We report angle-resolved photoemission spectroscopy (ARPES) studies combined with $\mathrm{DFT}+\mathrm{DMFT}$ calculations to confirm that the magnetic ordering vector transition from $\mathrm{Q}=(1/2,0,0)$ to $\mathrm{Q}=(1/3,0,0)$ in the metallic triangular antiferromagnets ${\mathrm{Co}}_{1/3\pm \epsilon }{\mathrm{TaS}}_2$ ($\epsilon \approx 0.007$) is induced by the electronic structure change in the system. The ARPES-measured Fermi surface (FS) maps of ${\mathrm{Co}}_{0.325}{\mathrm{TaS}}_2$ show two hexagonal and one circular holelike FSs around $\mathrm{\Gamma }$, which matches well with the triple-Q state by taking into account the contribution of nesting vectors occurring between Co $3d$ and Ta $5d$ orbitals. In the case of ${\mathrm{Co}}_{0.340}{\mathrm{TaS}}_2$, a new electron pocket around K appears, and the FS geometry changes as a result of the correlation effect of ${\mathrm{Co}}_4{\mathrm{S}}_{18}$ tripods forming in the system. The magnetic susceptibility calculations based on the charge-self-consistent $\mathrm{DFT}+\mathrm{DMFT}$ band structures and the random phase approximation indicate that the most stable magnetic ordering vector (1/2,0,0) split into (1/6,0,0) and (1/2,0,0), which is consistent with the magnetic phase transition around $x=1/3$ in ${\mathrm{Co}}_x{\mathrm{TaS}}_2$.

We investigate electron pairing in a super clean kagome superconductor ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ with a residual resistivity ratio (RRR) of 290. Using the dilution-refrigerator-based scanning tunneling microscopy (STM) at the Synergetic Extreme Condition User Facility (SECUF), we find that the pairing gap exhibits chiral 2 × 2 modulations, and their chirality can be controlled by magnetic field training. We introduce nonmagnetic impurities to observe the complete suppression of 2 × 2 pairing modulations in presence of persistent 2 × 2 charge order. This nonmagnetic pair-breaking effect provides phase-sensitive evidence for pair-density-wave (PDW)-induced pairing modulations. Our results support switchable chiral 2 × 2 PDW in this super clean kagome superconductor.

We demonstrate that including the second viscosity of an electron gas in the hydrodynamic model allows for highly accurate modeling of the optical response of heavily doped semiconductors. In our setup, which improves resonance visibility compared to previous approaches, plasmon resonances become more distinct, allowing for a detailed analysis of the underlying physics. With advanced fitting techniques based on a physics-informed cost function and a tailored optimization algorithm, we obtain a close agreement between simulations and experimental data across different sample thicknesses. This enhanced resonance visibility, combined with our integrated approach, shows that key parameters such as doping level and effective electron mass, as well as the second viscosity of the electron gas, can be retrieved from a single optical measurement. The spatial dispersion taken into account in the hydrodynamic framework is essential for accurately describing the optical response of plasmonic materials in this frequency range and is likely to become a standard modeling approach.

Bernstein modes—hybrid magnetoplasmon excitations arising from the coupling between cyclotron motion and collective oscillations in two-dimensional electron systems—offer direct access to nonlocal electrodynamics. These modes can exhibit rich nonlinear behavior akin to strong-coupling phenomena in cavity quantum electrodynamics, but reaching the nonlinear regime has remained experimentally challenging. Here we report the observation of nonlinear Bernstein modes in graphene using terahertz excitation with near-field enhancement from embedded metallic contacts. Photoresistance spectroscopy reveals sharp resonances at $B_c/2$ and $B_c/3$ that saturate at radiation intensities nearly an order of magnitude lower than the cyclotron resonance. We ascribe this to strong local heating of the electron gas due to resonant excitation of high-amplitude Bernstein magnetoplasmons, associated with a combination of the field-concentration effect of the near field and plasmonic amplification that is resonantly enhanced in the region of Bernstein gaps. Polarization-resolved measurements further confirm the near-field origin: Bernstein resonances are insensitive to circular helicity but strongly depend on the angle of linear polarization, in sharp contrast to the cyclotron resonance response. Our results establish graphene as a platform for nonlinear magnetoplasmonics, opening opportunities for strong-field manipulation of collective electron dynamics, out-of-equilibrium electron transport, and solid-state analogues of cavity quantum electrodynamics.

