This study investigated the formation and relaxation dynamics of nuclear spin polarization in three ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ bulk samples with different aluminum concentrations $x$ of 0.00, 0.05, and 0.15. The time-resolved Kerr rotation technique was primarily used. The samples with $x=$ 0.15 and 0.05 exhibited anomalously large nuclear magnetic fields $B_{\mathrm{N}}$ exceeding 1 T, approximately twice the applied magnetic field. Further investigations revealed that $B_{\mathrm{N}}$ formation occurred in two stages: a rapid initial rise followed by a gradual increase toward a saturation value. Relaxation measurements revealed that the relaxation time of $B_{\mathrm{N}}$ was longer for AlGaAs than for GaAs. The comparison of the results obtained under strong and weak magnetic fields indicated the suppression of quadrupole-induced relaxation. We modified the dynamics model of nuclear spin polarization and explained the two-stage formation and the accompanying large $B_{\mathrm{N}}$ in AlGaAs bulks.

Head-to-head ferroelectric domain walls are intrinsically charged, and are typically compensated by a mix of oppositely charged defects and free electrons. The free electrons form a two-dimensional electron gas (2DEG) along the domain wall. In many cases, inversion symmetry is broken at the wall, which implies that the 2DEG is subject to nontrivial spin-orbit coupling. Here, we use symmetry arguments to construct a generic six-band tight-binding electronic Hamiltonian for a ${90}^{\circ }$ head-to-head ferroelectric domain wall. The model, which includes spin-orbit physics and has a multiorbital $t_{2g}$ band structure that is common to transition-metal perovskites, is applied to ${\mathrm{BaTiO}}_3$. We find that the 2DEG develops an Ising spin texture, with spins aligned perpendicular to the domain wall. We contrast this with the Rashba spin texture that should emerge at weakly conducting ${90}^{\circ }$ head-to-tail domain walls. We then show that the head-to-head domain walls should have a measurable Edelstein effect (that is, a current-induced magnetization), even in the dilute limit and at room temperature, and describe a simple experiment to measure it.

In commensurate twisted homobilayers, purely radial Rashba spin-orbit fields can emerge. We employ first-principles calculations to investigate the band structures and the spin-orbit fields close to the high-symmetry points K and $\mathrm{\Gamma }$ of several commensurate twisted transition-metal dichalcogenide homobilayers: ${\mathrm{WSe}}_2, {\mathrm{NbSe}}_2$, and ${\mathrm{WTe}}_2$. The observed in-plane spin textures are mostly radial, and the main features are successfully reproduced using a model Hamiltonian based on two effective-mass models, including spin-orbit coupling, and a general (spin-conserving) interlayer coupling. Extracting the model Hamiltonian parameters through fitting of several twisted supercells, we find a twist angle dependency of the magnitude of the radial Rashba field, which is symmetric not only around the untwisted cases ($\mathrm{\Theta }=0^{\circ }$ and $\mathrm{\Theta }={60}^{\circ }$), but also around $\mathrm{\Theta }={30}^{\circ }$. Furthermore, we observe that the interlayer coupling between the $\text{K}\left(\text{or}{\text{K}}^{\prime }\right)$ points of the two layers decreases with the increase of the size of the commensurate supercells. Hence, peaks of high interlayer coupling can occur only for twist angles, where small commensurate supercells are possible. Exploring different lateral displacements between the layers, we confirm that the relevant symmetry protecting the radial Rashba is an in-plane ${180}^{\circ }$ rotation axis. We additionally investigate the effects of atomic relaxation and modulation of the interlayer distance. Our calculations on ${\mathrm{WTe}}_2$ bilayers show that their lack of $C_3$ symmetry results in spin textures that are neither radial nor tangential. Our results offer fundamental microscopic insights that are particularly relevant to engineering spin-charge conversion schemes based on twisted layered materials.

In one dimension, any disorder is traditionally believed to localize all states. We show that this paradigm breaks down under disorder with enhanced hyperuniformity, which suppresses long-wavelength fluctuations and interpolates between random and periodic potentials. In tight-binding chains, strong hyperuniformity induces a sharp delocalization transition and the emergence of mobility edges. The transition is identified by the generalized fractal dimension and corroborated by the scaling of localization length and transmittance. Hyperuniform disorder thus provides a general mechanism for engineering mobility edges and controlling transport in low dimensions.

