We propose a theory for nonlocal measurements of the nonlinear valley Hall effect (VHE). In contrast to the well-known reciprocity between direct and inverse VHEs in linear responses, we unveil that the nonlinear inverse VHE needed to generate nonlocal voltage is not reciprocal to the nonlinear VHE it probes, but rather has distinct symmetry characters and physical origins. Particularly, while all previously studied VHEs are valley-odd, the nonlinear inverse VHE is valley-even. This fundamental distinction gives rise to unprecedented scaling behaviors in valley mediated nonlocal transport, providing essential experimental hallmarks of nonlinear valley responses. Layer groups that permit the proposed nonlocal valley responses are identified. Sizable nonlocal signals from nonlinear valley responses are revealed in bilayer $T_d\text{−}{\mathrm{WTe}}_2$ through first-principles calculations. Our work lays a foundation for all-electrical detection of nonlinear valley-charge interconversion, which constitutes the basis of emerging nonlinear valleytronics.

We investigate the high-pressure polymorphism of $\mathrm{Ti}{\mathrm{O}}_2$ under laser-shock compression from 54(5) to 137(7) GPa using in situ femtosecond x-ray diffraction. Our results provide experimental evidence of the $Pca{2}_1$-type distorted fluorite structure formed from polycrystalline $\mathrm{Ti}{\mathrm{O}}_2$ dynamically compressed to 54(5) GPa. Upon higher compression, we observe the direct formation of the ninefold coordinated $\mathrm{F}{\mathrm{e}}_2\mathrm{P}$-type phase at 68(4) and 78(3) GPa in polycrystalline and [001]-oriented $\mathrm{Ti}{\mathrm{O}}_2$, respectively. This represents an unprecedented 100 GPa reduction in the shock synthesis pressure of the $\mathrm{F}{\mathrm{e}}_2\mathrm{P}$-type structure relative to quasihydrostatic loading conditions. On pressure release, the $\mathrm{F}{\mathrm{e}}_2\mathrm{P}$-type phase transforms to the α-$\mathrm{Pb}{\mathrm{O}}_2$ structure and, at later times, reverts to rutile. Thus, the rutile $\to \mathrm{F}{\mathrm{e}}_2\mathrm{P}$ and α-$\mathrm{Pb}{\mathrm{O}}_2\to \mathrm{rutile}$ transformations are both observed to occur on nanosecond timescales. Our results highlight the unique ability of high-strain-rate uniaxial compression to synthesize novel high-pressure phases and also indicate the importance of in situ atomic-level probes in developing pressure-temperature phase diagrams.

Quantum scars have recently been directly visualized in graphene quantum dots [Nature (London) 635, 841 (2024)], revealing their resilience and influence on electron dynamics in mesoscopic systems. Here, we examine variational scarring in two-dimensional quantum dots and demonstrate that these states remain robust even in an open system. We show that controlled perturbations enable modulation of electronic transmission via scarred states, presenting a viable approach to tuning quantum transport. These findings provide insights into the role of scarring in mesoscopic transport and open pathways for experimental realization in quantum devices.

The high thermal sensitivity of upconversion nanoparticles (UCNPs) luminescence at the nanoscale enables precise and rapid temperature detection, advancing nano-thermometry applications in quantum optics, catalytic reactions, and nanophotonics. Here, the phonon-assisted upconversion luminescence with a high thermal sensitivity is achieved by rare-earth doped UCNPs (${\mathrm{NaYF}}_4:{\mathrm{Yb}}^{3+}/{\mathrm{Er}}^{3+}$) in vacuum optical tweezers. We found that an essential contribution of phonon-assisted luminescence processes leading to high thermal sensitivity is not only from increasing the absorption cross section of ${\mathrm{Yb}}^{3+}$ but also from facilitating efficient energy transfer from ${\mathrm{Yb}}^{3+}$ to ${\mathrm{Er}}^{3+}$ ions. The increase in thermal sensitivity with the number of phonons involved in the progress of phonon-assisted luminescence can be well described by the temperature-dependent phonon population model. The study reveals how phonon-assisted processes influence thermal sensitivity in UCNPs, contributing to a deeper understanding of nanoscale temperature-dependent luminescence.

