We develop an accurate interlayer pairwise potential derived from the ab initio calculations and investigate the thermal transport of silicene bilayers within the framework of equilibrium molecular dynamics simulations. The electronic properties are found to be sensitive to the temperature with the opening of the band gap in the $\mathrm{\Gamma }\to M$ direction. The calculated phonon thermal conductivity of bilayer silicene is surprisingly higher than that of monolayer silicene, contrary to the trends reported for other classes of two-dimensional materials like graphene and hBN bilayers. This counterintuitive behavior of the bilayer silicene is attributed to the interlayer interaction effects and inherent buckling, which lead to a higher group velocity in the ${\mathrm{LA}}_1/{\mathrm{LA}}_2$ phonon modes. The thermal conductivity of both the mono- and bilayer silicene decreases with temperature as $\kappa \sim T^{-0.9}$ because of the strong correlations between the characteristic timescales of heat current autocorrelation function and temperature $(\tau \sim T^{-0.75})$. The mechanisms underlying phonon thermal transport in silicene bilayers are further established by analyzing the temperature induced changes in acoustic group velocity.

The calculation of optical properties of ${\mathrm{LiCoO}}_2$ presented in Phys. Rev. B 104, 115120 (2021) are revisited with improved accuracy calculations. An error in the polarization dependence in the previous work is pointed out and corrected here. The exciton binding energies are found to be somewhat smaller than previously reported—about 0.9 eV rather than 2 eV. The spatial extent of the excitons is analyzed here and shows interesting differences between dark and bright excitons. The effects of the local field effects are also clarified and found to be largest in the direction perpendicular to the layers.

Higher-order topological insulators (HOTIs) are a novel class of materials that exhibit exotic boundary states. The finite size effect induced hybridization between the boundary state HOTIs, however, remains largely unexplored. In this work, we analytically and numerically study the hybridization in films of three-dimensional chiral second-order topological insulators (SOTIs). We show that the gaps of two gapped surfaces (Chern insulators) are increased upon reducing the film thickness to a finite value. Meanwhile, in the presence of the hybridization of chiral edge states, we find that the gapless chiral hinge states (i.e., the boundary states of the surface Chern insulators) can be gapped, destroying the SOTIs. Moreover, we show that the gapless surfaces would in general be gapped due to the hybridization of surface states for a finite-sized system. Our work demonstrates that HOTIs exhibit a rich variety of physics under the influence of finite-size effects.

Vacancies are commonly introduced in the preparation of transition metal dichalcogenide (TMD) heterostructures, severely affecting photogenerated carrier dynamics. Herein, we systematically explore the carrier dynamics of TMD heterostructures (metal: Mo, W; chalcogen: S, Se, Te) with the most common chalcogen vacancies by nonadiabatic molecular dynamics simulations. For TMD monolayers, a chalcogen vacancy induces defect states around the conduction band minimum forming a recombination center, while in heterostructures, locations of defect states in different sublayers exhibit clear work function dependence. Specifically, the defect states of a sublayer with a high work function are pushed down and act as photogenerated carrier recombination centers, while those of a sublayer with a low work function are pushed up and defect tolerant. This is because the delocalized charge distribution weakens the overlap of photogenerated electrons and holes. This indicates that repairing the defects of sublayers with a high work function can eliminate the adverse effect of vacancies. As a further demonstration, oxygen passivation is employed to repair the defects of sublayers with a high work function, which fully recovers the 10 ns scale of the photogenerated carrier lifetime. In this paper, we provide an in-depth understanding of defective carrier dynamics and guidelines for high-performance TMD heterostructure photoelectric devices.

