The electronic and magnetic structure of ${\mathrm{CrPS}}_4$, a two-dimensional (2D) magnetic semiconductor is examined by employing the SCAN meta-GGA density functional. We find the resulting magnetic moment and band gap are in excellent agreement with experiment. From the bulk magnetic configurations, we confirm the experimentally observed A-type antiferromagnetic (A-AFM) ordered ground state with a magnetic moment of $2.78{\mu }_{\mathrm{B}}$ per chromium atom and band gap of 1.34 eV. To gain insight into the evolution of the ground state with layers, the total energy of each magnetic configuration is calculated for a variety of thicknesses. Monolayer ${\mathrm{CrPS}}_4$ is predicted to be a ferromagnetic insulator with a band gap of 1.37 eV, and A-AFM for bilayer and trilayer, with band gaps of 1.35 and 1.30 eV, respectively. The electronic structure is reported for the single-, two-, and three-layer and bulk ${\mathrm{CrPS}}_4$. Finally, we explore the optical properties of the 2D structure and report the dielectric tensor components and Kerr parameters for the monolayer.

One-dimensional anyonic models of the Hubbard type show intriguing ground-state properties, effectively transmuting between Bose-Einstein and Fermi-Dirac statistics. The simplest model that one can investigate is an anyonic version of the bosonic Josephson junction, the repulsive anyon-Hubbard dimer. In the following we find an exact duality relation to the Bethe-solvable Bose-Hubbard dimer, which is well known from quantum optics and information theory and has interesting connections to spin squeezing and entangled coherent states. Conversely, we show that the anyonic Hubbard dimer has nontrivial coherence properties that emerge from the anyonic statistics. In particular, we find that coherences can be suppressed and amplified and show that these features are remarkably robust against additional repulsive on-site interactions highlighting the distinct nature of anyons.

The introduction of magnetism has profound impacts on the thermodynamics of the carriers and thus thermoelectric (TE) performance, broadening the scope of traditional TEs. Herein, it is demonstrated that the large magnetic entropy induced by the interplanar magnetic frustration of an antiferromagnetic kagome lattice significantly enhances the Seebeck coefficient $S$ in a broad temperature range, from around 50 K to above 380 K, which far exceeds those of other materials reported so far. Specifically, at 300 K and 9 T, the $S$ and power factor (PF) is increased by $9.5\mathrm{\mu }\mathrm{V}{\mathrm{K}}^{-1}$ and $57\%$, respectively. In addition, a phonon drag contribution to the $S$ is also observed around the phonon peak, which is confirmed by the fitting of thermal conductivity and heat capacity. Combining the contribution of both magnetic frustration and phonon drag, the maximum PF reaches $10.5\mathrm{\mu }\mathrm{W}\mathrm{c}{\mathrm{m}}^{-1}{\mathrm{K}}^{-2}$ and is comparable to those of state-of-the-art conventional TE materials. This work supports the enhancement of TE performance through the magnetic frustration of an antiferromagnetic kagome lattice and phonon drag, and also discovers a potential new TE material system.

Motivated by the discoveries on the two-dimensional (2D) ${\mathrm{MoSi}}_2{\mathrm{N}}_4$ family [Y. L. Hong et al., Science 369, 670 (2020)] that can be regarded as 2D transition-metal dinitrides ($M{\text{N}}_2$) attached by $\mathrm{Si}\text{−}\mathrm{N}$ layers on its two sides, in this work, we propose an emerging 2D $M{\text{SiN}}_3$ family, where only one side of $M{\text{N}}_2$ monolayer passivated via a $\mathrm{Si}\text{−}\mathrm{N}$ layer. Among them, $1T\text{−}{\mathrm{VSiN}}_3$ and $1T\text{−}{\mathrm{NbSiN}}_3$ monolayer are determined to be direct-gap semiconductors with a bandgap of 1.24 and 2.92 eV, respectively, calculated on the Heyd-Scuseria-Ernzerhof hybrid functional level. In addition, $1T\text{−}{\mathrm{VSiN}}_3$ and $1T\text{−}{\mathrm{NbSiN}}_3$ monolayer are nonmagnetic within its ground state, and can maintain the chemical, dynamical, thermal, as well as mechanical stability. In the framework of quantum many-body perturbation theory, $1T\text{−}{\mathrm{VSiN}}_3$ and $1T\text{−}{\mathrm{NbSiN}}_3$ monolayer display strong light-harvesting ability in visible-light region, and have a long carrier lifetime up to the nanosecond level, suggestive of a high light-to-current conversion efficiency. This finding broadens the 2D semiconductors with excellent optical features, and would trigger more interest in this emerging family of $M{\text{SiN}}_3$ materials.