Although the relationship between magnetostriction and magnetic damping is often described phenomenologically, their intrinsic connection remains unclear. In this Letter, we demonstrate that the magnitude of magnetic damping depends on the sign of magnetostriction in (${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{)}_4\mathrm{N}$ and ${\mathrm{Ni}}_{1-y}{\mathrm{Co}}_y$ alloys across various compositions, consistent with experimental observations. This behavior is attributed to strain-induced changes in exchange splitting, which shift the minority-spin density of states near the Fermi level, thereby affecting both magnetostriction and damping through spin-conserving transitions. Additionally, the presence of locally degenerate orbitals plays a crucial role in determining magnetostriction. These findings suggest that magnetization dynamics and magnetostriction can be intrinsically controlled, facilitating the design of magnetic materials for applications such as flexible spintronics.

We conducted x-ray absorption (XAS) and magnetic circular dichroism (XMCD) measurements at the Co $L_{2,3}$ edges on single crystals of the Kitaev candidate honeycomb lattice compound ${\mathrm{BaCo}}_2{\left({\mathrm{AsO}}_4\right)}_2$. The measurements employed the inverse partial fluorescence yield technique, which is ideal for acquiring reliable x-ray absorption spectra from highly insulating samples, enabling precise quantitative analysis. Our experimental results revealed a significant linear dichroic signal, indicating strong trigonal distortion in the ${\mathrm{CoO}}_6$ octahedra in ${\mathrm{BaCo}}_2{\left({\mathrm{AsO}}_4\right)}_2$. We performed a detailed analysis of the experimental XAS and XMCD spectra using a full-multiplet configuration-interaction cluster model. This analysis unveiled that the $t_{2g}$ hole density is predominantly localized in the $a_{1g}$ orbital. Through XMCD sum rules and theoretical calculations, we quantified both the spin and orbital magnetic moments. Our study demonstrates that the local electronic structure of the ${\mathrm{CoO}}_6$ octahedra displays an effective trigonal distortion of approximately $-0.114$ eV. This distortion is larger than the Co $3d$ spin-orbit coupling constant, emphasizing the crucial impact of local structural distortions on the electronic and magnetic properties of ${\mathrm{BaCo}}_2{\left({\mathrm{AsO}}_4\right)}_2$.

Continuum atomic relaxation models for twisted bilayer graphene involve minimization of the sum of intralayer elastic energy and interlayer adhesion energy. The elastic energy favors a rigid twist, i.e., no distortion in the twisted honeycomb lattices, while the adhesion energy favors Bernal stacking and breaking the relaxation into triangular AB- and BA-stacked domains. We compare the results of two relaxation models with the published Bragg interferometry data, finding good agreement with one of the models. We then provide a method for finding a highly accurate approximation to the solution of this model which holds above the twist angle of $\approx 0.7^{\circ }$ and thus covers the first magic angle. We find closed-form expressions in the absence as well as in the presence of external heterostrain. These expressions are not written as a Taylor series in the ratio of adhesion and elastic energy because, as we show, the radius of convergence of such a series is too small to access the first magic angle.

We propose a bosonic quantum breakdown Hubbard model, which generalizes the Bose-Hubbard model by adding an asymmetric breakdown interaction turning one boson into two between adjacent sites. When the normal hopping is zero, this model has a global exponential U(1) symmetry, and we show that the ground state undergoes a first-order phase transition from a Mott insulator (MI) to a spontaneously symmetry breaking (SSB) breakdown condensate as the breakdown interaction increases. Surprisingly, the SSB breakdown condensate does not have a gapless Goldstone mode, which invalidates the Mermin-Wagner theorem and leads to stable SSB in one dimension. Moreover, we show that the quench dynamics of a boson added to MI exhibits a dynamical transition from dielectric to breakdown phases, which happens at a larger breakdown interaction than the ground-state phase transition. Between these two transitions, the MI (dielectric) state is a false vacuum stable against dynamical breakdown. Our results reveal that quantum models with unconventional symmetries such as the exponential symmetry can exhibit unexpected properties.

