We report an intermediate regime in the quench time, ${\tau }_q$, separating the usual validity of the Kibble-Zurek mechanism (KZM) and its breakdown for rapid quenches in open systems under finite quench protocols. It manifests in the power-law scaling of the transition time with ${\tau }_q$ as the system appears to enter the adiabatic regime, even though the ramp is already terminated and the final quench value is held constant. This intermediate regime, which we dub the delayed KZM, emerges due to the dissipation, preventing the system from freezing in the impulse regime. This results in a large delay between the actual time the system undergoes a phase transition and the time inferred from a threshold-based criterion for the order parameter, as done in most experiments. We demonstrate using the open Dicke model and its one-dimensional lattice version that this phenomenon is a generic feature of open systems that can be mapped onto an effective coupled oscillator model. We also show that the phenomenon becomes more prominent near criticality, and its effects on the transition time measurement can be further exacerbated by large threshold values for an order parameter. Due to this, we propose an alternative method for threshold-based criterion which uses the spatiotemporal information, such as the system's defect number, for identifying the transition time.

The spread of valence-band Wannier functions in semiconductors and insulators is a characteristic property that gives a rough estimation of how insulating is the material. We elaborate that the gauge-invariant part of the spread can be extracted experimentally from optical conductivity and absorbance, owing to their equivalence to the quantum metric of the valence band states integrated over momentum. Because the quantum metric enters the matrix element of optical conductivity, the spread of valence-band Wannier functions in the gapped 3D materials can be obtained from the frequency-integration of the imaginary part of the dielectric function. We demonstrate this practically for typical semiconductors like Si and Ge, and for topological insulators like ${\mathrm{Bi}}_2{\mathrm{Te}}_3$. In 2D materials, the spread of Wannier functions in the valence bands can be obtained from the absorbance divided by frequency and then integrated over frequency. Applying this method to graphene reveals a finite spread caused by intrinsic spin-orbit coupling, which may be detected by absorbance in the microwave range. The absorbance of twisted bilayer graphene in the millimeter wave range can be used to detect the formation of the flat bands and quantify their quantum metric. Finally, we apply our method to hexagonal transition metal dichalcogenides $M{X}_2$ ($M=\mathrm{Mo},\mathrm{W}$; $X=\mathrm{S},\mathrm{Se},\mathrm{Te}$) and demonstrate how other effects like excitons and higher energy bands can affect the spread of Wannier function.

The nonlinear Hall effect has recently attracted significant interest due to its potential as a promising spectral tool and for device applications. A theory of the nonlinear Hall effect on a disordered lattice is a crucial step towards explorations in realistic devices, but has not yet been addressed. We study the nonlinear Hall response on a lattice, which allows us to introduce strong disorder numerically. We reveal a disorder-induced Berry curvature that was not discovered in previous perturbation theories. The disorder-induced Berry curvature induces a fluctuation of the nonlinear Hall conductivity, which anomalously increases as the Fermi energy moves from the band edges to higher energies. More importantly, the fluctuation may explain those observations in recent experiments. We also find signatures of localization of the nonlinear Hall effect. This Letter shows a territory of the nonlinear Hall effect yet to be explored.

We investigate the near-field radiative heat transfer between a normally and/or laterally shifted nanoparticle and a planar fused silica slab coated with a strip graphene grating. For this study, we develop and use a scattering matrix approach derived from Fourier modal method augmented with local basis functions. We find that adding a graphene sheet coating on the slab can already enhance the heat flux by about 85%. We show that by patterning the graphene sheet coating into a grating, the heat flux is further increased, and this happens thanks to the a topological transition of the plasmonic modes from circular to hyperbolic one, which allows for more energy transfer. The lateral shift affects the accessible range of high-$k$ modes and thus affects the heat flux, too. By moving the nanoparticle laterally above the graphene grating, we can obtain an optimal heat flux with strong chemical potential dependence above the strips. For a fixed graphene grating period ($D=1\mu \mathrm{m}$) and not too large normal shift (separation $d<800\mathrm{nm}$), two different types of lateral shift effects (e.g., enhancement and inhibition) on heat transfer have been observed. As the separation $d$ is further increased, the lateral shift effect becomes less important. We show that the lateral shift effect is sensitive to the geometric factor $d/D$. Two distinct asymptotic regimes are proposed: (1) the inhibition regime ($d/D<0.85$), where the lateral shift reduces the heat transfer and (2) the neutral regime ($d/D\ge 0.85$) where the effect of the lateral shift is negligible. In general, we can say that the geometric factor $d/D\approx 0.85$ is a critical point for the lateral shift effect. Our predictions can have relevant implications to the radiative heat transfer and energy management at the nano/microscale.

