We show that neural quantum states based on very deep (4–16-layered) neural networks can outperform state-of-the-art variational approaches on highly frustrated quantum magnets, including quantum-spin-liquid candidates. We focus on group convolutional neural networks that allow us to efficiently impose space-group symmetries on our ansätze. We achieve state-of-the-art ground-state energies for the $J_1-J_2$ Heisenberg models on the square and triangular lattices, in both ordered and spin-liquid phases, and discuss ways to access low-lying excited states in nontrivial symmetry sectors. We also compute spin and dimer correlation functions for the quantum paramagnetic phase on the triangular lattice, which do not indicate either conventional or valence-bond ordering.

Noninteracting particles in non-Hermitian quasicrystals display localization-delocalization and spectral phase transitions in complex energy plane that can be characterized by point-gap topology. Here we investigate the spectral and dynamical features of two interacting particles in a non-Hermitian quasicrystal, described by an effective Hubbard model in an incommensurate sinusoidal potential with a complex phase, and unravel some intriguing effects without any Hermitian counterpart. Owing to the effective decrease of correlated hopping introduced by particle interaction, doublon states, i.e., bound particle states, display a much lower threshold for spectral and localization-delocalization transitions than single-particle states, leading to the emergence of mobility edges. Remarkably, since doublons display longer lifetimes, two particles initially placed in distant sites tend to bunch and stick together, forming a doublon state in the long time limit of evolution, a phenomenon that can be dubbed non-Hermitian particle bunching.

Realization of tunable superconductivity with concomitant nontrivial band topology is conceptually intriguing and highly desirable for next-generation nanoscale superconducting devices. Based on first-principles calculations, here we present a prediction of simultaneously tunable superconducting transition temperature ($T_{\mathrm{c}}$) and band topology in a superconducting $\mathrm{Ir}{\mathrm{Te}}_2$ overlayer on a ferroelectric ${\mathrm{In}}_2{\mathrm{Se}}_3$ monolayer. We first demonstrate that the $T_{\mathrm{c}}$ is substantially enhanced from that of $\mathrm{Ir}{\mathrm{Te}}_2$ nanoflakes ($T_{\mathrm{c}}\sim 3$ K) due to significant charge repartitioning around the Fermi level. More importantly, the $T_{\mathrm{c}}$ is shown to sensitively depend on the ${\mathrm{In}}_2{\mathrm{Se}}_3$ polarization, with the higher $T_{\mathrm{c}}$ of $\sim$(8–10) K attributed to enhanced interlayer electron-phonon coupling when the polarization is downward. The band topology is also switched from trivial to nontrivial as the polarization is reversed from upward to downward. These findings provide physically realistic platforms for simultaneously tuning superconductivity and band topology in two-dimensional heterobilayers and related heterostructures using a reversible and nonvolatile approach, with immense application potential for superconducting diodes and topological quantum computation.

A combination of competing exchange interactions and substitutional disorder gives rise to magnetic inhomogeneities in the $\left[{\mathrm{Y}}_{2-x}{\mathrm{Mn}}_x\right]{\mathrm{Mn}}_6{\mathrm{O}}_{12}x=0.23$ and 0.16 quadruple perovskite manganites. Our neutron powder scattering measurements show that both the $x=0.23$ and 0.16 samples separate into two distinct magnetic phases; below $T_1$ = 120 $\pm$ 10 K the system undergoes a transition from a paramagnetic phase to a phase characterized by short-range antiferromagnetic clusters contained in a paramagnetic matrix, and below $T_2\approx 65$ K the system is composed of well-correlated long-range collinear ferrimagnetic order, punctuated by short-range antiferromagnetic clusters. A sharp increase in the antiferromagnetic phase fraction is observed below $\approx 33$ K, concomitant with a decrease in the ferrimagnetic phase fraction. Our results demonstrate that the theoretically proposed antiferromagnetic phase is stabilized in the $\left[{\mathrm{Y}}_{2-x}{\mathrm{Mn}}_x\right]{\mathrm{Mn}}_6{\mathrm{O}}_{12}$ manganites in the presence of dominant $B\text{−}B$ exchange interactions, as predicted.