The interaction between graphene and hexagonal boron nitride (hBN) plays a pivotal role in determining the electronic and structural properties of graphene-based devices. In this work, we employ quantum Monte Carlo (QMC) to study the interlayer interactions and stacking-fault energy (SFE) between graphene and hBN. We generated QMC energies for several rigid bilayer stacking configurations and fitted these data to the Kolmogorov-Crespi type interlayer potential (ILP) model. Our QMC-derived potential offers a more reliable alternative to conventional density functional theory methods, which are prone to errors in predicting properties in van der Waals materials. This study enables highly accurate predictions of structural and electronic properties in graphene/hBN heterostructures. The resulting ILP-QMC potential is made available for further use in simulating complex systems, such as twisted bilayer graphene (TBG) on hBN.

Although the strongly interacting flat bands in twisted bilayer graphene (TBG) have been approached using the minimal Bistritzer-MacDonald Hamiltonian, there is mounting evidence that strain and lattice relaxation are essential in correctly determining the order of the correlated insulator ground states. These effects can be incorporated in an enhanced continuum model by introducing additional terms computed from the relaxation profile. To develop an analytical and physical understanding of these effects, we include strain and relaxation in the topological heavy fermion (HF) model of TBG. We find that strain and relaxation are very well captured in first-order perturbation theory by projection onto the fully symmetric HF Hilbert space and remarkably do not alter the interacting terms in the periodic Anderson model. Their effects are fully incorporated in the single-particle HF Hamiltonian and can be reproduced in a minimal model with only four symmetry-breaking terms. Our results demonstrate that the heavy fermion framework of TBG is an efficient and robust representation of the perturbations encountered in experiment.

The topological heavy fermion (THF) model of twisted bilayer graphene is a framework for treating its strongly interacting topological flat bands. In this work, we employ the THF model with heterostrain and particle-hole symmetry-breaking corrections to study its symmetry-broken ground states. We find that the heterostrain correction motivates a specific parent-state wave function which dictates the presence or absence of an incommensurate Kekulé spiral (IKS) at each integer filling by invoking Dirac node braiding and annihilation as a mechanism to achieve low-energy gapped states. We then show that one-shot Hartree-Fock faithfully replicates the numerical results of fully self-consistent states and motivates an analytical approximation for the IKS wave vector. We can also account for the particle-hole asymmetry in the correlated insulator gaps. In particular, the THF model predicts stronger correlated states on the electron side rather than hole side in agreement with magic angle experiments, despite the electron side being more dispersive in the single-particle band structure. This work demonstrates that we can analytically explain even the more subtle symmetry-breaking order properties observed in experiments where heterostrain, relaxation, and interactions together determine the ground state.

We introduce a computationally efficient method to calculate the quasiparticle band structure of general van der Waals (vdW) heterostructures. A layer-projected scissors (LAPS) operator, which depends on the one-body density matrix, is added to the density functional theory Hamiltonian. The LAPS operator corrects the band edges of the individual layers for self-energy effects (both intralayer and interlayer) and unphysical strain fields stemming from the use of model supercells. The LAPS operator is treated self-consistently whereby charge redistribution and interlayer hybridization occurring in response to the band energy corrections are properly accounted for. We present several examples illustrating both the qualitative and quantitative performance of the method, including ${\mathrm{MoS}}_2$ films with up to 20 layers, bilayer ${\mathrm{MoS}}_2$ in an electric field, lattice-matched ${\mathrm{MoS}}_2\text{/}{\mathrm{WS}}_2$ and ${\mathrm{MoSe}}_2\text{/}{\mathrm{WSe}}_2$ bilayers, and ${\mathrm{MoSe}}_2\text{/}{\mathrm{WS}}_2$ moiré structures. Our work opens the way for predictive modeling of electronic, optical, and topological properties of experimentally relevant vdW materials, including twisted homobilayers and incommensurate heterostructures.

