Enabling on-demand control of heat flow is key for the development of next-generation electronic devices, solid-state heat pumps, and thermal logic. However, precise and agile tuning of the relevant microscopic material parameters for adjusting thermal conductivities remains elusive. Here, we study several single crystals of lanthanum aluminate ($\mathrm{LaAl}{\mathrm{O}}_3$) with different domain structures and show that ferroelastic domain walls behave as boundaries that act like efficient controllers to govern thermal conductivity. At low temperature (3 K), we demonstrate a fivefold reduction in thermal conductivity induced by domain walls orthogonal to the heat flow and a twofold reduction when they are parallel to the heat flow. Atomistic calculations fully support this experimental observation. By breaking down phonon scattering mechanisms, we also analyze the temperature dependence of the thermal conductivity to derive a quantitative relation between thermal conductivity variations and domain wall organization and density.

The skyrmion core, percolating the volume of the magnet, forms a skyrmion string—a topological Dirac-string-like object. Here we analyze the nonlinear dynamics of a skyrmion string in a low-energy regime by means of the collective variables approach, which we generalized for the case of strings. Using the perturbative method of multiple scales (both in space and time), we show that the weakly nonlinear dynamics of the translational mode propagating along the string is captured by the focusing-type nonlinear Schrödinger equation. As a result, the basic “planar-wave” solution, which has the form of a helix-shaped wave, experiences modulational instability. The latter leads to the formation of cnoidal waves. Both types of cnoidal waves, dn- and cn-waves, as well as the separatrix soliton solution, are confirmed by micromagnetic simulations. Beyond the class of traveling-wave solutions, we found Ma-breather propagating along the string. Finally, we proposed a generalized approach that enables one to describe the nonlinear dynamics of the modes of different symmetries, e.g., radially symmetrical or elliptical.

The interplay between charge order (CO) and nontrivial band topology has spurred tremendous interest in understanding topological excitations beyond the single-particle description. In a quasi-one-dimensional nonsymmorphic crystal ${\mathrm{TaTe}}_4$, the ($2\mathrm{a}\times 2\mathrm{b}\times 3\mathrm{c}$) charge ordered ground state drives the system into a space group where the symmetry indicators feature the emergence of Dirac fermions and unconventional double Dirac fermions. Using angle-resolved photoemission spectroscopy and first-principles calculations, we provide evidence of the CO induced Dirac fermion-related bands near the Fermi level. Furthermore, the band folding at the Fermi level is compatible with the new periodicity dictated by the CO, indicating that the electrons near the Fermi level follow the crystalline symmetries needed to host double Dirac fermions in this system.

Strain engineering provides a powerful means to tune the properties of two-dimensional materials. Accordingly, numerous studies have investigated the effect of bi- and uniaxial strain. Yet, the strain fields in many systems such as nanotubes and nanoscale wrinkles are intrinsically inhomogeneous and the consequences of this symmetry breaking are much less studied. Understanding how this affects the electronic properties is crucial, especially since wrinkling is a powerful method to apply strain to two-dimensional materials in a controlled manner. In this paper, we employ density functional theory to understand the correlation between the atomic and the electronic structure in nanoscale wrinkles and nanotubes of the prototypical transition metal dichalcogenide ${\mathrm{WSe}}_2$. Our research shows that the symmetry breaking in these structures leads to strong Rashba-like splitting of the bands at the $\mathrm{\Gamma }$ point and they thus may be utilized in future tunable spintronic devices. The inhomogeneous strain reduces the band gap and leads to a localization of the band edges in the highest-curvature region, thus funneling excitons there. Moreover, we show how wrinkles can be modeled as nanotubes with the same curvature and when this comparison breaks down and further inhomogeneities have to be taken into account.

