We find that the presence of strong non-Abelian symmetries can lead to highly entangled stationary states even for unital quantum channels. We derive exact expressions for the bipartite logarithmic negativity, Rényi negativities, and operator space entanglement for stationary states restricted to one symmetric subspace, with focus on the trivial subspace. We prove that these apply to open quantum evolutions whose commutants, characterizing all strongly conserved quantities, correspond to either the universal enveloping algebra of a Lie algebra or the Read-Saleur commutants. The latter provides an example of quantum fragmentation, whose dimension is exponentially large in system size. We find a general upper bound for all these quantities given by the logarithm of the dimension of the commutant on the smaller bipartition of the chain. As Abelian examples, we show that strong U(1) symmetries and classical fragmentation lead to separable stationary states in any symmetric subspace. In contrast, for non-Abelian $\mathrm{SU}(N)$ symmetries, both logarithmic and Rényi negativities scale logarithmically with system size. Finally, we prove that, while Rényi negativities with $n>2$ scale logarithmically with system size, the logarithmic negativity (as well as generalized Rényi negativities with $n<2$) exhibits a volume-law scaling for the Read-Saleur commutants. Our derivations rely on the commutant possessing a Hopf algebra structure in the limit of infinitely large systems and, hence, also apply to finite groups and quantum groups.

In this work, we investigated glassy lattice thermal conductivity in high-symmetry $\mathrm{BaTi}{\mathrm{S}}_3$ crystals, with a particular focus on the critical interplay between orbital electrons and lattice dynamics. Strong orbital-lattice coupling was found to induce spontaneous symmetry breaking through the Ti-S octahedral distortions, leading to the formation of a unique 1D order–2D disorder lattice structure. With neuroevolution potentials, molecular dynamics simulation of this structure successfully reproduced the glasslike in-plane lattice thermal conductivity observed in experiments. The predicted out-of-plane thermal conductivity decreases with temperature, exhibiting a crystalline trend that is consistent with our measurements. Our findings provide fundamental insights into the mechanism of anomalous amorphous thermal conductivity in single crystals, which arises from the coexistence of overall high symmetry and local structural disorder in specific regions.

We present a mathematical theory of metastable pure states in closed many-body quantum systems with finite-dimensional Hilbert space. Given a Hamiltonian, a pure state is defined to be metastable when all sufficiently local operators either stabilize the state or raise its average energy. We prove that short-range-entangled metastable states are necessarily eigenstates (scars) of a perturbatively close Hamiltonian. Given any metastable eigenstate of a Hamiltonian, in the presence of perturbations, we prove the presence of prethermal behavior: Local correlation functions decay at a rate bounded by a timescale nonperturbatively long in the inverse metastability radius, rather than Fermi’s golden rule. Inspired by this general theory, we prove that the lifetime of the false vacuum in certain $d$-dimensional quantum models grows at least as fast as $\exp ({\epsilon }^{-d})$, where $\epsilon \to 0$ is the relative energy density of the false vacuum; this lower bound matches, for the first time, explicit calculations using quantum field theory. We identify metastable states at finite energy density in the PXP model, along with exponentially many metastable states in “helical” spin chains and the two-dimensional Ising model. Our inherently quantum formalism reveals precise connections between many problems, including prethermalization, robust quantum scars, and quantum nucleation theory, and applies to systems without known semiclassical and/or field-theoretic limits.

Ferroelectric domain walls are not just static geometric boundaries between polarization domains; they are, in fact, dynamic and functional interfaces with the potential for diverse technological applications. While the roles of ferroelectric domain walls in dielectric and piezoelectric responses are better understood, their impact on pyroelectric response remains underexplored. Here, the pyroelectric response of (001)-, (101)-, and (111)-oriented epitaxial heterostructures of the tetragonal ferroelectric ${\mathrm{PbZr}}_{0.2}{\mathrm{Ti}}_{0.8}{\mathrm{O}}_3$ is probed. These differently oriented heterostructures exhibit the same type of 90° ferroelastic domain walls, but their geometry and density vary with orientation. In turn, piezoresponse force microscopy and direct pyroelectric measurements reveal that (111)-oriented heterostructures exhibit both the highest density of domain walls and pyroelectric coefficients. By varying the thickness of these (111)-oriented heterostructures (from 100 to 280 nm), the density of domain walls can be varied, and a direct correlation between domain-wall density and pyroelectric coefficients is found. Molecular-dynamics simulations confirm these findings and reveal a novel domain-wall contribution to pyroelectric response in that the volume of the material in or near the domain walls exhibits a significantly higher pyroelectric coefficient as compared to the bulk of the domains. Analysis suggests that the domain-wall material has a higher responsivity of the polarization to both external fields and temperature. This study sheds light on the microscopic origin of domain-wall contributions to pyroelectricity and provides a pathway to controlling this effect.