We formulate a general framework to study the flow of the electron liquid in two dimensions past a random array of impenetrable obstacles in the presence of a magnetic field. We derive a linear-response formula for the resistivity tensor $\widehat{\rho }$ in hydrodynamics with obstacles, which expresses $\widehat{\rho }$ in terms of the vorticity and its harmonic conjugate, both on the boundary of obstacles. In the limit of rare obstacles, in which we calculate $\widehat{\rho }$, the contributions of the flow-induced electric field to the dissipative resistivity from the area covered by the liquid and the area inside obstacles are shown to be equal to each other. We demonstrate that the averaged electric fields outside and inside obstacles are rotated by Hall viscosity from the direction of flow. For the diffusive boundary condition on the obstacles, this effect exactly cancels in $\widehat{\rho }$. By contrast, for the specular boundary condition, the total electric field is rotated by Hall viscosity, which means the emergence of a Hall-viscosity-induced effective—proportional to the obstacle density—magnetic field. Its effect on the Hall resistivity is particularly notable in that it leads to a deviation of the Hall constant from its universal value. We show that the applied magnetic field enhances hydrodynamic lubrication, giving rise to a strong negative magnetoresistance. We combine the hydrodynamic and electrostatic perspectives by discussing the distribution of charges that create the flow-induced electric field around obstacles. We provide a connection between the tensor $\widehat{\rho }$ and the disorder-averaged electric dipole induced by viscosity at the obstacle. This establishes a conceptual link between the resistivity in hydrodynamics with obstacles and the notion of the Landauer dipole. We show that the viscosity-induced dipole is rotated from the direction of flow by Hall viscosity.

A ferroelectric distorted (d) 1T (d1T)-phase characterized by out-of-plane (OOP) polarization was previously predicted in monolayer transition-metal dichalcogenides, such as ${\mathrm{MoS}}_2$. A phenomenological model was proposed to explain the centrosymmetric 1T (c1T)-to-d1T transition; however, this model fails to account for polarization reversal. In this work, we reassess the underlying physical origin of spontaneous OOP polarization in the 1T-phase of monolayer ${\mathrm{MoTe}}_2$. Through first-principles calculations and the incorporation of a phenomenological model approach utilizing an alternative set of basis states, we confirm that OOP polarization emerges due to its interaction with twofold degenerate, nonpolar primary order parameters. Additionally, we investigate the unique coupling between the polar mode and the primary mode. Notably, employing our newly defined basis sets, we clarify the hybrid improper ferroelectric mechanism, the coupling behavior of lattice modes, phase transitions, and polarization switching within a proposed subspace. Furthermore, our findings highlight that the ferroelectric phase transition can occur even in the absence of unstable phonon modes within the highly symmetric c1T-phase, attributed to the higher order coupling between lattice modes.

The multifunctional manipulation of magnetic topological textures such as skyrmions and bimerons in energy-efficient ways is of great importance for spintronic applications, but it is still a big challenge. Here, by first-principles calculations and atomistic simulations, the creation and annihilation of skyrmions/bimerons, as key operations for the reading and writing of information in spintronic devices, are achieved in a van der Waals magnetoelectric $\mathrm{CrISe}\text{/}{\mathrm{In}}_2{\mathrm{Se}}_3$ heterostructure via perpendicular strain or electric field without an external magnetic field. In addition, bimeron-skyrmion conversion, size modulation, and reversible magnetization switching from in plane to out of plane could also be realized in magnetic-field-free ways. Moreover, the topological charge and morphology can be precisely controlled by a small magnetic field. The strong Dzyaloshinskii-Moriya interaction and tunable magnetic anisotropy energy in a wide window are found to play vital roles in such energy-efficient multifunctional manipulation, and the underlying physical mechanisms are elucidated. Our work predicts the $\mathrm{CrISe}\text{/}{\mathrm{In}}_2{\mathrm{Se}}_3$ heterostructure to be an ideal platform to address this challenge in spintronic applications, and theoretically guides the low-dissipation multifunctional manipulation of magnetic topological textures.

Semiconductor artificial graphene nanostructures where the Hubbard model parameter $U/t$ can be of the order of 100, provide a highly controllable platform to study strongly correlated quantum many-particle phases. We use accurate variational and diffusion Monte Carlo methods to demonstrate a transition from antiferromagnetic to metallic phases for an experimentally accessible lattice constant $a=50$ nm in terms of lattice site radius $\rho$, for finite-sized artificial honeycomb structures nanopatterned on GaAs quantum wells containing up to 114 electrons. By analyzing spin-spin correlation functions for hexagonal flakes with armchair edges and triangular flakes with zigzag edges, we show that edge type, geometry, and charge nonuniformity affect the steepness and the crossover $\rho$ value of the phase transition. For triangular structures, the metal-insulator transition is accompanied with a smoother edge polarization transition.