In a topological system with an edge, the number of edge states is typically determined by the bulk topological invariant, as per the well-established bulk-boundary correspondence. However, anomalous topological edge states can arise in periodically driven Floquet systems even when the bulk Chern number is zero. A topological invariant (winding number) has been introduced to establish the appropriate bulk-boundary correspondence in such systems. Nevertheless, manipulating the topological properties of systems, whether static or driven, remains a significant challenge. In photonic systems, optical fields with diverse inherent dimensions such as frequency and polarization offer a natural avenue for topological-phase manipulation. In this paper, we present an alternative scheme for switching the topological invariant (winding number) of the system by changing the optical-waveguide mode within an integrated waveguide-based scattering network. With creating a bonding interface, this topological-phase switching facilitates the transmissions of the interface state in different directions. Experimental validation of this concept is demonstrated using a ring-optical-waveguide device. In this paper, we introduce an alternative strategy for tailoring the topological properties of optical systems.

The thermal transport of magnons has attracted substantial attention as an energy-efficient alternative to the transport of electrons. Most theoretical studies so far have been carried out within the frame of the linear spin-wave theory, which dramatically fails upon increasing the temperature and in the presence of competing interactions. In this work, we consider the impact of three- and four-magnon interactions in a honeycomb antiferromagnet, where such interactions are remarkably strong even at zero temperature. Using a combination of quantum field theory and mean-field theory, we compute the band structure of the interacting magnons and investigate the spin Nernst effect. We find that in the presence of in-plane Dzyaloshinskii-Moriya Interaction, the three-magnon interaction induces a nonreciprocal band splitting, even at zero temperature, leading to an enhancement of the spin Nernst conductivity. In contrast, the four-magnon interaction renormalizes the magnon spectrum at high temperatures, leading to a reduction of the overall magnon spin Nernst effect. These results suggest that interactions can massively influence the transport properties of magnons in antiferromagnets, even at zero temperature, and should be taken into account for predictive modeling.

In the monolayer limit, $1T^{\prime }$ ${\mathrm{WTe}}_2$ is a two-dimensional topological insulator exhibiting the quantum spin Hall effect and is believed to host an excitonic insulator ground state. However, theoretical analysis of this system is complicated by the difficulty of obtaining descriptions of the single-quasiparticle band structure consistent with experimental measurement within conventional first-principles techniques. Previous band-structure calculations using the Perdew-Burke-Ernzerhof functional and a one-shot $GW$ approximation result in a semimetallic band structure, while calculations with hybrid functionals appear to open a band gap. Here, we demonstrate that self-consistently updating wave functions within a static $GW$ approximation (static COHSEX) can reproduce the insulating band structure experimentally observed by angle-resolved photoemission spectroscopy without resorting to mechanisms beyond the quasiparticle picture. Finally, a finite-momentum Bethe-Salpeter equation calculation on top of self-consistent $GW$ results in negative exciton excitation energies, leaving open the possibility of excitonic instability in $1T^{\prime }$ monolayer ${\mathrm{WTe}}_2$.

The design of better exchange-correlation functionals for density functional theory (DFT) is a central challenge of modern electronic structure theory. However, current developments are limited by the mathematical form of the functional, with efficient semilocal functionals being inaccurate for many technologically important systems and the more accurate hybrid functionals being too expensive for large solid-state systems because of the use of the exact exchange operator. In this paper, we use machine learning combined with exact physical constraints to design an exchange functional that is both orbital dependent and nonlocal, but which can be evaluated at roughly the cost of semilocal functionals and is significantly faster than hybrid DFT in plane-wave codes. By training functionals with several different feature sets, we elucidate the roles of orbital-dependent and nonlocal features in learning the exchange energy and determine that both types of features provide vital and independently important information to the model. Having trained our exchange functional with an expressive, nonlocal feature set, we substitute it into existing hybrid functionals to achieve hybrid-DFT accuracy on thermochemical benchmark sets and improve the accuracy of band gap predictions over semilocal DFT. To demonstrate the scalability of our approach as well as the practical benefits of improved band gap prediction, we compute charged defect transition levels in silicon using large supercells. Because of its transferability and computational efficiency for both molecular and extended systems, our model overcomes the cost-accuracy trade-off between semilocal and hybrid DFT, and our general approach provides a feasible path toward a universal exchange-correlation functional with post-hybrid DFT accuracy and semilocal DFT cost.

