Tellurium (Te) has recently attracted research interest for its outstanding electronic and optoelectronic properties and interesting topological physical related to topological Weyl cones in its band structure. To explore its high field transport and optoelectronic properties, the study of the ultrafast dynamics of photoexcited carriers plays a pivotal role. In this work, we study the ultrafast dynamics of photoexcited carriers in Te at the vicinity of Weyl nodes using transient reflection spectroscopy with 2-µm pump and 4-µm probe. We find that the relaxation process in a 100-nm Te flake is dominated by a 30-ps Auger recombination at high carrier density and a 200-ps multiphonon emission process at low carrier density. In contrast, the relaxation process in a 20-nm flake is ten times faster than the 100-nm flake due to the assistance of surface defects. The reflection modulation depth of the 20-nm sample is up to 60%, promising great potential for fast optical switch and optical modulator applications in the midinfrared region.

Lattice formation of swirling textures is ubiquitous in solid-state materials, such as a magnetic skyrmion lattice in chiral magnets. In the magnetic skyrmion lattices, their moving states and dynamics under external perturbations are still unrevealed, although a detailed understanding of the dynamics is crucial to realizing spintronic applications, such as magnetic domain-wall racetrack memory [Parkin et al., Science 320, 190 (2008); Fert et al., Nat. Nanotechnol. 8, 152 (2013)]. Here, we report in detail on the transient state of a moving magnetic skyrmion lattice in bulk single-crystalline MnSi under alternating current (AC) using small-angle neutron scattering. A rotation and concomitant broadening of the spot width in the azimuthal direction of the magnetic skyrmion reflections originating from the plastic deformation of the magnetic skyrmion lattice were found only at low AC frequencies [Okuyama et al., Commun. Phys. 2, 79 (2019)], whereas above the threshold AC frequency ($f_{\mathrm{t}}\sim 0.12\mathrm{Hz}$) the rotation was not observed, and the spot width becomes sharper. The observed complex response of the magnetic skyrmion reflections can be explained by the change in dislocation density in the magnetic skyrmion lattice. At frequencies higher than $f_{\mathrm{t}}$, the magnetic skyrmions oscillate removing the dislocations, indicating that the dislocation density is controlled by the AC frequency.

Antiferromagnetic superlattices (AFSL) are proposed based on the buckled hexagonal two-dimensional materials, which can be realized by the proximity effect of the periodically deposited antiferromagnets. It is found that the AF proximity effect can give rise to valley-polarized minibands and conductance, which are not held under ferromagnetic proximity. The spin degeneracy and valley degeneracy are lifted simultaneously in the presence of AF proximity and electric field. In consequence, both minibands and conductance could be spin-valley polarized completely in AFSL. The symmetry of spin-valley polarization is analyzed by considering the pseudospin rotation operations and spatial inversion operations. Furthermore, AFSL also induce a highly anisotropic band structure due to the spin-orbit coupling (SOC). In particular, the group velocity parallel to the periodic direction of AFSL is greatly renormalized, while the velocity perpendicular to the periodic direction remains unaffected, contrary to that observed in graphene superlattices. With the increase of SOC, the anisotropy becomes more prominent, leading to flattened band and electron supercollimation. The direction of anisotropy can be regulated by adjusting the potential and SOC. These findings offer an alternative approach to engineering anisotropic two-dimensional materials. As an application, the AFSL may well work as a symmetry-protected spin-valley valve easily controlled by the gate voltages.

Accurate prediction of electron transport coefficients is crucial for understanding warm dense matter. Utilizing density functional theory (DFT) with the Kubo-Greenwood formula is widely used to evaluate the electrical and thermal conductivities of electrons. By adding the nonlocal potential correction term that appears in the dynamic Onsager coefficient and using two different norm-conserving pseudopotentials, we predict the electrical and thermal conductivities of electrons for liquid Al (1000 K) and warm dense Al (0.2 to 10 eV). We systematically investigate the effects of nonlocal terms in the pseudopotentials and the frozen-core approximation on the conductivities. We find that taking into account the nonlocal potential correction and validating the frozen core approximation is essential for accurately calculating the electrical and thermal transport properties of electrons across a wide range of temperatures.