Intercalation in two-dimensional materials is an important degree of freedom to modify the structural and electronic properties. In this paper, the atomic arrangements at the interface between single-layer graphene and Co(0001) with intercalated Ag and Au atoms were investigated using total-reflection high-energy positron diffraction. A structure analysis on the basis of dynamical diffraction theory demonstrates that by intercalation at annealing temperatures above $400{}^{\circ }\mathrm{C}$, the graphene height from the underlying atoms is shifted from 2.04 Å to 3.24 Å for Ag and 3.32 Å for Au. The Debye temperatures of graphene are changed from 430 K to 320 K for Ag and 368 K for Au by the intercalation. These changes indicate that intercalation results in the transformation to quasifreestanding graphene via a van der Waals interaction. For Ag, further annealing at $700{}^{\circ }\mathrm{C}$ enables the graphene height and the Debye temperature to restore the pristine values by Ag deintercalation. However, for Au, the graphene height remains high up to $900{}^{\circ }\mathrm{C}$, which indicates the absence of deintercalation. The obtained results reveal a reversible structure change in the graphene/Co(0001) interface by Ag intercalation and deintercalation. The difference in the deintercalation between Ag and Au atoms can be explained by the activation energy of thermal desorption.

Nanographene triangulenes with an $S=1$ ground state have been used as building blocks of topological antiferromagnetic Haldane spin chains. Using inelastic electron spectroscopy, it was found that the intermolecular exchange was described by the bilinear-biquadratic Hamiltonian. Starting from a Hubbard model, we analytically derive these effective spin interactions using perturbation theory, up to fourth order. For chains with more than two units, other interactions arise that entail second neighbor linear and three-site nonlinear exchange. We discuss the extension to general $S=1$ molecules and give numerical results for the strength of the nonlinear exchange for several nanographenes.

We propose a continuum model for the theoretical study of hybridized moiré excitons in transition-metal dichalcogenides heterobilayers, and we use a variational method to solve the exciton wave function and calculate the optical absorption spectrum. The exciton continuum model is built by the charge continuum model for electrons and holes in moiré superlattices, thereby preserving the moiré periodicity and lattice symmetry from the charge continuum model. The momentum-space shift of interlayer electron-hole distribution is included, and thus the indirect nature of interlayer excitons is described. The spin and valley degrees of freedom and related interactions are omitted, except for the spin-orbit energy splitting of A and B excitons. This continuum model is applied to the simulation of optical absorption by hybridized moiré excitons in ${\text{WSe}}_2/{\text{WS}}_2$ and ${\text{MoSe}}_2/{\text{WS}}_2$ heterobilayers. Twist-angle and electric-field dependencies of absorption spectra are studied. Calculated spectra are compared with experimental observations in the literature, and correspondences of signatures are found. The deficiency and the potential of the present model are discussed.