Quasiparticles with Weyl dispersion can display an abundance of novel topological, thermodynamic, and transport phenomena, which is why novel Weyl materials and platforms for Weyl physics are being intensively looked for in electronic, magnetic, photonic, and acoustic systems. We demonstrate that conducting materials in magnetic fields generically host Weyl excitations due to the hybridization of phonons with helicons, collective neutral modes of electrons interacting with electromagnetic waves propagating in the material. Such Weyl excitations are, in general, created by the interactions of helicons with longitudinal acoustic phonons. An additional type of Weyl excitation in polar crystals comes from the interaction between helicons and longitudinal optical phonons. Such excitations can be detected in x-ray and Raman scattering experiments. The existence of the Weyl excitations involving optical phonons in the bulk of the materials also leads to the formation of topologically protected surface arc states that can be detected via surface plasmon resonance.

The recent interest in topological states localized to disclinations has highlighted the bulk-disclination correspondence, which links the reciprocal-space topology of a lattice to the emergence of such states. In this work, we extend such correspondence to the real-space topology. We construct a lattice with trivial reciprocal-space topology, in which the emergence of topological states and spectral flows at disclinations is induced by real-space topology. We experimentally verify the correspondence between real-space topology and disclination states in electrical circuit systems. Our findings offer insight into the real-space topology and enrich the family of topological states with their realization in topolectrical circuits.

The size of the Cooper pair ${\xi }_{\mathrm{pair}}$ is one of the basic characteristics of a superconductor, but it is not possible to measure it directly. It might be argued that ${\xi }_{\mathrm{pair}}$ can be determined from the value ${\xi }_{\mathrm{slope}}$ extracted from the measurement of the slope of $H_{c2}$ close to $T_c$. Taking into account both pair-conserving and pair-breaking scattering on impurities within the recently developed theory of Dynes superconductors, we perform an explicit calculation of ${\xi }_{\mathrm{pair}}$ and ${\xi }_{\mathrm{slope}}$. We show that the two quantities agree only in clean superconductors. In particular, when the pair-breaking disorder approaches the quantum critical point, ${\xi }_{\mathrm{pair}}$ stays finite, whereas ${\xi }_{\mathrm{slope}}$ diverges.

The geometric phase plays a key role both in the rotation of the Foucault pendulum and in the anomalous Hall effect (AHE), where an accelerated wave packet shows a transverse motion induced by the Berry curvature. Here, we show that the motion of quantum particles described by a spin-orbit-coupling Hamiltonian showing nonzero Berry curvature and placed in a two-dimensional harmonic-oscillator potential in real space exhibits Foucault precession. The plane of the oscillations rotates with time. The rotating pendulum configuration enhances the spatial deviation with respect to the AHE case, simplifying its observation and allowing high-precision measurements of the Berry curvature. We show how the nonadiabaticity and anharmonicity determine the maximal rotation angle and find the optimal conditions for the observations.

Dynamical buildup of spin-singlet correlations between the two quantum dots is investigated by means of the time-dependent numerical renormalization group method. By calculating the time evolution of the spin-spin expectation value upon a quench in the hopping between the quantum dots, we examine the timescales associated with the development of an entangled spin-singlet state in the system. Interestingly, we predict a nonmonotonic buildup of entanglement between the two dots. In particular, we find that in short timescales the effective exchange interaction between the quantum dots is of ferromagnetic type, favoring spin-triplet correlations, as opposed to the long-time limit, when strong antiferromagnetic correlations develop and eventually an entangled spin-singlet state is formed between the dots. We also numerically determine the relevant timescales and show that the physics is generally governed by the interplay between the Kondo correlations on each dot and exchange interaction between the spins of both quantum dots.

We have studied the superconducting properties of the isostructural ternary noncentrosymmetric superconductors $\mathrm{Ta}X\mathrm{Si}$ $(X=\mathrm{Re}, \mathrm{Ru})$ with the help of muon spin rotation/relaxation $\left(\mu \mathrm{SR}\right)$ and density functional theory calculations. Our transverse-field $\mu \mathrm{SR}$ measurements indicate isotropic $s$-wave superconductivity in TaReSi and multigap superconductivity in TaRuSi. Zero-field $\mu \mathrm{SR}$ measurements, highly sensitive to very small magnetic fields, find no evidence of spontaneous fields in the superconducting state of TaReSi, whereas we observe small spontaneous fields that onset with superconductivity indicating broken time-reversal symmetry (TRS) superconductivity in TaRuSi. Using density functional theory calculations, we find that spin-orbit coupling is relatively weak in TaRuSi and strong in TaReSi. Using symmetry analysis, we attribute the broken TRS in TaRuSi to a nonunitary triplet pairing state. Such a state is not allowed in the presence of strong spin-orbit coupling: our finding of no evidence for broken TRS in TaReSi is consistent with this expectation.