We investigate the Josephson diode effect in a superconducting interferometer under nonequilibrium conditions. In contrast to its thermodynamic counterpart, which requires the simultaneous breaking of time-reversal and inversion symmetry, we demonstrate that a diodelike asymmetry of the critical current can emerge solely due to a dissipative current in the normal region of an otherwise symmetric Josephson junction. This effect is driven entirely by the nonequilibrium conditions, without the need for additional inversion symmetry breaking. Using the standard quasiclassical Keldysh Green's function formalism, we explicitly calculate the diode coefficient from the supercurrent-phase relation of the interferometer. Remarkably, within certain ranges of control parameters, such as applied voltage, temperature, and the geometric aspect ratio of the device, the diode coefficient can exceed its nominal perfect value.

Searching for a ferromagnetic (FM) quantum critical point of a compositionally tuned system, we have investigated experimentally the quantum phase transition (QPT) in the ${\mathrm{FeGa}}_{3-x}{\mathrm{Ge}}_x$ ($x=0.0-0.16$) metallic quantum ferromagnet, by using crystallographically oriented single-crystalline samples with the Ge contents $x$ below and within the quantum critical regime. Performing the measurements of dc and ac magnetic susceptibility and $M\left(H\right)$ curves down to the temperature of 0.4 K in low magnetic fields 0.01–25 mT, and the measurements of electrical resistivity and specific heat down to 0.35 K, we found that there is no direct, continuous transition from the paramagnetic to the FM state and consequently no FM quantum critical point in the compositional phase diagram, but the QPT involves an intermediate canonical spin glass (SG) state at $x\approx$ 0.12–0.16. The compositional region of the SG state is narrow, it is formed at low temperatures (the spin freezing temperatures are in the range 1.1–1.8 K) and the coupling between the spins is weak, so that the SG ordering is fragile with respect to the external magnetic field. The analysis of the ac susceptibility via the Cole-Cole diagrams in the QPT region has revealed that the slowing-down spin dynamics of the SG state remains thermally activated down to the lowest investigated temperature of 0.4 K, so that the regime of quantum fluctuations is not yet entered. The employed experimental conditions have enabled us to follow the formation of fragile magnetic ordering in the ${\mathrm{FeGa}}_{3-x}{\mathrm{Ge}}_x$ compositionally tuned system in its infancy state.

Topological magnons, quantized spin waves featuring nontrivial boundary modes, present a promising route toward lossless information processing. Realizing practical devices typically requires magnons excited in a controlled manner to enable precise manipulation of their topological phases and transport behaviors. However, their inherent charge neutrality and high-frequency nature pose a significant challenge for nonvolatile control, especially via electric means. Herein, we propose a general strategy for electrical control of topological magnons in bilayer ferromagnetic insulators. With strong spin-layer coupling, an applied vertical electric field induces an interlayer potential imbalance that modifies intralayer Heisenberg exchanges between adjacent layers. This electric-field-driven modulation competes with the bilayer's intrinsic Dzyaloshinskii-Moriya interaction, enabling the accurate tuning of the band topology and nonreciprocal dynamics of magnons. More importantly, such an electric control mechanism exhibits strong coupling with external magnetic fields, unveiling different perspectives on magnetoelectric coupling in charge-neutral quasiparticles.

We report the observation of spin-dependent force induced by nitrogen vacancy (NV) centers embedded in a diamond crystal attached to a tethered oscillator. This result was obtained using a spin-dependent torque generated by a microdiamond containing billions of NV centers, placed at the end of a commercially available silicon cantilever. Our experiment highlights the intricate interplay between the spin properties of NV centers and the mechanical behavior of the oscillator in the presence of a pure torque, providing a set of tools for exploring quantum effects in state-of-the-art micromechanical systems.