Graphene-based plasmonic devices are regarded to be suitable for a plethora of applications, ranging from midinfrared to terahertz frequencies. In this regard, among the peculiarities associated with graphene, it is well known that plasmons are tunable and tend to show stronger confinement as well as a longer lifetime than in the noble-metal counterpart. However, due to the two-dimensional specificity of graphene, the presence of defects might induce stronger effects than in bulky noble metals. Here we theoretically investigate the impact of structural defects hosted by graphene on selected figures of merit associated to localized plasmons, which are of key technological importance for plasmon-based molecular sensing. By considering an optimized graphene nanostructure, we provide a comparative analysis intended to shed light on the impact of the type of defect on graphene localized plasmons, that regards distinct types of defects commonly arising from fabrication procedures or exposure to radiation. This understanding will help industry and academia in better identifying the most suitable applications for graphene-based molecular sensing.

We study the appearance of topological Floquet flat bands in alternating-twist multilayer graphene, which has alternating relative twist angle $\pm \theta$ near the first magic angle. While the system hosts both flat bands and a steep Dirac cone in the static case, the circularly polarized laser beam can open a gap at the Moiré $K$ point and create Floquet flat bands carrying nonzero Chern numbers. Considering recent lattice-relaxation results, we find that the topological flat band is well-isolated for the effective interlayer tunneling in $n=3,4,5$ layers. Such dynamically produced topological flat bands are potentially observed in the experiment and thus provide a feasible way to realize the fractional Chern insulator.

The three-dimensional (3D) quantum Hall effect (QHE) in topological semimetals has attracted much interest in recent years. We study the 3D QHE in Weyl semimetals combining the Chern number calculated from Landau levels and the Hall conductivity calculated using the Kubo formula from the bulk-edge correspondence. We derive the Chern numbers under magnetic field using topological analysis. We get the magnetic field and Fermi energy dependence of the Hall conductivity according to the correspondence between the Chern number and Hall conductivity in a Weyl semimetal slab with the periodic boundary condition from the perspective of bulk states. We numerically calculate the Hall conductivity using the Kubo formula in a Weyl semimetal slab with the open boundary condition. The results of the Hall conductivity using the periodic boundary condition and open boundary condition are consistent. Our study demonstrates the 3D QHE in Weyl semimetals from both the bulk states and edge states through the bulk-edge correspondence.

A Monte Carlo study of a crystalline membrane described by the Nelson-Peliti Hamiltonian is presented. The membrane is studied for vanishing in-plane strain, as well as in the presence of tensile biaxial strain. The equilibrium simulations are performed in the classical limit as a function of temperature and system size, with elastic constants appropriate to the description of graphene. In the long-wavelength limit, the out-of-plane fluctuations of the unstrained membrane are described by a nonanalytic function with a critical exponent $\eta =0.8$, in agreement with previous results based on the Nelson-Peliti Hamiltonian. Isotherm curves in the stress-strain plane allow us to identify the region of thermodynamic stability of the flat membrane. The most striking result is that the flat unstrained membrane, as described by the Nelson-Peliti Hamiltonian, is thermodynamically unstable at any finite temperature. A critical tensile biaxial stress is needed to reach the region of thermodynamic stability of the flat membrane. The critical strain is a monotonically increasing function of temperature. At any given temperature, the critical strain corresponds to the state with a minimum value of the real area of the membrane.

${\mathrm{PbZrO}}_3$ and ${\mathrm{CaTiO}}_3$ appear a priori as similar ${\mathrm{ABO}}_3$ oxide perovskites with very close tolerance factors. Amazingly, they show, however, significantly distinct properties: while ${\mathrm{PbZrO}}_3$ is a prototypical antiferroelectric, ${\mathrm{CaTiO}}_3$ is not. Here, we investigate the microscopic origin of their different behaviors. From the comparison of the harmonic and anharmonic interatomic force constants, we highlight that the key differences are in their $A\text{−}\mathrm{O}$ interactions. Then, specifically tuning the harmonic and anharmonic coefficients of these interactions, we show that it is possible to make the energy landscape of ${\mathrm{CaTiO}}_3$ very similar to that of ${\mathrm{PbZrO}}_3$.