Electric-field control of magnetism is one of the most promising routes for developing the energy-efficient magnetoresistive random access memory and spin-logic devices. Of particular interest is the electric-field-induced 180° perpendicular magnetization switching, which currently remains challenging. Here, we experimentally demonstrate the electric-field switching of perpendicular magnetization in a ferrimagnet FeTb in the absence of external magnetic fields. By utilizing ionic liquid gating at room temperature, the ferrimagnetic order can be reversibly switched as a result of the hydrogen injection or extraction under positive or negative gate voltages. Specifically, the hydrogen content pronouncedly modifies the spin and orbital magnetic moments of the Tb sublattice, which subsequently influences the relative magnitude and/or direction of the Fe and Tb sublattice magnetizations, resulting in the switching of ferrimagnetic order. More importantly, we demonstrate a prototype room-temperature three-terminal magnetoelectric memory device by incorporating the giant magnetoresistance effect with electric-field controllable ferrimagnetism. Our results reveal the prosperous aspects of ionic gating for enabling the electric-field-controllable magnetoelectric memory or logic devices.

Despite growing interest in beyond-group symmetries in quantum condensed matter systems, there are relatively few microscopic lattice models explicitly realizing these symmetries, and many phenomena have yet to be studied at the microscopic level. We introduce a one-dimensional stabilizer Hamiltonian composed of group-based Pauli operators whose ground state is a $G\times \mathrm{Rep}(G)$-symmetric state: the $G$-cluster state introduced by Brell [New J. Phys. 17, 023029 (2015)]. We show that this state lies in a symmetry-protected topological (SPT) phase protected by $G\times \mathrm{Rep}(G)$ symmetry, distinct from the symmetric product state by a duality argument. We identify several signatures of SPT order, namely, protected edge modes, string order parameters, and topological response. We discuss how $G$-cluster states may be used as a universal resource for measurement-based quantum computation, explicitly working out the case where $G$ is a semidirect product of Abelian groups.

Characterizing entanglement in quantum materials is crucial for advancing next-generation quantum technologies. Despite recent strides in witnessing entanglement in magnetic materials with distinguishable spin modes, quantifying entanglement in systems formed by indistinguishable electrons remains a formidable challenge. To solve this problem, we introduce a method to extract various four-fermion correlations by analyzing the nonlinearity in resonant inelastic x-ray scattering spectra. These correlations constitute the primary components of the cumulant two-particle reduced density matrix. We further derive bounds for its eigenvalues and demonstrate the linear scaling with fermionic entanglement depth, providing a reliable witness for entanglement. Using the material-relevant strongly correlated models as examples, we show how this entanglement witness can efficiently quantify multipartite entanglement across different phase regions, highlighting its advantage over quantum Fisher information.

We formulate a first-principles approach for calculating the topological Hall effect (THE) in magnets with noncollinear nanoscale spin textures. We employ a modeling method to determine the effective magnetic field induced by the spin texture, thereby circumventing the computational challenges associated with superlattice calculations. Based on these results, we construct a Wannier tight-binding Hamiltonian to characterize the electronic states and calculate the Hall conductivity. Applying this approach to the skyrmion material ${\mathrm{Gd}}_2{\mathrm{PdSi}}_3$ shows good agreement with experimental data. Our analysis in momentum space further reveals that the dominant contribution to the THE arises from the crossing points between the folded bands along high-symmetry lines in the Brillouin zone. This work advances numerical techniques for simulating a general magnetic system, exemplified by but not restricted to skyrmion lattice, and its result offering insights into the complex interplay between spin textures and electronic transport.

Chern insulators host topologically protected chiral edge currents with quantized conductance characterized by their Chern number. Switching the chirality of a Chern insulator, namely, the direction of the edge current, is highly challenging due to topologically forbidden backscattering but is of considerable importance for the design of topological devices. Nevertheless, this can be achieved by reversing the sign of the Chern number. Here, we report electrically switchable chirality in rhombohedral multilayer graphene-based Chern insulators through a topological phase transition. By introducing moiré superlattices in rhombohedral heptalayer graphene, we observe a cascade of topological phase transitions at quarter electron filling of a moiré band with the Chern number tunable from $-1$, 1, to 2. Furthermore, integrating monolayer tungsten diselenide at the moiréless interface of rhombohedral decalayer graphene and hexagonal boron nitride superlattices stabilizes the Chern insulators, enabling quantized anomalous Hall resistance of $h/2e^2$. Remarkably, the Chern number can be electrically switched using displacement fields, leading to a topological phase transition from $-1$ to 2. Our work establishes rhombohedral multilayer graphene moiré superlattices as a versatile platform for topological engineering, with switchable chirality offering significant promise for integrating chiral edge currents into topological electronic circuits.