Attosecond extreme-ultraviolet (XUV) transient absorption spectroscopy measurements on the Peierls-distorted phase of the semimetal antimony (Sb) are presented. After excitation by an ultrashort, broad band near-infrared (NIR) pulse, the distortion is (partly) lifted causing the well-known coherent phonon motion of the lattice. While the overall observed dynamics generally follow a displacive excitation model, a delayed onset of the pump-induced carrier dynamics because of hot-carrier thermalization is observed, as well as a large spectral phase dependence in the coherent phonon oscillation. This is attributed to significantly different carrier relaxation timescales for carrier energies above and near the Fermi level of the semimetal and corroborated by a simple theoretical model that considers the carrier relaxation timescales in the displacive phonon model to explain the observed dynamics. Our results provide direct experimental evidence about the role of carrier-relaxation in the origin of displacive coherent phonon motion.

The average structure of the solid solution ${\mathrm{LaMn}}_{1-x}{\mathrm{Ga}}_x{\mathrm{O}}_3$ (LMGO) has been investigated from a symmetry-motivated approach utilizing synchrotron x-ray and neutron powder diffraction techniques. We show experimentally that a trilinear coupling term $\left({\mathrm{\Gamma }}_5^{+}{\mathrm{M}}_2^{+}{\mathrm{M}}_3^{+}\right)$ between shear strain, octahedral rotation, and the $C$-type orbital ordering mode is responsible for driving the orthorhombic to pseudocubic phase transition occurring in the composition range $0.5<x<0.6$. Our Monte Carlo simulations elucidate the macroscopic origin of this coupling to shear strain, and point to its importance with respect to controlling the orbital order-disorder transitions. We find that the emergence of the pseudocubic state can be rationalized by considering the competition between this trilinear term and a linear-quadratic term of the out-of-phase octahedral tilting with strain $\left({\mathrm{\Gamma }}_5^{+}{\left({\mathrm{R}}_5^{-}\right)}^2\right)$. Illustrating the general nature of these results, we construct a simple function that captures the change in Landau free energy at the order-disorder transition, in parameters that are trivial to relate to the concentration of Jahn-Teller active species, temperature, tolerance factor, and unit cell strain, for a broad range of manganite perovskites. Our results point to the fact that far from the pseudocubic state being a symptom of orbital disorder, in many cases, it is more correctly to view it as a cause. The results have a broad impact on the study of orbital ordering physics in the perovskite materials and on chemical and physical control parameters through which to tune the richness of the intertwined physical properties.

$G$-type antiferromagnetic ${\mathrm{BiFeO}}_3$ is shown to be an altermagnet. We present the band structure using an unconventional scheme designed to highlight the distinctive spin splitting which is characteristic of altermagnets. We define and show plots of the spin-splitting function in reciprocal space. We show that the nodal surfaces of the spin-splitting function that follow from symmetry can be classified into two types, which we call symmetry enforced and continuity-enforced. We describe the spin-splitting function with a simple parametrization in a basis of symmetry-adapted plane waves. Using group-theory analysis based on irreducible representations of the crystallographic Laue group, we confirm that the altermagnetism of $G$-type ${\mathrm{BiFeO}}_3$ is $g$ wave and present a complete classification table for the general three-dimensional case. Finally, we discuss the effect of ferroelectric switching on the altermagnetic order, and identify three classes of ferroelectric altermagnets.

As an unconventional magnet, altermagnetism attracts great interest in condensed-matter physics and applies a new research platform for the spintronics. Since the tunneling magnetoresistance (TMR) effect is an important research aspect in spintronics, we theoretically propose a universal altermagnetic sandwich device to achieve the TMR effect and investigate its transport properties. Using the nonequilibrium Green's function method and the Landauer-Büttiker formula, we obtain the conductance and the TMR ratio. By systematically rotating the orientations of the altermagnet and spin, we investigate how the altermagnetic orientations affect the conductance and the TMR ratio and comprehensively demonstrate the dependence of the conductance and the TMR ratio on a range of parameters in the system. By tuning the altermagnetism strength and the Fermi energy, as well as rotating the orientations in the altermagnet, the TMR ratio can reach a value of over 1000%. In addition, we analyze the detailed symmetry relations of the conductance and the TMR ratio in our system. Our approach provides a new design concept for the next-generation information technologies based on the altermagnetic platform, paving the way for the development of spintronics applications.