The potential applications of two-dimensional (2D) molybdenum disulfide ($\mathrm{Mo}{\mathrm{S}}_2$) in ultrathin digital logic circuits have stimulated intensive research. When 2D $\mathrm{Mo}{\mathrm{S}}_2$ connects to external circuits to form field-effect transistors based on complementary metal-oxide-semiconductor technology, transport of both electrons ($n$ type) and holes ($p$ type) is necessary. However, the $p$-type ohmic contact to $\mathrm{Mo}{\mathrm{S}}_2$ remains challenging. In the current work, based on first-principles calculations, we propose metal surface oxidation, serving as a general route for the regulation of the Schottky barrier from $n$ to $p$ type and the suppression of Fermi-level pinning simultaneously at metal-$\mathrm{Mo}{\mathrm{S}}_2$ interfaces. By introducing an ultrathin insulating layer, the oxidized metal surface layer has the following four advantages: (1) Surface oxidation greatly increases the work function of the metal surface, which is crucial for the realization of a $p$-type contact. (2) Surface oxidation passivates the metal surface and hence suppresses the metal-induced gap states in $\mathrm{Mo}{\mathrm{S}}_2$. (3) Due to the out of plane $p_z$-orbital coupling between oxygen and sulfur, significant quasibonding-induced gap states appear above the valence-band maximum of $\mathrm{Mo}{\mathrm{S}}_2$, which is beneficial to the reduction of $p$-type Schottky barrier height. (4) Surface oxidation is easily achieved for common transitional precious metal substrates, which ensures that it is a viable way to achieve $p$-type ohmic contacts in metal-$\mathrm{Mo}{\mathrm{S}}_2$ heterojunctions. Hence, this study suggests that metal surface oxidation is a general route to $p$-type ohmic contacts for $\mathrm{Mo}{\mathrm{S}}_2$ and other 2D semiconductors with out of plane orbitals in the valence band.

We present the results of theoretical studies of the Purcell effect in infinite photonic crystal slabs without defects or cavities. First, we describe a theoretical model for calculating total and external Purcell factors in two-dimensional photonic crystal slabs in terms of dipole's emissivity to the near field and the far field. Then we apply this theory to silicon photonic crystal slabs with triangular lattice on silica substrate and study how the Purcell factor depends on the wavelength and the dipole's position. We show that by placing the dipoles in the hot spots of modes with the zero group velocity, one can greatly enhance the Purcell factor in comparison to a homogeneous silicon slab. We demonstrate that this effect is associated with Van Hove singularities. We also calculate partial contributions to the total Purcell factor from different energy dissipation channels.

The entanglement entropy (EE) of the ground state of a one-dimensional Hamiltonian at criticality has a universal logarithmic scaling with a prefactor given by the central charge $c$ of the underlying $(1+1)$-dimensional conformal field theory. When the system is probed by measurements, the entanglement in the critical ground state is inevitably affected due to wave-function collapse. In this paper, we study the effect of weak measurements on the entanglement scaling in the ground state of the one-dimensional critical transverse-field Ising model. For the measurements of the spins along their transverse spin axis, we identify interesting postmeasurement states associated with spatially uniform measurement outcomes. The EE in these states still satisfies the logarithmic scaling but with an alternative prefactor given by the effective central charge $c_{\mathrm{eff}}$. We derive the analytical expression of $c_{\mathrm{eff}}$ as a function of the measurement strength. With both numerical simulations and analytical studies, we show that for the EE averaged over all postmeasurement states based on their Born-rule probabilities, the effective central charge is independent of the measurement strength, contrary to the usual expectation that local and nonoverlapping measurements reduce the entanglement in the system. We also examine the behavior of the average EE under (biased) forced measurements where the measurement outcomes are sampled with a predetermined probability distribution without intersite correlations. In particular, we find an optimal probability distribution that can serve as a mean-field approximation to the Born-rule probabilities and lead to the same $c_{\mathrm{eff}}$ behavior. The effects of the measurements along the longitudinal spin axis and the postmeasurement correlation functions are also discussed.