Pyrochlore iridates are thought to be a fertile ground for the realization of topologically nontrivial and strongly correlated states of matter. Here, we observe a magnetically driven phase transition between the topological Weyl semimetal and a Luttinger semimetal with quadratic band touching in ${\mathrm{Nd}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ using Raman scattering. We also find evidence of quadratic band touching in ${\mathrm{Pr}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$, with the chemical potential further away from the node. Our theoretical analysis of the Raman scattering from Weyl and Luttinger quasiparticles agrees with experimental observations and enables a characterization of the material parameters while revealing interaction and disorder effects through a relatively short quasiparticle lifetime.

Thermal equilibrium states of local quantum many-body systems are notorious for their spatially decaying correlations, which place severe restrictions on the types of many-body entanglement structures that may be observed at finite temperatures. These restrictions may, however, be defied when an out-of-equilibrium steady state is considered instead. In this paper, we study the entanglement properties of free fermions on a one-dimensional lattice that contains a generic charge- and energy-conserving noninteracting impurity, and that is connected at its edges to two reservoirs with different equilibrium energy distributions. These distributions may differ in either temperature, chemical potential, or both, thereby inducing an external bias. We analytically derive exact asymptotic expressions for several quantum information measures—the mutual information, its Rényi generalizations, and the fermionic negativity—that quantify the correlation and entanglement between two subsystems located on opposite sides of the impurity. We show that all these measures scale (to a leading order) linearly with the overlap between one subsystem and the mirror image of the other (upon reflection of the latter about the impurity), independently of the distance between the subsystems. While a simple proportionality relation between the negativity and Rényi versions of the mutual information is observed to hold at zero temperature, it breaks down at finite temperatures, suggesting that these quantities represent strong long-range correlations of different origins. Our results generalize previous findings that were limited to the case of a chemical-potential bias at zero temperature, rigorously demonstrating that the effect of long-range volume-law entanglement is robust at finite temperatures.

We present a theoretical study of the intrinsic plasmonic properties of twisted bilayer graphene (TBG) as a function of the twist angle $\theta$ (and other microscopic parameters such as temperature and filling factor). Our calculations, which rely on the random phase approximation, take into account four crucially important effects, which are treated on equal footing: (i) the layer-pseudospin degree of freedom, (ii) spatial nonlocality of the density-density response function, (iii) crystalline local field effects, and (iv) Hartree self-consistency. We show that the plasmonic spectrum of TBG displays a smooth transition from a strongly coupled regime (at twist angles $\theta \lesssim 2^{{}^{\circ }}$), where the low-energy spectrum is dominated by a weakly dispersive intraband plasmon, to a weakly coupled regime (for twist angles $\theta \gtrsim 2^{{}^{\circ }}$) where an acoustic plasmon clearly emerges. This crossover offers the possibility of realizing tunable mid-infrared subwavelength cavities, whose vacuum fluctuations may be used to manipulate the ground state of strongly correlated electron systems.

When subjected to quasiperiodic driving protocols, superconducting systems have been found to harbor robust time-quasiperiodic Majorana modes, extending the concept beyond static and Floquet systems. However, the presence of incommensurate driving frequencies results in dense energy spectra, rendering conventional methods of defining topological invariants based on band structure inadequate. In this work, we introduce a real-space topological invariant capable of identifying time-quasiperiodic Majorana modes by leveraging the system's spectral localizer, which integrates information from both Hamiltonian and position operators. Drawing insights from non-Hermitian physics, we establish criteria for constructing the localizer and elucidate the robustness of this invariant in the presence of dense spectra. Our numerical simulations, focusing on a Kitaev chain driven by two incommensurate frequencies, validate the efficacy of our approach.

Low-temperature magnetoelectroluminescence (MEL) of organic light-emitting diodes (OLEDs) reveals a near-complete suppression of electroluminescence at strong magnetic fields due to the high degree of thermal spin polarization (TSP) arising when the Zeeman energy exceeds the thermal energy. In addition to TSP, spin mixing within the Coulombically bound carrier pairs can arise, as can interactions between triplet excitons or triplet excitons and charge carriers. These effects also depend on the applied magnetic field strength. We report on the surprisingly nonmonotonic MEL in the intermediate magnetic-field region of up to 230 mT at temperatures down to 1.5 K, and explore the effect of deuteration to distinguish between triplet-excitonic and carrier-pair effects. A narrow MEL feature is observed in the field region of ±3 mT, which is inverted upon deuteration and can therefore be clearly assigned to spin mixing mediated by the hyperfine fields. At larger fields, a broader MEL feature is identified, which shows discrete substructure assigned to the zero-field splitting of the triplet exciton. The resolution of this substructure is enhanced by deuteration. Quantitative modeling of the MEL by solving the stochastic Liouville equation in the density-matrix formalism provides excellent agreement with the experimental results and demonstrates that the triplet excitonic feature arises from delayed fluorescence generated by triplet-triplet annihilation (TTA). The microscopic simulations reveal that TTA occurs preferentially when the axes of the two triplets in the amorphous $\pi$-conjugated polymer are close to parallel to each other, illustrating an alternative spectroscopic approach to investigating the underlying physics of TTA.