α-${\mathrm{Li}}_3\mathrm{N}$ is a promising $\mathrm{L}{\mathrm{i}}^{+}$ superionic conductor for battery applications due to its high ionic conductivity. The superionic behavior emerges with the breakdown of anisotropy in $\mathrm{L}{\mathrm{i}}^{+}$ diffusion. This study investigates the role of different Li sites in the hexagonal layered structure in influencing the ionic transport using machine-learned molecular dynamics simulation. We found that below 600 K, the Li at $2c$ Wyckoff site contribute dominantly to diffusion in the $\mathit{ab}$ plane. However, at elevated temperatures, Li at both the Wyckoff sites ($2c$ and $1b$) becomes equally mobile, leading to three-dimensional superionic behavior. Further, we identified that the superionic transition is associated with the strong anharmonicity in low-energy phonon modes dominated by $\mathrm{L}{\mathrm{i}}^{+}$ dynamics. We find that this transition leads to a marked reduction in lattice thermal conductivity, highlighting the role of strongly anharmonic low-energy phonons in suppressing thermal transport.

Magnetic chirality describes the fixed correspondence between polarization and rotation of noncollinear spin twists. It remains a fundamental challenge in determining the chirality of Néel-twisted nanometric spin textures. This work presents a theoretical framework for determining the chirality of Néel-type spin twists via magnetic force microscopy (MFM). By modeling the magnetostatic interaction between a nanometric magnetic tip and the stray field of a two-dimensional magnetic film, our analytical model and numerical simulations demonstrate that the magnetic fields generated by distinct moment components of a left-handed Néel-type spin twist mutually cancel, resulting in a vanishing phase shift. In contrast, a right-handed twist exhibits a twofold enhancement in the phase shift. The influence of magnetic anisotropy on this phase-shift contrast is also discussed. Our findings provide a critical advancement for skyrmion imaging using conventional MFM and establish guidelines for experimental chirality determination.

Magnetic solitons such as skyrmions and bimerons show great promise for both fundamental research and spintronic applications. Stabilizing and controlling topological spin textures in atomically thin van der Waals (vdW) materials has gained tremendous attention due to high tunability, enhanced functionality, and miniaturization. Here, we present an efficient spin-spiral approach based on first-principles calculations, a method for mapping magnetic interactions from collective models onto arbitrary lattice symmetries, such as hexagonal and honeycomb lattices. Using atomistic spin models parametrized from first principles, we predict the emergence of multiple topological spin textures in an all-magnetic vdW heterostructure ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2/{\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ (FGT/CGT)–an experimentally feasible system. Interestingly, the FGT layer favors out-of-plane magnetization, whereas the CGT layer prefers in-plane magnetocrystalline anisotropy. Néel-type nanoscale skyrmions are formed at zero field in the FGT layer due to interfacial Dzyaloshinskii-Moriya interaction (DMI), while nanoscale bimerons and antibimerons can co-exist in the CGT layer by the interplay between exchange frustration and DMI. Using our collective approach, we reveal significant discretization effects in hexagonal and honeycomb lattices. In particular, we demonstrate that the lifting of geometric exchange frustration on the honeycomb lattice significantly affects soliton barriers and pinning energetics. These fundamental results not only highlight the importance of spin simulations in discrete models for topological magnetism, especially in 2D materials, but may also help to pave the way for solitonic devices based on atomically thin vdW heterostructures.