Quantum anomalous Hall insulators provide an intriguing platform to study emergent magnetic topological phenomena, but the low critical temperature is a weighty obstacle for practical applications. We theoretically propose that the monolayer $A\mathrm{Ti}X$ family (KTiSb, KTiBi, RbTiSb, SrTiSn) are potential candidates for large-gap quantum anomalous Hall insulators with high Chern number $\mathcal{C}=2$. Both the topology and the magnetism in these materials are from $3d$ orbitals of Ti. We further construct the tight-binding model from orbital projected band structure and symmetry analysis to reveal the origin of topology. Remarkably, quite different from the conventional $s\text{−}d$ band inversion, here the topological band inversion within $3d$ orbitals is due to the crystal field and electron hopping, while spin-orbit coupling only trivially gaps out the Dirac cone at Fermi level. The general physics from the $3d$ orbitals here applies to a large class of transition metal compounds with the space group $P4/nmm$ or $P\text{−}42m$ and their subgroups. These notable predictions, if realized experimentally, could greatly promote the research and application of topological quantum physics.

We take advantage of the polariton bistability in semiconductor microcavities to estimate the interaction strength between lower exciton-polariton and dark exciton states. We combine the quasiresonant excitation of polaritons and the nominally forbidden two-photon excitation (TPE) of dark excitons in a GaAs microcavity. To this end, we use an ultranarrow linewidth cw laser for the TPE process that allows us to determine the energy of dark excitons with high spectral resolution. Our results evidence a sharp drop in the polariton transmission intensity and width of the hysteresis cycle when the TPE process is resonant with the dark exciton energy, highly compromising the bistability of the polariton condensate. This behavior demonstrates the existence of a small symmetry breaking such as that produced by an effective in-plane magnetic field, allowing us to directly excite the dark reservoir. We numerically reproduce the collapse of the hysteresis cycle with the increasing dark exciton population, treating the evolution of a polariton condensate in a one-mode approximation, coupled to the exciton reservoir via polariton-exciton scattering processes.

We study the dynamics of the one-dimensional Ising model in the presence of a static symmetry-breaking boundary field via the two-time autocorrelation function of the boundary spin. We find oscillatory correlations that decay as power laws. We uncover a phase diagram of dynamical responses where, upon tuning the strength of the boundary field, we observe distinct power laws that directly correspond to changes in the number of edge modes as the boundary and bulk magnetic field are varied. We suggest how the universal physics can be demonstrated in current experimental setups, such as Rydberg chains.

We present a micromagnetic theory of compact magnetic skyrmions under applied magnetic field that accounts for the full dipolar energy and the interfacial Dzyaloshinskii-Moryia interaction (DMI) in the thin-film regime. Asymptotic analysis is used to derive analytical formulas for the parametric dependence of the skyrmion size and rotation angle, as well as the energy barriers for collapse and bursting, two processes that lead to a finite skyrmion lifetime. We demonstrate the existence of a regime at low DMI, in which the skyrmion is stabilized by a combination of nonlocal dipolar interaction and a magnetic field applied parallel to its core, and discuss the conditions for an experimental realization of such field-stabilized skyrmions.

In this paper, we demonstrate the ultrafast reduction of conductivity in an incommensurate crystal structure within hundreds of femtoseconds. This phenomenon stands in stark contrast to most prior experimental investigations where incident light pulses led to increased conductivity. We achieve this by selectively targeting a specific atomic bond using near-infrared light pulses. Our investigation focuses on misfit layered chalcogenide (${\mathrm{LaS})}_{1.196}{\mathrm{VS}}_2$, known as ${\text{LaVS}}_3$, a semimetal with incommensurability along one crystallographic direction. Our time-resolved electron dynamics investigation reveals that the conductivity decreases as photoexcited electrons are promoted into localized energy states within vanadium clusters due to the incommensurate structure. These findings offer insights into the potential for controlling electronic properties at femtosecond time scales, with implications for the development of ultrafast electronic devices.