The kinetic energy of electrons in a magnetic field is quenched resulting in a discrete set of highly degenerate Landau levels (LLs) which gives rise to fascinating phenomena such as the de Haas–van Alphen effect (dHvAe) or the integer and fractional quantum Hall effects. The latter is a result of interactions partially lifting the degeneracy within a given LL while inter-LL interactions are usually assumed to be unimportant. Here, we study the LL spectrum of the Hatsugai-Kohmoto model, a Hubbard-like model which is exactly soluble on account of infinite-range interactions. For the doped Mott insulator phase in a magnetic field we find that the degeneracy of LLs is preserved but inter-LL interactions are important leading to a nonmonotonous reconstruction of the spectrum. As a result, strong LL repulsion leads to aperiodic quantum oscillations of the dHvAe in contrast to Onsager's famous relation connecting oscillation frequencies with the Fermi surface areas at zero field. In addition, we find unconventional temperature dependencies of quantum oscillations with interaction-induced effective mass renormalizations. We discuss the general importance of inter-LL interactions for understanding doped Mott insulators in magnetic fields.

Direct experimental detection of anyonic exchange statistics in fractional quantum Hall systems by braiding the excitations and measuring the wave-function phase is an enormous challenge. Here, we use a small, noisy quantum computer to emulate direct braiding within the framework of a simplified model applicable to a thin cylinder geometry and measure the topological phase. Our algorithm first prepares the ground state with two quasiholes. It then applies a unitary operation controlled by an ancilla, corresponding to a sequence of adiabatic evolutions that take one quasihole around the other. We finally extract the phase of the wave function from measuring the ancilla with a compound error mitigation strategy. Our results point out an avenue for studying braiding statistics in fractional Hall states.

We investigate correlations between the spin-splitting energy and the orbital angular momentum (OAM) of the $L$-gap surface states on $\mathrm{Au}\left(110\right)\text{−}(1\times 1)$ and $-(1\times 2)$ surfaces by first-principles calculations. It is revealed that their spin-splitting energies can be expressed approximately as $\mathrm{\Delta }\varepsilon \approx 2|{\xi }_p{\mathbf{L}}^{\left(p\right)}+{\xi }_d{\mathbf{L}}^{\left(d\right)}|$, where ${\mathbf{L}}^{\left(p\right)}$ and ${\mathbf{L}}^{\left(d\right)}$ are the $p$- and $d$-orbital contributions to the OAM in the absence of spin-orbit interaction (SOI), and ${\xi }_p$ and ${\xi }_d$ are the atomic SOI parameters for the $p$ and $d$ orbitals of Au. For all the surface states studied, the major component ${\mathbf{L}}^{\left(d\right)}$ exhibits rather weak anisotropy in the space of two-dimensional wave vector $\mathbf{k}$, whereas the minor component ${\mathbf{L}}^{\left(p\right)}$ is more anisotropic between the $\left[\overline{1}10\right]$ and [001] directions. Since ${\xi }_p$ for Au is several times larger than ${\xi }_d$, $\mathrm{\Delta }\varepsilon$ becomes more strongly anisotropic than the total OAM $\mathbf{L}$ in the $\mathbf{k}$ space. In particular, the Rashba parameter in the [001] direction of the unoccupied surface band on the ($1\times 1$) surface becomes nearly vanishing due to strong cancellation of the $p$- and $d$-orbital contributions. We also discuss how the OAM of surface states calculated without SOI is modified when SOI is turned on.

Heat transport by acoustic phonons in two-dimensional (2D) materials is fundamentally different from that in 3D crystals because the out-of-plane phonons propagate in a unique way that strongly depends on tension and bending rigidity. Here, using optomechanical techniques, we experimentally demonstrate that the heat transport time in freestanding graphene membranes is significantly higher than the theoretical prediction, and decreases by as much as $33\%$ due to an electrostatically induced tension of 0.07 $\mathrm{N}/m$. Using phonon scattering and Debye models, we explain these observations by the tension-enhanced acoustic impedance match of flexural phonons at the boundary of the graphene membrane. Thus, we experimentally elucidate the tunability of phononic heat transport in 2D materials by tension, and open a route towards electronic devices and circuits for high-speed control of temperature at the nanoscale.

The quantum dynamics of spin systems with uniform all-to-all interaction are often studied in the totally symmetric space (TSS) of maximal total spin. However, the TSS states are atypical in the full many-body Hilbert space. In this paper, we explore several aspects of the all-to-all quantum dynamics away from the TSS, and reveal surprising features of the “deep Hilbert space” (DHS). We study the out-of-time order correlator (OTOC) in the infinite-temperature ensemble of the full Hilbert space. We derive a phase-space representation of the DHS OTOC and show that the OTOC can have a superexponential initial growth in the large $N$ limit, due to the fast dynamics in an unbounded phase space (in finite systems, we observe numerically that the superexponential growth ends precociously and gives way to a power-law one until saturation). By a similar mechanism, the Krylov complexity grows explosively. We also study the entanglement growth in a quantum quench from a DHS product state, i.e., one of nonaligned spins that resemble the DHS infinite-temperature ensemble with respect to the statistics of the collective spins. Using a field-theoretical method, We exactly calculate the entanglement entropy in the large $N$ limit. We show that, in the DHS, fast OTOC growth does not imply fast entanglement growth, in contrast to the Zurek-Paz relation derived in the TSS.