Quantum anomalies are the breakdowns of classical conservation laws that occur in a quantum-field theory description of a physical system. They appear in relativistic field theories of chiral fermions and are expected to lead to anomalous transport properties in Weyl semimetals. This includes a chiral anomaly, which is a violation of the chiral current conservation that takes place when a Weyl semimetal is subjected to parallel electric and magnetic fields. A charge pumping between Weyl points of opposite chirality causes the chiral magnetic effect that has been extensively studied with electrical transport. On the other hand, if the thermal gradient, instead of the electrical field, is applied along the magnetic field, then as a consequence of the gravitational (also called the thermal chiral) anomaly an energy pumping occurs within a pair of Weyl cones. As a result, this is expected to generate anomalous heat current contributing to the thermal conductivity. We report an increase of both the magnetoelectric and magnetothermal conductivities in the semiclassical regime of the magnetic Weyl semimetal NdAlSi. Our work also shows that the anomalous electric and heat currents, which occur due to the chiral magnetic effect and gravitational anomalies respectively, are still linked by a 170-year-old relation called the Wiedemann-Franz law.

Recent breakthroughs in topological Anderson insulators (TAIs) have revealed the counterintuitive possibility that sufficiently strong disorder can induce nontrivial topology from a trivial phase. Previous experimental research on TAIs has mainly focused on Chern-type and higher-order systems. However, the observation of spin-Chern-type TAI hosting disorder-induced spin-dependent boundary states remains unexplored. Here, we report on the experimental realization of a spin-Chern-type TAI in a two-dimensional bilayer phononic crystal. We directly observe evidence of TAI through disorder-induced pseudospin-dependent helical boundary modes from a trivial insulator and further demonstrate their robustness. By extending topological descriptions to disordered supercells and capturing the spin-Bott index, we confirm the topological Anderson phase transition. This work opens different perspectives for the realization of interesting topological phases in optics, circuits, and cold atom systems.

The Z method is a popular atomistic simulation method for determining the melting temperature where a sequence of microcanonical molecular dynamics runs are carried out to target the lowest system energy where the solid always melts. Homogeneous melting at the limit of critical superheating $T_{\text{h}}$ is accompanied by a drop in temperature as kinetic energy is converted to potential energy and the equilibrium melting temperature $T_{\text{m}}$ can be calculated directly from the liquid state. Implementation of the Z method interfaced with modern ab initio electronic structure packages often use Hellmann-Feynman forces to propagate the ions in the microcanonical ensemble where the Mermin free energy plus the ionic kinetic energy is conserved. The electronic temperature $T_{\text{el}}$ is therefore kept fixed along the trajectory which may introduce some spurious ion-electron interactions in molecular dynamics runs with large changes in temperatures such as often seen in homogeneous melting and freezing processes in the microcanonical ensemble. We estimate possible systematic errors in the calculated melting temperature to choice of $T_{\text{el}}$ for two main mantle components: the insulators ${\mathrm{SiO}}_2$ and ${\mathrm{CaSiO}}_3$ at high pressure. Comparison of the calculated melting temperature from runs where $T_{\text{el}}=T_{\text{h}}$ and $T_{\text{el}}=T_{\text{m}}$, representing reasonable upper and lower boundaries, respectively, to choice of $T_{\text{el}}$, shows that the difference in melting temperature is 200–300 K (3%–5% of the melting temperature) for our two test systems. Our results are in good agreement with previous large-size coexistence method and thermodynamic integration calculations, suggesting that ${\mathrm{CaSiO}}_3$ and ${\mathrm{SiO}}_2$ melt at around 6500 K (100 GPa) and 6000 K (160 GPa), respectively. The melting temperature decreases with increasing $T_{\text{el}}$ due to the increasing entropic stabilization of the liquid and the systems melt typically about three times faster in molecular dynamics runs with $T_{\text{el}}=T_{\text{h}}$ compared to runs where $T_{\text{el}}=T_{\text{m}}$. A careful choice of electron temperature in Born-Oppenheimer molecular dynamics simulations where the ions are propagated using Hellmann-Feynman forces with the Mermin free energy $+$ the ionic kinetic energy being conserved is therefore essential for the critical evaluation of the Z method and, in particular, at very high temperatures.