We construct a dual symmetry-protected topological Hamiltonian for the U(1) symmetric anisotropic spin-$\frac{1}{2}$ Heisenberg chain, a model that has traditionally been used to study spontaneous symmetry breaking in both ferromagnetic and antiferromagnetic phases, with an intervening extended Luttinger liquid phase. By performing a nonlocal unitary transformation, we explicitly construct a local fermionic Hamiltonian that exhibits two nontrivial topological phases separated by an extended Luttinger liquid regime. We demonstrate the topological nature of these phases by analyzing the entanglement structure, deriving a nonlocal string order parameter and constructing an exact zero-mode operator that connects states in different fermionic parity sectors.

Electron-magnon coupling at the interface between a normal metal and a magnetically ordered insulator modifies the electrical conductivity of the normal metal, an effect known as spin-Hall magnetoresistance. It can also facilitate magnon-mediated electric current drag, the nonlocal electric current response of two normal metal layers separated by a magnetic insulator. Additionally, spin and heat transport are coupled both in the magnetic insulator and across the interfaces to normal metals. In this article, we present a theory of these spintronic and spin-caloritronic effects for time-dependent applied electric fields $E\left(\omega \right)$, with driving frequencies $\omega$ up to the terahertz regime. Our model describes how the dominant transport mechanism, coherent or incoherent magnons, evolves with the driving frequency $\omega$.

Controlling magnetic order on ultrafast timescales, driven by spintronic and recording applications, is one of the main directions of current research in magnetism. Despite major advances in understanding the temporal evolution of magnetic order upon its emergence or quenching, experimental demonstration of the local link between microstructure and dynamic nucleation is missing. Here, taking advantage of the high structural and magnetic resolution of in situ transmission electron microscopy, we observe that cumulative laser irradiation significantly alters the nucleation pathway of the first-order antiferromagnetic to ferromagnetic phase transition of FeRh thin films, causing the transition to switch from homogeneous to heterogeneous nucleation. This leads to a decrease of 20 K in transition temperature and the emergence of submicron magnetic vortices as preferential nucleation motifs. These vortices are pinned in the film by underlying dislocation networks. We observe that the dislocation networks are formed and rearranged upon repeated crossing of the phase transition using both femtosecond and picosecond laser pulses. Our results establish a direct link between defect formation and the microscopic morphology of the nucleated ferromagnetic phase, with broad implications for ultrafast stroboscopic experiments and defect-mediated phase transitions in functional materials.

Soft phonon modes in tetragonal CeCoSi, which undergoes a structural transition at $T_0=12\mathrm{K}$ followed by antiferromagnetic order at $T_{\text{N}}=9.5\mathrm{K}$, have been investigated using high-resolution inelastic x-ray scattering. Pronounced softening was detected in the transverse acoustic modes corresponding to the $(yz+zx)$-type monoclinic distortion, consistent with the experimentally determined triclinic structure. Remarkably, the softening persists up to the zone boundary along $(0,0,q)$, indicating a short correlation length of the lattice instability. This instability, characterized by a Curie-type strain susceptibility, is interpreted as a consequence of the on-site $4f\text{−}5d$ hybridization, which is intrinsic to this crystal structure due to the lack of inversion symmetry at the two Ce sites.

We systematically investigate whether classical hydrodynamic field theories can predict the long-time dynamics of many-particle quantum systems. We study both numerically and analytically the time evolution of a chain of spins (or qubits) subjected to stroboscopic dynamics. The time evolution is implemented by a sequence of local and nearest-neighbor gates that conserve the total magnetization. The long-time dynamics of such a system is believed to be describable by a hydrodynamic field theory, which, importantly, includes the effect of noise. Based on a field theoretical analysis and symmetry arguments, we map each operator in the spin model to the corresponding fields in hydrodynamics. This allows us to predict which expectation values decay exponentially and which decay with a hydrodynamic long-time tail. We illustrate these findings by studying the time evolution of all 255 Hermitian operators that can be defined on four neighboring sites. All operators not protected by hydrodynamics decay exponentially, while the others show a slow hydrodynamic decay. While most hydrodynamic power laws seem to follow the analytical predictions, we also discuss cases where there is an apparent discrepancy between analytics and the finite-size numerical data.