We have used a combination of linear response time-domain THz spectroscopy (TDTS) and nonlinear THz pump-probe spectroscopy to separately probe the electronic momentum and energy relaxation rates, respectively, of the Dirac semimetal ${\mathrm{Cd}}_3{\mathrm{As}}_2$. We find, consistent with prior measurements, that ${\mathrm{Cd}}_3{\mathrm{As}}_2$ has enormous nonlinearities in the THz frequency range. We extract the momentum relaxation rate of ${\mathrm{Cd}}_3{\mathrm{As}}_2$ using Drude fits to the optical conductivity. We also conduct THz-range 2D coherent spectroscopy and find that the dominant response is a pump-probe signal, which allows us to extract the energy relaxation rate. We find that the rate of energy relaxation decreases down to the lowest measured temperatures. We connect this to ${\mathrm{Cd}}_3{\mathrm{As}}_2$'s anomalous lattice dynamics, evidence for which is found in its low thermal conductivity and soft phonons in Raman scattering. We believe the lack of a peak in the energy relaxation rate as a function of T is related to the linear-in-T dependence of the current relaxation at low T; e.g., the phonon scattering is elastic from the lowest measured temperature, 5 K, to at least as high as 120 K.

The spin Jahn-Teller effect, where a degeneracy of magnetic ground states is spontaneously lifted by structural distortions, lends itself to exploitation by the implied magnetoelastic coupling. ${\mathrm{CoTi}}_2{\mathrm{O}}_5$ hosts a frustrated topology of magnetic interactions that could realize such coupling, since the observed magnetic ground state requires a spontaneous monoclinic distortion of the crystal. Using resonant elastic x-ray scattering to simultaneously probe the magnetic structure and lattice distortion of ${\mathrm{CoTi}}_2{\mathrm{O}}_5$, we demonstrate near complete magnetic domain switching by an applied uniaxial stress that is conjugate to the monoclinic strain. Our results, supported by density functional theory calculations, confirm that ${\mathrm{CoTi}}_2{\mathrm{O}}_5$ displays spin Jahn-Teller effects that can be understood in terms of modulated Heisenberg exchange interactions, and demonstrate the potential functionality of spin Jahn-Teller materials in spintronic devices sensitive to uniaxial strain.

At zero temperature, a Josephson junction coupled to an ohmic environment displays a quantum phase transition between superconducting and insulating phases, depending on whether the resistance of the environment is below or above the resistance quantum. This so-called Schmid transition, representative of the effect in a broad class of quantum impurity problems, turns into a crossover at finite temperatures. We determine the conditions under which the temperature dependence of the thermal conductance, which characterizes heat flow from a hot to cold resistor across the Josephson junction, displays a universal scaling characteristic of the Schmid transition. We also discuss conditions for heat rectification to happen in the circuit. Our work can serve as a guide for identifying signatures of the Schmid transition in heat transport experiments.

The development of interatomic potentials that can accurately capture a wide range of phys ical phenomena and diverse environments is of significant interest, but it presents a formidable challenge. This challenge arises from the numerous structural forms, multiple phases, complex intramolecular and intermolecular interactions, and varying external conditions. In this paper, we present a method to construct environment-adaptive interatomic potentials by adapting to the local atomic environment of each atom within a system. The collection of atomic environments of interest is partitioned into several clusters of atomic environments. Each cluster represents a distinctive local environment and is used to define a corresponding local potential. We introduce a many-body many-potential expansion to smoothly blend these local potentials to ensure global continuity of the potential energy surface. This is achieved by computing the probability functions that determine the likelihood of an atom belonging to each cluster. We apply the environment-adaptive machine learning potentials to predict observable properties for Ta element and InP compound, and compare them with density functional theory calculations.