A central theoretical issue at the core of the current research on many-body localization (MBL) consists in characterizing the statistics of rare long-range resonances in many-body eigenstates. This is of paramount importance to understand: (i) the critical properties of the MBL transition and the mechanism for its destabilization through quantum avalanches; (ii) the unusual transport and anomalously slow out-of-equilibrium relaxation when the transition is approached from the metallic side. To study and characterize such long-range rare resonances, we develop a large-deviations approach based on an analogy with the physics of directed polymers in random media, and in particular with their freezing glass transition on infinite-dimensional graphs. The basic idea is to enlarge the parameter space by adding an auxiliary parameter (which plays the role of the inverse temperature in the directed polymer formulation) which allows us to fine-tune the effect of anomalously large outliers in the far-tails of the probability distributions of the transmission amplitudes between far-away many-body configurations in the Hilbert space. We first benchmark our approach onto two noninteracting paradigmatic toy models, namely the single-particle Anderson model on the (loopless) Cayley tree and the Rosenzweig-Porter random matrix ensemble, and then apply it to the study of a class of disordered quantum spin chains in a transverse field. This analysis shows the existence of a broad disorder range in which rare, long-distance resonances, that in finite-size systems may only form for a few specific realizations of the disorder and a few specific choice of the random initial state, are expected to destabilize the MBL phase in the thermodynamic limit, while the genuine MBL transition is shifted to much larger values of the disorder than originally thought.

The manipulation of magnetization dynamics by a focused laser with thermal and nonthermal mechanisms is a fascinating research topic. While such phenomena in ferromagnets have been intensively studied, the laser-induced magnetization dynamics in antiferromagnets remain to be clarified. In the present paper, we theoretically investigate both thermal and nonthermal mechanisms in CrSBr films, a van der Waals antiferromagnet with triaxial magnetic anisotropy, which has attracted great research interest recently. In contrast to previous reports on ferromagnet, the magnetization dynamics in CrSBr show several distinct features, especially for the thermal mechanism associated with the shear stress, where the presence and symmetry of spatial patterns of the dynamical magnetization strongly rely on the nonuniformity of shear stress and the strength of the in-plane magnetic field. From the nonthermal excitation, we find an isotropic spin-wave propagation at zero field, as well as an enhanced anisotropy introduced by the magnetic field. Our results could be useful for the laser-induced spin dynamics in antiferromagnets.

The quantum geometric tensor (QGT) is a fundamental concept for characterizing the local geometry of quantum states. After casting the geometry of pure quantum states and extracting the QGT, we generalize the geometry to mixed quantum states via the density matrix and its purification. The gauge-invariant QGT of mixed states is derived, whose real and imaginary parts are the Bures metric and the Uhlmann form, respectively. In contrast to the imaginary part of the pure-state QGT that is proportional to the Berry curvature, the Uhlmann form vanishes identically for ordinary physical processes. Moreover, there exists a Pythagorean-like equation that links different local distances and reflect the underlying fibration. The Bures metric of mixed states is shown to reduce to the corresponding Fubini-Study metric of the ground states as temperature approaches zero, establishing a correspondence despite the different underlying fibrations. We also present two examples with contrasting local geometries and discuss experimental implications.