We explore the statistical properties of energy transfer in ensembles of doubly driven random-matrix Floquet Hamiltonians based on universal symmetry arguments. The energy-pumping efficiency distribution $P\left(\overline{E}\right)$ is associated with the Hamiltonian parameter ensemble and the eigenvalue statistics of the Floquet operator. For specific Hamiltonian ensembles, $P\left(\overline{E}\right)$ undergoes a transition which cannot be associated with a symmetry breaking of the instantaneous Hamiltonian. The Floquet eigenvalue spacing distribution indicates the considered ensembles constitute generic nonintegrable Hamiltonian families. As a step towards Hamiltonian engineering, we develop a machine-learning classifier to understand the relative parameter importance in resulting high-conversion efficiency. We propose random Floquet Hamiltonians as a general framework to investigate frequency conversion effects in a class of generic dynamical processes beyond adiabatic pumps.

The structure of the material responsible for the room temperature and near ambient pressure superconductivity reported in an N-doped lutetium hydride [Nature (London) 615, 244 (2023)] has not been conclusively determined. Herein, density functional theory calculations are performed in an attempt to uncover what it might be. Guided by a range of strategies including crystal structure prediction and modifications of existing structure types, we present an array of Lu-N-H phases that are dynamically stable at experimentally relevant pressures. Although none of the structures found are thermodynamically stable, and none are expected to remain superconducting above $\sim 17$ K at 10 kbar, a number of metallic compounds with fcc Lu lattices—as suggested by the experimental x-ray diffraction measurements of the majority phase—are identified. The system whose calculated equation of states matches best with that measured for the majority phase is fluorite-type ${\mathrm{LuH}}_2$, whose 10 kbar superconducting critical temperature was estimated to be 0.1 K using the Allen-Dynes modified McMillan equation.

Despite the huge recent interest towards chiral magnetism related to the interfacial Dzyaloshinskii-Moriya interaction (i-DMI) in layered systems, there is a lack of experimental data on the effect of i-DMI on the spin-wave eigenmodes of laterally confined nanostructures. Here, we exploit Brillouin light scattering (BLS) to analyze the spin-wave eigenmodes of noninteracting circular and elliptical dots, as well as of long stripes, patterned starting from a Pt(3.4-nm)/CoFeB(0.8-nm) bilayer, with lateral dimensions ranging from 100 to 400 nm. Our experimental results, corroborated by micromagnetic simulations based on the graphic processing units-accelerated mumax3 software package, provide evidence for a strong suppression of the frequency asymmetry $\mathrm{\Delta }f$ between counterpropagating spin waves (corresponding to either Stokes or anti-Stokes peaks in BLS spectra), when the lateral confinement is reduced from 400 to 100 nm, i.e., when it becomes lower than the light wavelength. Such an evolution reflects the modification of the spin-wave character from propagating to stationary and indicates that the BLS-based method of quantifying the i-DMI strength from the frequency difference of counterpropagating spin waves is not applicable in the case of magnetic elements with lateral dimension below about 400 nm.

We report the temperature evolution of the Dirac band in semiconducting zirconium pentatelluride (${\mathrm{ZrTe}}_5$) using magnetoinfrared spectroscopy. We find that the band gap is temperature independent at low temperatures and increases with temperature at elevated temperatures. Although such an observation seems to support a weak topological insulator phase at all temperatures and defy the previously reported topological phase transition (TPT) at an intermediate temperature in ${\mathrm{ZrTe}}_5$, we show that it is also possible to explain the observation by considering the effect of conduction-valence band mixing and band inversion with a strong topological insulator phase at low temperatures. Our work provides an alternative picture of the band gap evolution across TPT.