Helical trilayer graphene (hTG) exhibits a supermoiré pattern with large domains centered around stacking points ABA and BAB, where two well-separated low-energy bands appear with different total Chern numbers at each valley, forming a Chern mosaic pattern. In the chiral limit, the low-energy bands become exactly flat at zero energy for magic-angle twists. Here we investigate these zero-energy flat bands and their topological properties in the presence of a perpendicular magnetic field. We show that hTG retains the precise flatness of the zero-energy bands, even at finite magnetic fields. We find topological phase transitions at fields corresponding to unit and half magnetic flux leading to an emergence of higher Chern number flat bands. Consequently the Chern mosaic gets modified for finite magnetic fields. We further find the analytical forms of zero-energy wave functions and identify a set of hidden wave functions, which gives crucial insights into both the topological transitions and enhancement of Chern numbers across them. We also find topological transitions away from the chiral limit with finite corrugations and at different magic angles.

Isolated flat bands with significantly quenched kinetic energy of electrons could give rise to exotic strongly correlated states from electron-electron interactions. More intriguingly, the interplay between topology and flat bands can further lead to richer physical phenomena, which have attracted much interest. Here, taking advantage of the recently proposed intertwined Dirac states induced from the anisotropic coupling between the top and bottom surface states of an antiferromagnetic topological insulator thin film, we show the emergence of a high-Chern-number (quasi)flat-band state through moiré engineering of the surface states. Remarkably, the flat bands are isolated from other bands and located near the Fermi level. Furthermore, topological phase transitions between trivial and nontrivial flat-band states can be driven by tuning the out-of-plane electric field. Our work not only proposes a scheme to realize high-Chern-number flat-band states, but also highlights the versatility of the intertwined Dirac-cone states.

The anisotropic thermal conductivity and the phonon mean free path (mfp) in monoclinic β-${\mathrm{Ga}}_2{\mathrm{O}}_3$ single crystals and homoepitaxial (HE) films of several microns were determined using the $3\omega$ method in the temperature range from 10 to 300 K. The measured effective thermal conductivity of both single-crystal and HE films are on the order of 20 W/(mK) at room temperature; <30 K, it increases with a maximum of 1000 to 2000 $\mathrm{W}/\left(\mathrm{mK}\right)$ and decreases with $T^3<25\mathrm{K}$. Analysis of the phonon mfp shows the dominance of phonon-phonon-Umklapp scattering >80 K, below which the influence of point-defect scattering is observed. Below 30 K, the phonon mfp increases until it is limited by the total β-${\mathrm{Ga}}_2{\mathrm{O}}_3$ sample size. A crossover from resistive to ballistic phonon transport is observed <20 K, and boundary effects of the total sample size become dominant. This reveals that the HE film-substrate interface is highly phonon transparent. The resistive and ballistic phonon transport regimes in β-${\mathrm{Ga}}_2{\mathrm{O}}_3$ are discussed corresponding to the models of Callaway and Majumdar, respectively.

There is growing interest in strongly correlated insulator thin films because the intricate interplay of their intrinsic and extrinsic state variables causes memristive behavior that might be used for biomimetic devices in the emerging field of neuromorphic computing. In this study we find that laser irradiation tends to drive ${\mathrm{V}}_2{\mathrm{O}}_3$ from supercooled/superheated metastable states toward thermodynamic equilibrium, most likely in a nonthermal way. We study thin films of the prototypical Mott-insulator ${\mathrm{V}}_2{\mathrm{O}}_3$, which show spontaneous phase separation into metal-insulator herringbone domains during the Mott transition. Here, we use low-temperature microscopy to investigate how these metal-insulator domains can be modified by scanning a focused laser beam across the thin film surface. We find that the response depends on the thermal history: When the thin film is heated from below the Mott transition temperature, the laser beam predominantly induces metallic domains. On the contrary, when the thin film is cooled from a temperature above the transition, the laser beam predominantly induces insulating domains. Very likely, the ${\mathrm{V}}_2{\mathrm{O}}_3$ thin film is in a superheated or supercooled state, respectively, during the first-order phase transition, and the perturbation by a laser beam drives these metastable states into stable ones. This way, the thermal history is locally erased. Our findings are supported by a phenomenological model with a laser-induced lowering of the energy barrier between the metastable and equilibrium states.