Anomalous thermalization, particularly the crossings of temperature trajectories from different initial states termed Mpemba crossings (MPCs), has intrigued scientists for decades. While recent studies in quantum systems suggest that initial conditions play a decisive role in their emergence, they offer limited insight into MPCs in complex, highly nonequilibrium systems. In this study, we investigate temperature dynamics in the strongly coupled, quantum chaotic Sachdev-Ye-Kitaev (SYK) model, which is dual to the low-energy dynamics of 2D dilaton gravity. Our findings reveal a dynamically driven nonequilibrium mechanism underlying MPCs during rapid thermalization, with implications for gravitational systems. We explore quench dynamics in SYK systems under three conditions: coupling to a single SYK thermal bath, coupling to two thermal baths at different temperatures, and dissipative SYKs modeled by the Lindblad equation. We find that strong system-bath coupling induces oscillating effective temperatures and trajectory crossings in transient states due to nonequilibrium statistics, phenomena absent in quasistatic thermodynamics and Lindbladian SYKs. These MPCs highlight a unique feature of anomalous thermalization of strongly coupled quantum chaotic systems driven far from equilibrium. Besides, the results also provide qualitative insights into the nonequilibrium thermodynamics of black holes strongly interacting with their environment, such as primordial black holes in the early universe.

The interplay between electronic and lattice degrees of freedom in the insulator-to-metal transition (IMT) in rare-earth nickelates is a long-standing question. In the present work, broadband ultrafast transient reflectivity (TR) spectroscopy is applied to study the photoinduced IMT in ${\mathrm{NdNiO}}_3$. Both coherent and incoherent terms of the TR signal show discontinuous behavior around the same pump fluence value. A drastic drop in the sample reflectivity appearing at $\sim 100$ fs timescale in the high excitation density regime indicates the closing of the gap across the IMT. In this regime, coherent phonons associated with the low-temperature crystal phase are not observed even at early time delays, indicating an ultrafast transformation of the lattice potential. A detailed analysis of the coherent phonons indicates a strong coupling between some phonon modes, electronic excitations, and possibly the magnetic order. In this study, we provide insights into the ultrafast dynamics of rare-earth nickelates.

We present a combined theoretical and experimental study of angle-dependent parametric pumping of magnons in yttrium iron garnet films, with a focus on the mechanisms that transfer parametrically injected magnons toward the spectral minimum where Bose-Einstein condensation occurs. Using a classical Hamiltonian formalism, we analyze the threshold conditions for parametric instability as a function of the angle ${\alpha }_p$ between the microwave pumping field ${\mathit{h}}_p\left(t\right)$ and the external magnetic field ${\mathit{H}}_{\mathrm{ext}}$, continuously tracing the transition between parallel (for ${\alpha }_p=0$) and transverse pumping (for ${\alpha }_p=\pi /2$). We also describe two competing four-magnon scattering mechanisms that transfer parametric magnons toward the bottom of their frequency spectrum: The step-by-step Kolmogorov-Zakharov cascade, which is allowed for all values of $H_{\mathrm{ext}}$, and the kinetic instability mechanisms that provide a much more efficient single-step channel in transferring magnons directly to the lowest-energy states but occurs within specific regions of ${\alpha }_p$ and $H_{\mathrm{ext}}$ where the conservation laws permit it. In the experimental part, we employ microfocused Brillouin light scattering spectroscopy in combination with a vector magnet, allowing for angle-resolved mapping of the magnon population spectrum under controlled pumping angle ${\alpha }_p$. We observe that transverse pumping, although characterized by a higher instability threshold, yields a markedly stronger population at the spectral minimum compared to parallel pumping. These observations demonstrate that the kinetic instability channel plays a dominant role in transferring magnons to the spectral minimum under such conditions. These results reveal the crucial role of pumping geometry in shaping the magnon distribution and provide guidelines for optimizing the flux of magnons into the condensate, thereby advancing the control of magnon Bose-Einstein condensation in magnetic insulators.