Rhombohedral ${\mathrm{CoTiO}}_3$ is a Dirac-type antiferromagnet with unique nodal magnetic dynamics, a strong Zeeman-phonon effect, and multiple spin-orbital excitations. The intricate interaction between these excitations makes it difficult to precisely understand its properties. We report the observation of strong spin-phonon coupling for several phonon modes, as well as the Zeeman splitting of the degenerate phonon modes with $E_g$ symmetry. These effects were observed in magnetic fields applied perpendicular to and within the $ab$-plane, and they evidence a phonon magnetic moment reaching up to a giant value of 1.63 ${\mu }_B$. Some of these modes experience extremely strong coupling with spin-orbital excitations of the ${\mathrm{Co}}^{2+}$ ions, manifested as avoided crossings, which leads to an underestimated magnitude of the phonon splitting at low fields. Both acoustic and optical magnon branches were observed in the antiferromagnetic phase, along with a two-magnon continuum within the 10–22 meV range, closely reflecting the magnon density of states. Observation of new magnetic modes above the spin-orientation transition at 17 T challenges the applicability of the flavor wave model to the spin dynamics in ${\mathrm{CoTiO}}_3$. Strong renormalization of the magnon and phonon modes (including zone-folded ones) suggests a low-field spin-orientation transition around 1 T within the $ab$-plane.

A fundamental understanding of ultrafast charge transfer dynamics is essential for the rational design of functional optoelectronic and photovoltaic devices. However, the underlying microscopic mechanisms of the charge transfer process at the heterojunction interface remain unclear, particularly regarding the specific role of the interfacial dipole moment and external electric field (EEF). Here, we systematically investigate the ultrafast interfacial charge transfer of $\mathrm{Hf}{\mathrm{S}}_2$/$\mathrm{Mo}X\mathrm{Se}$ ($X=\mathrm{S}$, Se) heterostructures using ab initio nonadiabatic molecular dynamics. The results demonstrate that the $\mathrm{Hf}{\mathrm{S}}_2/{\mathrm{MoSe}}_2$ heterostructure exhibits strong nonadiabatic coupling, and ultrafast interlayer hole and electron transfer occurring within 116 and 136 fs, respectively. Notably, the hole transfer dynamics are significantly suppressed in the symmetry-broken $\mathrm{Hf}{\mathrm{S}}_2$/MoSSe heterostructure, which is primarily attributed to the interfacial dipole substantially modifying both the charge distribution and interlayer coupling strength. Furthermore, within the $\mathrm{Hf}{\mathrm{S}}_2/{\mathrm{MoSe}}_2$ system, we elucidated the asymmetric modulation mechanism of EEFs on interlayer charge transfer dynamics: a negative EEF significantly slows down charge transfer, while a positive EEF accelerates electron transfer but suppresses hole transfer. The elucidation of this specific mechanism provides important theoretical insights for developing high-performance optoelectronic devices based on two-dimensional transition metal dichalcogenide heterostructures.

First-principles real-time time-dependent density-functional theory (rt-TDDFT) calculations reveal the existence of ballistic photocurrents generated by Coulomb scattering, which has not previously been considered as a mechanism for the bulk photovoltaic effect. With monolayer GeS as an example, it is predicted that ballistic currents can be comparable to shift currents under experimentally accessible conditions.

We show that periodically time-modulating the Dzyaloshinskii-Moriya interaction (DMI) in a two-dimensional magnon insulator may induce a topological phase transition that results in the presence of robust edge modes. To this end, we study a square lattice of spins interacting via an XXZ Heisenberg model with a ferromagnetic longitudinal coupling and antiferromagnetic transverse coupling, as well as the aforementioned time-modulated DMI. The topologically protected edge states of this system are composed of coherent superpositions of single-magnon excitations and two-magnon bound states. Furthermore, we show that the chirality of the edge states can be controlled by adjusting the relative phase for the drive on the DMI associated with nearest neighbors in the $x$ and $y$ directions.