Barium zirconate $\left({\mathrm{BaZrO}}_3\right)$ is one of the very few perovskites that is claimed to retain an average cubic structure down to $0\mathrm{K}$, while being energetically very close to an antiferrodistortive phase obtained by condensation of a soft phonon mode at the $R$ point of the Brillouin zone boundary. In this work, we report a combined experimental and theoretical study of the temperature dependence of this soft phonon mode. Inelastic neutron and x-ray scattering measurements on single crystals show that it softens substantially from $9.4\mathrm{meV}$ at room temperature to $5.6\mathrm{meV}$ at $2\mathrm{K}$. In contrast, the acoustic mode at the same $R$ point is nearly temperature independent. The effect of the anharmonicity on the lattice dynamics is investigated nonperturbatively using direct dynamic simulations as well as a first-principles-based self-consistent phonon theory, including quantum fluctuations of the atomic motion. By adding cubic and quartic anharmonic force constants, quantitative agreement with the neutron data for the temperature dependence of the antiferrodistortive mode is obtained. The quantum fluctuations of the atomic motion are found to be important to obtain the proper temperature dependence at low temperatures. The mean-squared displacements of the different atoms are determined as function of temperature and are shown to be consistent with available experimental data. Adding anharmonicity to the computed fluctuations of the Ba-O distances also improves the comparison with available extended x-ray absorption fine structure data at $300\mathrm{K}$.

Recent advances in material synthesis made it possible to realize two-dimensional monolayers of candidate materials for a quantum spin liquid (QSL) such as $\alpha -{\mathrm{RuCl}}_3$, 1T-${\mathrm{TaSe}}_2$, and 1T-${\mathrm{TaS}}_2$. In this work, we propose an experimental setup that exploits nonlocal electrical probes to gain information on the transport properties of a gapless QSL. The proposed setup is a spinon-mediated drag experiment: a current is injected in one of the two layers and a voltage is measured on the second metallic film. The overall momentum transfer mechanism is a two-step process mediated by Kondo interaction between the local moments in the quantum spin liquid and the spins of the electrons. In the limit of negligible momentum relaxed within the QSL layer, we calculate the drag relaxation rate for Kitaev, ${\mathbb{Z}}_2$, and U(1) QSLs using Aslamazov-Larkin diagrams. We find, however, that the case of dominant momentum relaxation within the QSL layer is far more relevant and thus develop a model based on the Boltzmann kinetic equation to describe the proposed setup. Within this framework we calculate the low-temperature scaling behavior of the drag resistivity, both for U(1) and ${\mathbb{Z}}_2$ QSLs with Fermi surfaces. In some regimes we find a crossover in the temperature scaling that is different between the ${\mathbb{Z}}_2$ and U(1) QSL because of the non-Fermi-liquid nature of the latter, which reflects itself both in altered kinematic constraints for the momentum transfer as well as in the qualitative aspects of momentum relaxation within the QSL layer. Our findings suggest that parameters of the system can be tuned to make the spinon-mediated drag a significant fraction of the total transresistance.

Recent theoretical and experimental studies suggest that van der Waals heterostructures with $n$- and $p$-doped bilayers of transition metal dichalcogenides are promising facilitators of exciton superfluidity. Exciton superfluidity in these bilayer systems is often modelled by solving a mean-field gap equation defined for only the conduction and valence band of the electron and hole material, respectively. A key quantity entering the gap equation is the effective Coulomb potential acting as the bare interaction in the subspace of the model. Since the model only includes a few bands around the Fermi energy, the effective model interaction is partially screened. Although the screening is a material-dependent quantity it has, in previous studies, been accounted for in an ad hoc manner, by assuming a static dielectric constant of 2 for a wide range of different materials. In this paper we show that the effective model interaction can be derived from first principles using open source code frameworks. We show that the material dependent screening, accounted for by this ab initio downfolding procedure, has a large influence on the exciton binding energies and superfluid properties. By applying the method to 336 different heterostructures comprised of transition metal dichalcogenides and transition metal oxides we show that the proposed downfolding method yields qualitatively different trends of the exciton binding energies and superfluid properties compared to the standard assumptions with a single dielectric constant. Additionally, we propose material platforms of both transition metal oxides and dichalcogenides with superior properties compared to conventional devices with two transition metal dichalcogenide layers.