The emergence of a charge density wave (CDW) in a Weyl semimetal—a correlated topological phase—is exceptionally rare in condensed matter systems. In this context, the quasi-one-dimensional type-III Weyl semimetal (${\mathrm{TaSe}}_4{)}_2\mathrm{I}$ undergoes a CDW transition at $T_{\text{CDW}}\approx 263\mathrm{K}$, providing an exceptional platform to investigate correlated topological CDW states. Here, we uncover an additional hidden-order phase transition at $T^{*}\sim 100\mathrm{K}$, well below the CDW onset, using low-frequency resistance noise spectroscopy, electrical transport, and thermoelectric measurements. This transition is characterized by a sharp enhancement in the noise exponent ($\alpha$) and variance of resistance fluctuations. Analysis of higher-order statistics of resistance fluctuations reveals the correlated dynamics underlying the transition. A pronounced anomaly in the Seebeck coefficient near $T^{*}$ further suggests a Fermi surface reconstruction. First-principles calculations reveal a structural distortion from the high-symmetry $I422$ phase to a low-symmetry $C2$ phase, via an intermediate $I4$ symmetry. This leads to renormalization of the electronic structure near the Fermi level and opening of a bandgap in the hidden-order phase. These findings demonstrate a previously unidentified correlated phase transition in the topological CDW-Weyl semimetal (${\mathrm{TaSe}}_4{)}_2\mathrm{I}$, enriching the phase diagram of this material and establishing it as an ideal platform for studying intertwined electronic and structural orders.

We report a symmetry-resolved study of longitudinal magnetoelastoresistance (MER) in the multivalley material bismuth, with the current, uniaxial stress, and magnetic field all applied along the binary axis. The magnitude of MER exhibits a steep increase at low magnetic fields, reaches a peak, and then gradually decreases at higher fields. By decomposing the strain response into symmetric and antisymmetric symmetry channels, we reveal contrasting magnetic-field dependencies. Despite the overall nonmonotonic field dependence of the MER, the symmetric component remains nearly constant under magnetic fields, suggesting that the valleys in bismuth preserve a rigid-band nature against strain even in the presence of a magnetic field. In contrast, the antisymmetric component, associated with mobility anisotropy, dominates the MER response in a magnetic field. At low magnetic fields, the applied field effectively modifies the apparent mobility of each valley, leading to an enhancement in the magnitude of the antisymmetric MER. At higher fields, field-induced valley polarization further modifies this mobility anisotropy by altering the contributions from each valley's mobility, accounting for the moderate suppression of the MER. These findings demonstrate that symmetry-resolved MER serves as a powerful probe of valley-dependent electronic states and provides a fundamental platform for understanding the interplay between magnetic field, strain, and charge transport.

We study the effect of the electron wave function on Kohn-Luttinger superconductivity. The role of the wave function is encoded in a complex form factor describing the topology and geometry of the bands. We show that the wave function significantly impacts the superconducting transition temperature and superconducting order parameter. We illustrate this using the lowest Landau-level form factor and find exponential enhancement of $T_c$ for the resulting topological superconductor. This enhancement of $T_c$ is due to the resonance between the Berry flux enclosed by the Fermi surface and Cooper pair angular momentum. We find that the ideal band geometry, which favors a fractional Chern insulator in the flat band limit, has an optimal $T_c$. Finally, we apply this understanding to a model relevant to rhombohedral graphene multilayers and unravel the importance of the band geometry for achieving robust superconductivity.