We develop an analytic theory to describe the interaction between electrons and $K$ phonons and study its influence on superconductivity in the bare bands of twisted bilayer graphene (TBG). We find that, due to symmetry and the two-center approximation, only one optical $K$ phonon ($\sim 160\text{meV}$) of graphene is responsible for the intervalley electron-phonon interaction. This phonon has recently been found in angular-resolved photoemission spectroscopy to be responsible for replicas of the TBG flat bands. By projecting the interaction to the TBG flat bands, we perform the full symmetry analysis of the phonon-mediated attractive interaction and pairing channels in the Chern basis, and show that several channels are guaranteed to have gapless order parameters. From the linearized gap equations, we find that the highest $T_c$ pairing induced by this phonon is a singlet gapped $s$-wave inter-Chern-band order parameter, followed closely by a gapless nematic $d$-wave intra-Chern-band order parameter. We justify these results analytically, using the topological heavy-fermion mapping of TBG which has allowed us to obtain an analytic form of a phonon-mediated attractive interaction and to analytically solve the linearized and $T=0$ gap equations. For the intra-Chern-band channel, the nematic state with nodes is shown to be stabilized in the chiral flat-band limit. While the flat-band Coulomb interaction can be screened sufficiently enough—around the Van Hove singularities—to allow for electron-phonon based superconductivity, it is unlikely that this effect can be maintained in the lower density of states excitation bands around the correlated insulator states.

We propose a route to detect Majorana bound states (MBSs) by coupling a topological superconductor to quantum dots (QDs) in a $pnp$ junction. Here two MBSs are coherently coupled to electrons on two QDs, which recombine with holes to photons. We focus on the spectroscopy of cross-correlated shot noise and the polarization of the emitted photons. Our detection scheme allows us to probe the necessary condition for the emergence of MBSs, specifically, the existence of nonlocal triplet superconducting correlations and also the fundamental property that two MBSs comprise a single complex fermion. We compare our results to the ones obtained from nontopological quasi-MBSs (qMBSs) and establish a correspondence between the number of peaks in the cross-correlation with the number of MBSs in the system. Here we can identify a tunneling regime that facilitates differentiation between topological MBSs and trivial qMBSs. Additionally, we test the robustness of the detection scheme by the addition of uncorrelated quasiparticles.

Onsager's regression hypothesis connects the temporal relaxation of close-to-equilibrium systems with their dynamical correlation functions at thermal equilibrium. While the hypothesis is provably correct in classical systems, it is known to fail in the quantum regime. Here, we derive a suitably adjusted quantum version of Onsager's original hypothesis. Rigorous analytical results are complemented by a variety of numerical examples.

Within the framework of the recently introduced multinanoparticle power-series expansion method for the polarized small-angle neutron scattering (SANS) cross section, we present analytical expressions for the polarized SANS observables arising from dilute nanoparticle assemblies with antisymmetric vortex-type spin structures. We establish connections between the magnetic correlation coefficients and the magnetic field-dependent vortex-axes distribution function, which is related to the random orientations of the magnetocrystalline anisotropy axes of the nanoparticles. Our analytical results are validated through a comparative analysis with micromagnetic simulations. This framework contributes to a comprehensive understanding of polarized magnetic neutron scattering from spherical nanoparticle systems exhibiting vortex-type spin structures.

The recent discovery of a charge density (CDW) state in ${\mathrm{ScV}}_6{\mathrm{Sn}}_6$ at $T_{\text{CDW}}$ = 91 K offers new opportunities to understand the origins of electronic instabilities in topological kagome systems. By comparing to the isostructural non-CDW compound ${\mathrm{LuV}}_6{\mathrm{Sn}}_6$, we unravel interesting electrical transport properties in ${\mathrm{ScV}}_6{\mathrm{Sn}}_6$, above and below the charge-ordering temperature. We observed that by applying a magnetic field along the $a$ axis, the temperature behavior of the longitudinal resistivity in ${\mathrm{ScV}}_6{\mathrm{Sn}}_6$ changes from metal-like to insulator-like above the CDW transition. We show that in the charge-ordered state ${\mathrm{ScV}}_6{\mathrm{Sn}}_6$ follows the Fermi liquid behavior while above that, the resistivity varies sublinearly over a broad temperature range. The sublinear resistivity, which scales by $T^{3/5}$ is a common feature among other vanadium-containing kagome compounds exhibiting CDW states such as ${\mathrm{KV}}_3{\mathrm{Sb}}_5, {\mathrm{RbV}}_3{\mathrm{Sb}}_5$, and ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$. By contrast, the sublinear behavior of the resistivity does not occur in ${\mathrm{LuV}}_6{\mathrm{Sn}}_6$. Finally, we show anomalous Hall-like behavior in ${\mathrm{ScV}}_6{\mathrm{Sn}}_6$ below $T_{\text{CDW}}$, which is absent in the Lu compound. Comparing the transport properties of ${\mathrm{ScV}}_6{\mathrm{Sn}}_6$ and ${\mathrm{LuV}}_6{\mathrm{Sn}}_6$ is valuable to highlight the impacts of the unusual CDW in the Sc compound.