In this work we present a theoretical investigation into an antiferromagnetically coupled spin system, specifically, ${\mathrm{Cu}}_3\text{−}X$ $(X=\mathrm{As},\mathrm{Sb})$, which exhibits an isosceles triangular configuration or slightly distorted equilateral triangular configuration, as previously identified by Choi et al. [Phys. Rev. Lett. 96, 107202 (2006)]. This system can be effectively represented by the Heisenberg model on a triangular structure, taking into account the exchange interaction, the Dzyaloshinskii-Moriya interaction, $g$ factors, and external magnetic field, as delineated in the previous work. By using a numerical approach we explore both zero-temperature and finite-temperature behaviors of a ${\mathrm{Cu}}_3$-type antiferromagnetically coupled spin system. At zero temperature, the system displays a 1/3 quasiplateau magnetization when the magnetic field is varied. Moreover, we place particular emphasis on the magnetic properties, including magnetization, magnetic susceptibility, entropy, and specific heat at finite temperatures. Furthermore, we investigate the magnetocaloric effect as a function of an externally imposed magnetic field, oriented both parallel and perpendicular to the plane of the triangular structure. Interestingly, these configurations demonstrate remarkably similar behavior for both orientations of the magnetic field. Our investigation also includes an analysis of the adiabatic curve, the Grüneisen parameter, and the variation in entropy when magnetic field is applied or removed. The magnetocaloric effect is found to be more prominent in low the temperature region, typically at $T\sim 1\mathrm{K}$, for both parallel and perpendicular magnetic fields at $\sim 4.5$ and $\sim 5$ T, respectively.

We carried out numerical simulations of propagation of spin waves (magnons in quantum language) in an yttrium-iron garnet film. The numerical model is based on an original formalism. We demonstrated that a potential barrier for magnons, created by an Oersted field of a dc current flowing through a wire sitting on top of the film, can act as an electrically controlled partly transparent mirror for the magnons. We found that the mirror transparency can be set to 50% by properly adjusting the current strength, thus creating a semitransparent mirror. A strong Hong-Ou-Mandel effect for single magnons is expected in this configuration. The effect must be seen as two single magnons, launched simultaneously into the film from two transducers located from the opposite sides of the mirror, creating a two-microwave-photon state at the output port of one of the transducers. The probability of seeing those two-photon states at the output port of either transducer must be the same for both transducers.

We study indirect exchange interactions between localized spins of magnetic impurities in spin valley coupled systems described with the Kane-Mele model. Our model captures the main ingredients of the energy bands of the 1H transition metal dichalcogenide (TMD) monolayers, such as $1H\text{−}{\mathrm{MoS}}_2$ and $1H\text{−}{\mathrm{NbSe}}_2$. To obtain the effective interactions, we use the exact diagonalization of the Hamiltonian, avoiding momentum cutoffs. We start by comparing the standard perturbation expansion in terms of the Kondo exchange with the exact calculation of the interaction, treating the local spins classically. We find that perturbation theory works well even beyond the regime where the relevant figure of merit, the ratio between the exchange $J$ and the hopping $t$, is small. We verify that the effective indirect exchange Hamiltonian derived from perturbation theory also works in the nonperturbative regime. Additionally, we analyze the interplay between the symmetry of the different terms of the interaction (Heisenberg, Ising, and Dzyaloshinskii-Moriya), the Fermi-surface topology, and the crystallographic direction in which the impurities are placed. We show that the indirect exchange along the armchair direction is actually Heisenberg-like, due to the reflection symmetry of the crystal structure around this direction. Finally, we explore the exploitation of indirect exchange, combined with atomic manipulation, to engineer the Majumdar-Ghosh model. Our results show that TMDs provide an extremely versatile platform to engineer indirect exchange interactions.

The Landau-Lifshitz-Gilbert (LLG) equation is widely used to describe magnetization dynamics. We develop a unified framework of the microscopic LLG equation based on the nonequilibrium Green's function formalism. We present a unified treatment for expressing the microscopic LLG equation in several limiting cases, including the adiabatic, inertial, and nonadiabatic limits with respect to the precession frequency for a magnetization with fixed magnitude, as well as the spatial adiabatic limit for the magnetization with slow variation in both its magnitude and direction. The coefficients of those terms in the microscopic LLG equation are explicitly expressed in terms of nonequilibrium Green's functions. As a concrete example, this microscopic theory is applied to simulate the dynamics of a magnetic skyrmion driven by quantum parametric pumping. Our work provides a practical formalism of the microscopic LLG equation for exploring magnetization dynamics.