${\mathrm{EuMnSb}}_2$ is a unique and promising compound with complex magnetism (Eu and Mn sublattices) to realize magnetic control of topological quasiparticles. By applying pressure ($P$) to this system, we have observed a continuous pressure-induced valence change from ${\mathrm{Eu}}^{2+}$ to ${\mathrm{Eu}}^{3+}$. Remarkably, despite the significant valence change in Eu ions, an extremely large increase in the magnetic ordering temperature ($T_{\mathrm{N},\mathrm{Eu}}$) from 21 K at ambient pressure to 180 K at 28.4 GPa is detected. The substantial enhancement in $T_{\mathrm{N},\mathrm{Eu}}$ is likely the consequence of the reduced distance between neighboring Eu ions and the enhanced indirect exchange interaction through increased $4f-5d$ mixing under pressure. Furthermore, $T_{\mathrm{N},\mathrm{Eu}}$($P$) shows a discontinuous change above 8 GPa, although the orthorhombic crystal structure remains stable. Our experimental data indicate the record high value in the magnetic hyperfine field of ${}^{151}\mathrm{Eu}$, reaching 74.1 T at 28.4 GPa and 26 K. These unusual electronic and magnetic transitions revealed by pressure highlight that ${\mathrm{EuMnSb}}_2$ is a rich magnetic topological semimetal candidate for investigating a multitude of strong correlations.

Frustrated magnetic systems can host highly interesting phases known as classical spin liquids (CSLs), which feature extensive ground state degeneracy and lack long-range magnetic order. Recently, H. Yan et al. proposed a classification scheme of CSLs in the large-$\mathcal{N}$ (soft spin) limit [Phys. Rev. B 110, L020402 (2024); Phys. Rev. B 109, 174421 (2024)]. This scheme classifies CSLs into two categories: the algebraic CSLs and the fragile topological CSLs, each with their own correlation properties, low energy effective description, and finer classification frameworks. In this work, we further develop the classification scheme by considering the role of crystalline symmetry. We present a mathematical framework for computing the band representation of the flat bands in the spectrum of these CSLs, which extends beyond the conventional representation analysis. It allows one to determine whether the algebraic CSLs, which feature gapless points on their bottom flat bands, are protected by symmetry or not. It also provides more information on the finer classifications of algebraic and fragile topological CSLs. We demonstrate this framework via concrete examples and showcase its power by constructing a pinch-line algebraic CSL protected by symmetry.

The recent observation of fractional quantum anomalous Hall (FQAH) states in tunable moiré materials encourages study of several new phenomena that may be uniquely accessible in these platforms. Here, we show that an isolated localized anyon of the FQAH state will nucleate a “halo” of charge-density-wave (CDW) order around it. We demonstrate this effect using a recently proposed quantum Ginzburg-Landau theory that describes the interplay between the topological order of the FQAH state and the broken-symmetry order of a CDW. The spatial extent of the CDW order will, in general, be larger than the length scale at which the fractional charge of the anyon is localized. The strength and the decay length of the CDW order around anyons induced by doping or the magnetic field differ qualitatively from those nucleated by a random potential. Our results leverage a precise mathematical analogy to earlier studies of the superfluid-CDW competition of a system of lattice bosons which has been used to interpret the observed CDW halos around vortices in high-$T_c$ superconductors. We show that measurement of these patches of CDW order can give an indirect route to measuring the fractional charge of the anyon. Such a measurement may be possible by scanning tunneling microscopy in moiré systems.

We explore the possibilities for spin-singlet superconductivity in newly discovered altermagnets. Investigating $d$-wave altermagnets, we show that finite-momentum superconductivity can easily emerge in altermagnets even though they have no net magnetization, when the superconducting order parameter also has $d$-wave symmetry with nodes coinciding with the altermagnet nodes. Additionally, we find a rich phase diagram when both altermagnetism and an external magnetic field are considered, including superconductivity appearing at high magnetic fields from a parent zero-field normal state.