We use a combination of numerical matrix product states (MPS) and analytical approaches to investigate the phase diagram of the two-leg $t\text{−}J$ ladder in the region of low to intermediate fillings. We choose the same coupling strength along the leg and rung directions, but study the effect of adding a nearest-neighbor repulsion $V$. We observe a rich phase diagram and analytically identify a Lifshitz-type band-filling transition, which can be associated to a numerically observed crossover from $s$-wave- to $d$-wave-like superconducting quasi-long-range order. Due to the strong interactions, the Lifshitz transition is smeared into a crossover region which separates two distinct Luttinger theories with unequal physical meaning of the Luttinger parameter. Our numerically exact MPS results spotlight deviations from standard Luttinger theory in this crossover region and are consistent with Luttinger theory sufficiently far away from the Lifshitz transition. At very low fillings, studying the Friedel-type oscillations of the local density identifies a precursor region to a Wigner crystal at small values of the magnetic exchange interaction $J/t$. We discuss analytically how tuning parameters at these fillings modify the phase diagram, and find good agreement with MPS results.

We combine ab initio calculations based on many-body perturbation theory and the cumulant expansion with angle-resolved photoemission spectroscopy (ARPES) to quantify the electron-plasmon interaction in the highly doped semiconducting transition metal dichalcogenide $1T\text{−}{\mathrm{HfS}}_2$. ARPES reveals the emergence of satellite spectral features in the vicinity of quasiparticle excitations at the bottom of the conduction band, suggesting coupling to bosonic excitations with a characteristic energy of 200 meV. Our first-principles calculations of the photoemission spectral function reveal that these features can be ascribed to electronic coupling to carrier plasmons (doping-induced collective charge-density fluctuations). We further show that reduced screening at the surface enhances the electron-plasmon interaction and is primarily responsible for the emergence of plasmonic polarons.

One of the key features of graphene is the chirality of the electrons originating from the lattice symmetry. Conservation of the chirality leads to Klein and anti-Klein tunneling in graphene and AB-stacked bilayer graphene, respectively. Introduction of an external perturbation (electric field, strain, twist, layer mismatch, etc.) can change the chirality and therefore significantly modify the quantum transport. We explore the tunneling in AB-stacked bilayer graphene in the presence of an in-plane pseudomagnetic field (IPMF) and an out-of-plane electric field (OEF). Employing a low-energy effective model, we study the energy spectrum, chirality, and wave functions to evaluate the transmission and conductance in an $n\text{−}p\text{−}n$ junction. We demonstrate that the IPMF and OEF together induce an indirect band gap. The IMPF induces a layer polarization of propagating states (enhanced in the presence of the OEF) with the layer symmetry broken locally in a single valley but preserved globally. While the effect of the IPMF is negligible in the ballistic transport regime at the neutrality point, the OEF suppresses the ballistic transport (partially recovered in the presence of the IPMF). In the diffusive transport regime, the IPMF amplifies the Fabry-Pérot resonances, thus modifying the conductance, while the OEF induces an out-of-plane chirality component that enables Klein resonances.

We compute the perturbative expansion for the effective interaction $W$ of the half-filled 2-dimensional Hubbard model. We derive extensions of standard random-phase-approximation resummations that include arbitrarily high order contributions in the $W_{\uparrow \uparrow }$ and $W_{\uparrow \downarrow }$ basis. Using algorithmic tools we explore the static $Q$-dependent interaction as well as the same-time quantity both in momentum and real space. We emphasize the absence of screening in the Hubbard interaction where we find an enhanced repulsive local $W_{\uparrow \downarrow }$ with a nonzero attractive $W_{\uparrow \uparrow }$. Finally, starting from only a locally repulsive bare interaction we find an emergent nonlocal nearest-neighbor attraction for low temperatures at sufficiently large values of $U/t$ which may be key to understanding pairing processes in the model.