Magnetic topological materials exhibit large intrinsic anomalous Nernst effect (ANE) driven by Berry curvature near the Fermi surface, having great potential in manufacturing transverse thermoelectric devices for converting waste heat into electricity. However, candidate topological magnets with large anomalous Nernst thermopower, $S_{yx}$, remain rare. Moreover, it is extremely challenging to maximize the ANE for a given topological magnet because of their large and complex Fermi surfaces. Herein, we report a significant enhancement of ANE in an iron-based binary topological ferromagnet by trace doping of a rare-earth element. Through 0.3 at. % Ho doping, the room temperature $S_{yx}$ of $\mathrm{F}{\mathrm{e}}_3\mathrm{Ga}$ is raised remarkably from 3.6 $\mu \mathrm{V}{\mathrm{K}}^{-1}$ to 5.2 $\mu \mathrm{V}{\mathrm{K}}^{-1}$, an enhancement of approximately 44%, in addition to a large increase of approximately 60 K in the Curie temperature. Microstructure investigations reveal that the doping decreases the lattice parameter as well as the grain sizes, which also induces nanoprecipitates of Ga-enriched modified $\mathrm{D}{0}_3$ structure into the $\mathrm{D}{0}_3$ matrix. Density functional theory (DFT) calculations reveal that combined effects of Fermi-level shifting induced by Ga vacancies in the primary $\mathrm{D}{0}_3$ phase and the increased number of Berry curvature sources due to the reduced symmetry underlying the large enhancement in ANE. Our findings demonstrate a universal and effective approach for giant improvement of the ANE in $\mathrm{F}{\mathrm{e}}_{3}X$ ($X$ = Ga, Al, Pt, Sn, Si) topological materials, opening avenues for designing high-performance transverse thermoelectric devices for practical applications.

Majorana zero modes (MZMs) are highly sought-after states with a possible application in quantum computation. Here, we show that vortex-core MZMs can carry a nontrivial angular momentum. This establishes distinct ‘‘flavors’’ of Majorana modes, independent of the Chern classification of topological superconductors. The MZM angular momentum is explicitly calculated for a microscopic model of a $d+id$ superconductor placed on a three-dimensional topological insulator (the $d+id+\mathrm{Dirac}$ model) using both exact diagonalization and the Chebyshev expansion. We classify all possible quantum numbers of MZMs depending on the windings of the order parameter and underlying normal state. The topological protection of the MZM is set by the bulk gap, quasiparticle poisoning by trivial in-gap states, and its localization length. All these severely limit the stability of MZMs in the $d+id+\mathrm{Dirac}$ model, in contrast to earlier claims. Nevertheless, the possibility of having different flavors of MZM—in the form of angular momentum or something else—can provide a unique path forward for the study of MZMs.

We show that an effective gravitational field naturally emerges in quantum materials with long-wavelength spin (or pseudospin) textures. When the itinerant electrons' spin strongly couples to the background spin texture, it effectively behaves as a spinless particle in a curved space, with the curvature arising from quantum corrections to the electron's spin orientation. The emergent curved space gives rise to the electron lensing effect, an analog of the gravitational lensing. The lensing effect can appear in systems without (emergent) magnetic fields, such as those with coplanar spin textures. Our work shows that “gravitational” phenomena generically appear in quantum systems due to nonadiabaticity.

Tensor network states have enjoyed great success in capturing aspects of strong correlation physics. However, obtaining dynamical correlators at nonzero temperatures is generically hard even using these methods. Here, we introduce a practical approach to computing such correlators using minimally entangled typical thermal states (METTS). While our primary method directly computes dynamical correlators of physical operators in real time, we propose extensions where correlations are evaluated in the complex-time plane. The imaginary time component bounds the rate of entanglement growth and strongly alleviates the computational difficulty allowing the study of larger system sizes. To extract the physical correlators, one must take the limit of purely real-time evolution. We present two routes for obtaining this information: (i) via an analytic correlation function in complex time combined with a stochastic analytic continuation method to obtain the real-time limit and (ii) a Hermitian correlation function that asymptotically captures the desired correlation function quantitatively. We show that these numerical techniques capture the finite-temperature dynamics of the Shastry-Sutherland model—a model of interacting spins one-half in two spatial dimensions.

Impurities immersed in hard-core Bose gases offer exciting opportunities to explore polaron and bipolaron physics. We investigate the ground-state properties of a single and a pair of impurities throughout the superfluid and insulating (charge density wave) phases of the bosonic environment. In the superfluid phase, the impurity exhibits polaron-like behavior, forming a dressed quasiparticle. In contrast, in the insulating phase, the impurity regains its particle-like character, moving through a potential landscape shaped by the charge density wave order. Moreover, we show that two impurities can form a bound state even in the absence of an explicit impurity-impurity coupling. We establish the stability of this bound state within both the superfluid and insulating phases. Our results offer valuable insights for ongoing lattice polaron experiments with ultracold gases.