Motivated by the importance of studying topological superconductors beyond the mean-field approximation, we here investigate mesoscopic islands of time-reversal-invariant topological superconductors. We characterize the spectrum in the presence of strong order-parameter fluctuations in the presence of an arbitrary number of Kramers pairs of Majorana edge states and study the effect of coupling the Coulomb blockaded island to external leads. In the case of an odd fermionic parity on the island, we derive an unconventional Kondo Hamiltonian in which metallic leads couple to both topological Majorana degrees of freedom (which keep track of the parity in different leads) and the overall spin $\frac{1}{2}$ in the island. For the simplest case of a single wire (two pairs of Majorana edge states), we demonstrate that anisotropies are irrelevant in the weak coupling renormalization group flow. This permits us to solve the Kondo problem in the vicinity of a Toulouse-type point using Abelian bosonization. We demonstrate a residual ground-state entropy of $ln\left(2\right)$, which is protected by spin-rotation symmetry, but reduced to $ln\left(\sqrt{2}\right)$ (as in the spinless topological Kondo effect) by symmetry-breaking perturbations. In the symmetric case, we further demonstrate the simultaneous presence of both Fermi-liquid and non-Fermi-liquid-like thermodynamics (depending on the observable) and derive charge and spin transport signatures of the Coulomb blockaded island.

The negatively charged boron vacancy (${\text{V}}_B^{-}$) center in hexagonal boron nitride (hBN) is currently garnering considerable attention for the design of two-dimensional (2D) quantum sensing units. Such developments require a precise understanding of the spin-dependent optical response of ${\text{V}}_B^{-}$ centers, which still remains poorly documented despite its key role for sensing applications. Here we investigate the spin-dependent photodynamics of ${\text{V}}_B^{-}$ centers in hBN by a series of time-resolved photoluminescence (PL) measurements. We first introduce a robust all-optical method to infer the spin-dependent lifetime of the excited states and the electron spin polarization of ${\text{V}}_B^{-}$ centers under optical pumping. Using these results, we then analyze PL time traces recorded at different optical excitation powers with a seven-level model of the ${\text{V}}_B^{-}$ center and we extract all the rates involved in the spin-dependent optical cycles, both under ambient conditions and at liquid helium temperature. These findings are finally used to study the impact of a vector magnetic field on the optical response. More precisely, we analyze PL quenching effects resulting from electron spin mixing induced by the magnetic field component perpendicular to the ${\text{V}}_B^{-}$ quantization axis. All experimental results are well reproduced by the seven-level model, illustrating its robustness to describe the spin-dependent photodynamics of ${\text{V}}_B^{-}$ centers. This work provides important insights into the properties of ${\text{V}}_B^{-}$ centers in hBN, which are valuable for future developments of 2D quantum sensing units.

Topological flat moiré bands with nearly ideal quantum geometry have been identified in homobilayer transition metal dichalcogenide moiré superlattices, and are thought to be crucial for understanding the fractional Chern insulating states recently observed therein. Previous study proposed viewing the system using an adiabatic approximation that replaces the position dependence of the layer spinor with a nonuniform periodic effective magnetic field. When the local zero-point kinetic energy of this magnetic field cancels identically against that of an effective Zeeman energy, a Bloch-band version of Aharonov-Casher zero-energy modes, which we refer to as Aharonov-Casher band, emerges leading to ideal quantum geometry. Here, we critically examine the validity of the adiabatic approximation and identify the parameter regimes under which Aharonov-Casher bands emerge. We show that the adiabatic approximation is accurate for a wide range of parameters including those realized in experiments. Furthermore, we show that while the cancellation leading to the emergence of Aharonov-Casher bands is generally not possible beyond the leading Fourier harmonic, the leading harmonic is the dominant term in the Fourier expansions of the zero-point kinetic energy and Zeeman energy. As a result, the leading harmonic expansion accurately captures the trend of the bandwidth and quantum geometry, although it may fail to quantitatively reproduce more detailed information about the bands such as the Berry curvature distribution.