Motivated by the experiment by Budden et al. [Nat. Phys. 17, 611 (2021)], who observed signatures of long-lived photoinduced superconductivity, we develop an accurate analytical and computational approach to nonequilibrium superconductivity following a quench. We consider the BCS-Holstein model, which includes both integrable local electron-electron interactions and integrability-breaking electron-phonon coupling. We develop the Keldysh-Eliashberg theory on the Kadanoff-Baym contour, which enables us to describe nonequilibrium dynamics of the superconductor. We consider a quench in interactions, which results in a dynamic transition from the initial superconducting state to a normal thermal state in the end of the evolution. It is shown that the dynamics contain two stages: The early-time integrable behavior, involving coherent oscillations of the superconducting order parameter, crosses over to the late-time ergodic dynamics exhibiting a thermal decay into an equilibrium state. In the former regime, our computational approach both reproduces exact analytical results on the integrable dynamics of the order parameter (obtained for the zero-temperature initial state) and generalizes those to the case of an initial thermal state. The method also succeeds in describing both integrable-to-thermalizing crossover and the late-time thermal decay, which is shown to be consistent with the time-dependent Ginzburg-Landau theory (with the exponential decay time dependent on the density of quasiparticle excitations). We observe the electron distribution function approaching the Fermi-Dirac thermal distribution at final stages. The details of two-time nonequilibrium dynamics depend on the density of quasiparticles in the initial state and the integrability-breaking parameters, which under certain conditions may result in a long-lived transient superconductivity consistent with experiment.

We develop a method to construct simple and explicit variational approximations for the ground state of Kitaev's honeycomb model with a nontrivial ${\mathbb{Z}}_2$ flux configuration consisting of a pair of visons on neighboring plaquettes. The method consists of retaining only the largest singular values of the generator of the transformation between the vison pair and flux-free ground states. We compare physical quantities calculated using the approximate state to those obtained by extrapolating the results of exact diagonalization of finite lattices, finding them to be in very good agreement. We discuss ways to extend the method to more complicated flux configurations.

We formulate a path-integral Monte Carlo algorithm for simulating lattice systems consisting of fictitious particles governed by generalized exchange statistics. This method, initially proposed for continuum systems, introduces a continuous parameter $\xi$ in the partition function that interpolates between bosonic $(\xi =1)$ and fermionic $(\xi =-1)$ statistics. We generalize this approach to discrete lattice models and apply it to the two-dimensional Hubbard model of fictitious particles, including the Bose- and Fermi-Hubbard models as special cases. By combining reweighting and $\xi$-extrapolation techniques, we access both half-filled and doped regimes. In particular, we demonstrate that the method remains effective in strongly correlated, doped systems, offering a distinct advantage by naturally operating in the canonical ensemble. Our results validate the applicability of the fictitious-particle framework to lattice models and establish it as a promising, complementary tool for investigating strongly interacting fermionic systems.

${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{Se}$ is an emerging high-performance layered semiconductor with excellent stability. While experimental studies have explored carrier transport across various doping levels for both $n$-type and $p$-type conduction, a comprehensive theoretical understanding remains incomplete. In this work, we present parameter-free first-principles calculations of the electron and hole mobilities in ${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{Se}$, based on iterative solution of the Boltzmann transport equation that includes electron-phonon scattering and ionized impurity scattering on an equal footing. Intriguingly, we find that ${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{Se}$ exhibits high electron mobilities in both the in-plane and out-of-plane directions, whereas the hole mobilities are only significant in the in-plane direction, displaying a unique three-dimensional electron transport and two-dimensional hole transport behavior. At 300 K, the calculated intrinsic electron and hole mobilities along the in-plane direction are 447 ${\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ and 29 ${\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$, respectively, which are primarily affected by Fröhlich electron-phonon interactions. Due to its large static dielectric permittivity, ${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{Se}$ exhibits exceptionally high low-temperature electron mobilities above $1.0\times {10}^5{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$, and its electron mobilities above 50 K are robust against ionized impurity scattering over a wide range of impurity concentrations. By incorporating the Hall effect into our analysis, we predict an in-plane electron Hall mobility of 517 ${\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ at 300 K, in excellent agreement with experimental data. These results provide valuable insights into the carrier transport mechanisms in ${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{Se}$, and offer predictive benchmarks for future theoretical and experimental investigations.