The tunneling process, a prototypical phenomenon of nonperturbative dynamics, is a natural consequence of photocarrier generation in materials irradiated by a strong laser. Common treatments for Zener tunneling are based on a one-body problem with a field-free electronic structure. In the literature [Ikemachi et al., Phys. Rev. A 98, 023415 (2018)], the characteristic of gap shrinking or excitation can occur due to the electron-hole interaction for slow and strong time-varying electric fields. We have developed a theoretical framework called the quasi-Hartree-Fock (qHF) method to enable a more flexible imitation of the electronic structures and electron-hole attraction strength of materials compared with the original Hartree-Fock method. In the qHF framework, the band gap, reduced effective mass, and electron-hole interaction strength can be independently selected to reproduce common crystals. In this paper, we investigate the effect of electron-hole attraction on Zener tunneling subjected to a DC electric field for four different systems using the qHF method. Our findings demonstrate that the electron-hole attraction promotes the tunneling rates in all four material systems assumed as examples. Specifically, systems that have a strong electron-hole interaction show a few factor enhancements for tunneling rates under DC fields, while systems with a weak interaction show higher enhancements of a few tens of percent.

Magnon frequency comb (MFC), the spin-wave spectra composing of equidistant coherent peaks, is attracting much attention in magnonics. A terahertz (THz) MFC, combining the advantages of the THz and MFC technologies, is highly desired because it would significantly advance the MFC applications in ultrafast magnonic metrology, sensing, and communications. Here, we show that the THz MFC can be generated by nonlinear interactions between spin waves and skyrmions in antiferromagnets [Z. Jin et al., Phys. Rev. Lett. 131, 166704 (2023)]. It is found that the strength of the three-wave mixing between propagating magnons and breathing skyrmions follows a linear dependence on the driving frequency and the MFC signal can be observed over a broad driving frequency range. Our results extend the working frequency of MFC to the THz regime, which would have potential applications in ultrafast spintronic devices and promote the development of nonlinear magnonics in antiferromagnets.

The Dzyaloshinskii-Moriya interaction (DMI) is known to be responsible for multiple phenomena in magnetic materials. In the conventional description as a perturbation of the superexchange interaction by the spin-orbit coupling, the strength of the DMI is only weakly sensitive to the external fields, making its control difficult in spintronic applications. In this work, we show that an electrical modulation of the DMI may actually be possible in magnetic Weyl semimetals (WSMs). Specifically, it is theoretically illustrated that an antisymmetric indirect spin-spin interaction identified recently as an alternative mechanism for the DMI can result in the desired sensitivity to the external electric and magnetic fields via a redistribution of Weyl fermions among nodes of opposite chirality. This chiral anomaly enabled approach becomes particularly prominent in WSMs with inversion symmetry, in which the conventional DMI is not allowed. Numerical estimations suggest that moderate electric and magnetic fields of $\sim {10}^3–{10}^4\mathrm{V}/\mathrm{cm}$ and $\sim 1$ T can induce a sufficiently strong change in the DMI. The impact of this externally modulated DMI on the manipulation of magnetic textures, including skyrmions in WSMs, is also discussed.

We theoretically study spin pumping from a ferromagnetic insulator (FI) into a carbon nanotube (CNT). By employing the bosonization method, we formulate the Gilbert damping induced by the FI/CNT junction, which can be measured by ferromagnetic resonance. We show that the increase in the Gilbert damping has a temperature dependence characteristic of a Luttinger liquid and is highly sensitive to the Luttinger parameter of the spin sector for a clean interface. We also discuss the experimental relevance of our findings based on numerical estimates, using realistic parameters.