effect, anisotropic magnetoresistance, and negative longitudinal magnetoresistance. These in-plane results are a strong indication of chiral anomaly unrelated to the previously established type-II Dirac node, pointing to the presence of additional Dirac or Weyl nodes near the Fermi energy. Our density functional theory calculation reveals a type-I Dirac node $\sim 50$ meV below the Fermi energy that has previously been overlooked. We argue that the exotic transport phenomena observed in ${\mathrm{NbAl}}_3$ can be attributed to this identified type-I Dirac node.

The wavelike nature of electrons is evident from quantum interference effects observed during the photoemission process. When there are different nuclei in the unit cell of a crystal and/or structural distortions, photoelectron wave functions can interfere, giving rise to a peculiar intensity modulation of the spectrum, which can also hide energy states in a photoemission experiment. The $2H$ phase of transition metal dichalcogenides, with two nonequivalent layers per unit cell and charge density wave distortion, is an optimal platform for such effects to be observed. Here, we discover undetectable states in $2H\text{−}{\mathrm{TaS}}_2$, interpreting high-resolution angular resolved photoemission spectroscopy considering interference effects of the correlated electron wave functions. In addition, phase mismatching induced by charge density wave distortion results in an evident signature of the phase transition in the photoemission spectrum. Our results highlight the importance of quantum interference, electronic correlations, and structural distortion to understand the physics of layered materials.

In Fe-based kagome magnets, $\mathrm{F}{\mathrm{e}}_3\mathrm{Sn}$ has been predicted to have the largest intrinsic anomalous Hall conductivity (AHC) and the highest Curie temperature, $T_{\mathrm{C}}=743$ K. However, the current experimental results show that the total AHC is much lower than the predicted value due to the strong extrinsic contribution. To suppress the extrinsic contribution and thus enhance AHC, we increased the Fe content in the stoichiometric $\mathrm{F}{\mathrm{e}}_3\mathrm{Sn}$. We found that the extrinsic contribution is significantly suppressed, and the intrinsic contribution is dominant and approaches the theoretically predicted value of $555\mathrm{S}/\mathrm{cm}$ for polycrystals over the entire temperature range. The scaling relationship shows that the skew scattering mainly provides the extrinsic contribution and compensates with intrinsic contribution. Based on the formula for the skew scattering, we analyzed why the skew scattering was suppressed and found that the spin-orbit coupling strength provided by the impurity center was the key. The spin-orbit coupling strength provided by Fe as an impurity center in this study is much smaller than that of Sn as an impurity center in previous studies. Therefore, the AHC in Fe-rich $\mathrm{F}{\mathrm{e}}_3\mathrm{Sn}$ obeys the unified theory, while the AHC in Sn-rich $\mathrm{F}{\mathrm{e}}_3\mathrm{Sn}$ deviates from the unified theory. Our study provides a promising solution for regulating the extrinsic contribution to the anomalous Hall effect in kagome magnets.

We investigate the magnetic characteristics and tunneling signatures of a planar Josephson junction with Rashba spin-orbit coupling during the fusion of two Majorana vortices. By employing the topological phase diagram and conducting tight-binding simulations of the proposed device, we demonstrate that this fusion process induces a parity-dependent magnetic moment aligned with the junction axis. We further propose a method to probe the spin properties of the fusing Majorana zero modes through spin-resolved Andreev conductance measurements at the junction endpoints. To support our findings, we derive a low-energy effective Hamiltonian that provides a detailed microscopic description of the numerically observed phenomena. Our analysis enables the detection of Majorana fusion outcome from accessible spin current measurements, thus paving the way for future experimental verification and potential applications in topological quantum computation.

Band singularities in the momentum space, such as Weyl points, play an important role in topological physics. Their nontrivial topological properties provide a platform to investigate various intriguing phenomena associated with wave propagation inside or at the surfaces of the medium. Here we show that the evanescent waves near the band singularities can host a variety of non-Hermitian behaviors, in the absence of material gain or loss. Different from commonly investigated parity-time ($\mathit{PT}$)–symmetric systems, one can manipulate the $\mathit{PT}$-symmetric phase of the evanescent waves via tuning the temporal growing/decaying behavior of the excitation rather than the gain/loss or coupling. Interestingly, the distribution of non-Hermitian Berry curvature for the evanescent waves around a Weyl point can be confined inside an exceptional cone in the momentum space, which leads to the transition of the winding of the reflection phase around the Weyl point. Our findings provide a different way of manipulating the $\mathit{PT}$-symmetric photonic system and insight into the topological behavior of band singularities.

Itinerant-electron metamagnetism is investigated within the Hubbard model for various lattices having van Hove singularities (vHS) in the electronic spectrum: face-centered cubic and orthorhombic lattices. The remarkable itinerant-electron metamagnetic transition occurs provided that the Fermi level is in the region with a strong positive curvature of the density of electron states typically positioned between two close van Hove singularities. Orthorhombic distortion of a tetragonal lattice is a promising mechanism for generating two closely split vHS with strong density-of-states curvature between them. A phase diagram in terms of electron filling and Hubbard interaction parameter is presented, which shows the paramagnetic-metamagnetic-ferromagnetic phase transition and regions of saturated and nonsaturated magnetism. The standard Landau theory expansion based on the electron density of states in the vicinity of the Fermi level is demonstrated to be insufficient to describe the whole magnetic phase diagram including the itinerant-electron metamagnetic transition.