Understanding and engineering phonon transport in GaN/AlN superlattices (SLs) is critical for the thermal management of GaN high-electron-mobility transistors (HEMTs). While most studies suggest that interfacial roughness reduces the thermal conductivity of SLs, a few have reported its unexpected enhancement, yet the underlying mechanism, particularly concerning phonon coherence, remains elusive. In this study, we address this gap by developing ametric—the concentration degree of energy (CDE) in $k$-space—to directly quantify the degree of phonon coherence from atomic displacements obtained through lattice dynamics. Our simulations reveal that interfacial roughness can enhance the thermal conductivity of SLs, provided that the inherent periodicity of the SL is not disrupted. A mode-resolved coherence analysis based on the CDE metric reveals that while increased roughness leads to a rise in incoherent modes, it simultaneously enables more modes to propagate. Meanwhile, the preservation of periodicity is the decisive factor that significantly enhances phonon coherence. This insight successfully explains why SLs with moderate, periodicity-preserving roughness can achieve superior phonon coherence and higher thermal conductivity than ideal SLs. The coherence analysis framework is applicable to general SL systems.

Manipulating the valley degree of freedom in monolayer transition metal dichalcogenides (TMDs) is essential for advancing valleytronics-based information processing and transport. However, significant intervalley scattering causes valley signals to vanish at room temperature, posing a major challenge. To mitigate this, strategies involving magnetic fields, intense circularly polarized light, and in-plane electric fields have been explored to enhance valley polarization. Recently, increasing the excitons lifetimes using photonic nanostructures has emerged as a promising approach. In this work, we present a phenomenological model based on rate equations that describes and regulates the valley depolarization process in TMDs embedded within engineered photonic environments. Our findings demonstrate the potential to achieve near-unity valley polarization and enable negative valley polarization of dark excitons. Additionally, we identify extrinsically chiral nanocavities as effective platforms for generating chiral emission and spatially separating valley states. This research offers valuable insights for the nanophotonic design of devices with enhanced valley control, including manipulation of valley-polarized dark excitons, and paves the way for the development of advanced integrated valleytronic systems.

We study the structural relaxation and electronic properties of a one-dimensional (1D) moiré system composed of a zigzag graphene nanoribbon (GNR) placed on a hexagonal boron nitride $\left(h\text{−}\mathrm{BN}\right)$ substrate. Using an effective grid model derived from continuum elasticity theory, we calculate the relaxed atomic structure of the GNR/$h$-BN system for various twist angles and ribbon widths. The relaxation gives rise to a characteristic 1D domain structure consisting of alternating commensurate ${\mathrm{AB}}^{\prime }$ regions and two distinct types of domain boundaries. At finite twist angles, the ribbon adopts a wavy shape, locally tracing the $h$-BN zigzag direction but occasionally sliding to adjacent atomic rows. The resulting moiré potential strongly modulates the electronic structure: the zero-energy zigzag edge states are modulated by the local stacking, leading to densely packed subbands in the ${\mathrm{AB}}^{\prime }$ domains and sharply localized domain-wall states in the energy gaps between the domain subbands, which together realize gate-tunable one-dimensional arrays of quantum-confined electronic states. Our results demonstrate that moiré modulation in GNR/$h$-BN heterostructures provides a versatile platform for electronic structure engineering and the design of 1D moiré nanodevices.

An external orbital magnetic field applied perpendicular to the separation vector of a pair of Weyl points can couple them and induce a gap in the electronic spectrum. In this work, we investigate the gap-opening behavior in the presence of a lattice, revealing rich phenomenology absent in the continuum picture. Specifically, we address the emergence of layered Chern insulating states, examining how the anisotropy of the Weyl cone dispersion influences the sequence of phase transitions, and establishing connections to the continuum limit. We analyze the evolution of surface Fermi-arc states across these regimes, highlighting their distinct behaviors during the gap-opening transitions.