Isotope ${}^{121,123}\mathrm{Sb}$ nuclei with large electric quadrupole moments are applied to investigate the dynamics of orbital degrees of freedom in Sb-substituted iron(Fe)-based compounds. In the parent compound $\mathrm{La}\mathrm{Fe}\left({\mathrm{As}}_{0.6}{\mathrm{Sb}}_{0.4}\right)\mathrm{O}$, the nuclear-spin relaxation rate ${}^{121,123}\left(T_1^{-1}\right)$ at ${}^{121,123}\mathrm{Sb}$ sites was enhanced at structural transition temperature ($T_s\sim$ 135 K), which is higher than Néel temperature ($T_{\mathrm{N}}\sim 125$ K). The isotope ratio ${}^{123}\left(T_1^{-1}\right)/{}^{121}\left(T_1^{-1}\right)$ indicates that the electric quadrupole relaxation due to the dynamical electric field gradient at the Sb site increases significantly toward $T_s$. It is attributed to the critically enhanced nematic fluctuations of stripe-type arrangement of Fe-$3d_{xz}$ (or $3d_{yz}$) orbitals. In the lightly electron-doped superconducting (SC) compound $\mathrm{La}\mathrm{Fe}\left({\mathrm{As}}_{0.7}{\mathrm{Sb}}_{0.3}\right)\left({\mathrm{O}}_{0.9}{\mathrm{F}}_{0.1}\right)$, the nematic fluctuations are largely suppressed in comparison with the case of the parent compound; however, it remains a small enhancement below 80 K down to the $T_c$ ($\sim 20$ K). The results indicate that the fluctuations from both the spin and orbital degrees of freedom on the $3d_{xz}$(or $3d_{yz}$) orbitals can be seen in lightly electron-doped SC state of LaFeAsO-based compounds. We emphasize that isotope ${}^{121,123}\mathrm{Sb}$ quadrupole moments are a sensitive local probe to identify the dynamics of orbital degrees of freedom in Fe pnictides, which provides an opportunity to discuss the microscopic correlation between the superconductivity and both nematic and spin fluctuations simultaneously, even in the polycrystalline samples.

We investigate the optical magnetoelectric (ME) effects which lead to the nonreciprocity of photons arising from the enhanced dynamical ME coupling of the electromagnon resonances in the spin-cycloidal phases of multiferroic perovskite manganites ${\mathrm{Eu}}_{1-x}{\mathrm{Y}}_x{\mathrm{MnO}}_3$ ($x=0.3$, 0.4, 0.45, 0.5) by means of terahertz time-domain polarimetry. The correlation between the optical ME effects and the associated order parameters is examined by changing $x$. Nonreciprocal directional dichroism (NDD) is demonstrated in the $ab$- and $bc$-plane cycloidal phases. The resonantly enhanced NDD in the $ab$-plane cycloidal phase shows correlation with the relevant coupled order parameter composed of spontaneous polarization $\mathit{P}$ and magnetization $\mathit{M}$ as $\mathit{P}\times \mathit{M}$. The NDD is further enhanced in the $bc$-plane cycloidal phase due to the mode-mode coupling of electromagnons with distinct origins. The nonreciprocal optical rotation stemming from gyrotropic birefringence (GB) is also demonstrated in the $ab$-plane spin cycloidal phase. The magnitude of GB shows clear deviations from the relevant coupled order parameter of $\mathit{P}·\mathit{M}$. The GB is remarkably enhanced near the phase boundary ($x=0.3$) between the cycloidal phase and the canted antiferromagnetic phase. As a result, the dynamical diagonal ME coupling approaches 0.23 (770 ps/m) for $x=0.3$. These different $x$ dependences between NDD and the GB are discussed on the basis of the mechanisms of the dynamical ME coupling of collective spin excitations.

Quantum systems in $3+1$ dimensions that are invariant under gauging a one-form symmetry enjoy novel noninvertible duality symmetries encoded by topological defects. These symmetries are renormalization group invariants which constrain dynamics. We show that such noninvertible symmetries often forbid a symmetry-preserving vacuum state with a gapped spectrum. In particular, we prove that a self-dual theory with ${\mathbb{Z}}_N^{\left(1\right)}$ one-form symmetry is gapless or spontaneously breaks the self-duality symmetry unless $N=k^2\ell$ where $-1$ is a quadratic residue modulo $\ell$. We also extend these results to noninvertible symmetries arising from invariance under more general gauging operations including, e.g., triality symmetries. Along the way, we discover how duality defects in symmetry-protected topological phases have a hidden time-reversal symmetry that organizes their basic properties. These noninvertible symmetries are realized in lattice gauge theories, which serve to illustrate our results.

The electron-hole exchange interaction is a fundamental mechanism that drives valley depolarization via intervalley exciton hopping in semiconductor multivalley systems. Here, we report polarization-resolved photoluminescence spectroscopy of neutral excitons and negatively charged trions in monolayer ${\mathrm{MoSe}}_2$ and ${\mathrm{WSe}}_2$ under biaxial strain. We observe a marked enhancement (reduction) on the ${\mathrm{WSe}}_2$ triplet trion valley polarization with compressive (tensile) strain while the trion in ${\mathrm{MoSe}}_2$ is unaffected. The origin of this effect is shown to be a strain-dependent tuning of the electron-hole exchange interaction. A combined analysis of the strain-dependent polarization degree using ab initio calculations and rate equations shows that strain affects intervalley scattering beyond what is expected from strain-dependent band-gap modulations. The results evidence how strain can be used to tune valley physics in energetically degenerate multivalley systems.