Quantum entanglement in strongly correlated electron systems often leads to exotic elementary excitations. Quantum spin liquids provide a paradigmatic example, where the elementary excitations are described by fractional quasiparticles such as spinons. However, such fractional quasiparticles behave differently from electrons, making their experimental identification challenging. Here, we theoretically investigate the spin Seebeck effect, which is a thermoelectric response via a spin current, as an efficient probe of the fractional quasiparticles in quantum spin liquids, focusing on the Kitaev honeycomb model. By comprehensive studies using real-time dynamics, perturbation theory, and linear spin-wave theory based on the tunnel spin-current theory, we find that the spin current is induced by thermal gradient in the Kitaev spin liquid via the low-energy fractional Majorana excitations. This identification underscores the ability of Majorana fermions to carry spin current, despite lacking spin angular momentum. Furthermore, we find that the induced spin current changes its sign depending on the sign of the Kitaev interaction, indicating that the Majorana fermions contribute to the spin current with (up-) down-spin-like nature when the exchange coupling is (anti)ferromagnetic. Thus, in contrast to the negative spin current already found in a one-dimensional quantum spin liquid, our calculation reveals that the spin Seebeck effect can exhibit either positive or negative signals, contingent upon the nature of fractional excitations in the quantum spin liquids. We also clarify contrasting field-angle dependence between the Kitaev spin liquid in the low-field limit and the high-field ferromagnetic state, which is useful for the experimental identification. Our finding suggests that the spin Seebeck effect could be used not only to detect fractional quasiparticles emerging in quantum spin liquids but also to generate and control them.

Moiré heterostructures consisting of transition metal dichalcogenide (TMD) heterobilayers and homobilayers have emerged as a promising material platform to study correlated electronic states. Optical signatures of strong correlations in the form of Mott-Wigner states and fractional Chern insulators have already been observed in TMD monolayers and their twisted bilayers. In this work, we use a moiré substrate containing a twisted hexagonal boron nitride ($h$-BN) interface to externally generate a superlattice potential for the TMD layer: The periodic structure of ferroelectric domains in $h$-BN creates a purely electrostatic potential for charge carriers. We find direct evidence for the induced moiré potential in the emergence of new excitonic resonances at integer fillings and our observation of an enhancement of the trion binding energy by $\simeq 3\mathrm{meV}$. A theoretical model for exciton-electron interactions allows us to directly determine the moiré potential modulation of $30\pm 5\mathrm{meV}$ from the measured trion binding energy shift. We obtain direct evidence for charge order linked to electronic Mott-Wigner states at filling factors $\nu =1/3$ and $\nu =2/3$ through the associated exciton umklapp resonances.

Water wires, quasi-one-dimensional chains composed of hydrogen-bonded (H-bonded) water molecules, play a fundamental role in numerous chemical, physical, and physiological processes. Yet direct experimental detection of water wires has been elusive so far. Based on advanced ab initio many-body theory that includes electron-hole interactions, we report that optical absorption spectroscopy can serve as a sensitive probe of water wires and their ordering. In both liquid and solid water, the main peak of the spectrum is discovered to be a charge-transfer exciton. In water, the charge-transfer exciton is strongly coupled to the H-bonding environment where the exciton is excited between H-bonded water molecules with a large spectral intensity. In regular ice, the spectral weight of the charge-transfer exciton is enhanced by a collective excitation occurring on proton-ordered water wires, whose spectral intensity scales with the ordering length of water wire. The spectral intensity and excitonic interaction strength reaches its maximum in ice XI, where the long-range ordering length yields the most pronounced spectral signal. Our findings suggest that water wires, which widely exist in important physiological and biological systems and other phases of ice, can be directly probed by this approach.

The spin-$1/2$ kagome antiferromagnet (AFM) is one of the most studied models in frustrated magnetism since it is a promising candidate to host exotic spin-liquid states. However, despite numerous studies using both analytical and numerical approaches, the nature of the ground state and low-energy excitations in this system remains elusive. This challenge is related to the difficulty in determining the spin gap in various calculations. We present the results of our investigation of the kagome AFM using the recently developed group equivariant convolutional neural networks—a novel machine learning technique for studying strongly frustrated models. This approach, combined with variational Monte Carlo method, introduces significant improvement of the achievable results’ accuracy for frustrated spin systems in comparison with approaches based on other neural-network architectures. Contrary to the results obtained previously with various methods, which predicted ${\mathbb{Z}}_2$ or $U(1)$ Dirac spin-liquid states, our results strongly indicate that the ground state of the kagome lattice antiferromagnet is a spinon pair density wave that does not break time-reversal symmetry or any of the lattice symmetries. The state appears due to the spinon Cooper pairing instability close to two Dirac points in the spinon energy spectrum, and it resembles the pair density wave state studied previously in the context of underdoped cuprate superconductors in connection with the pseudogap phase. This state has significantly lower energy than the lowest-energy states found by the $SU(2)$ symmetric density matrix renormalization group calculations and other methods.

Detection and control of Andreev bound states (ABSs) localized at semiconductor-superconductor interfaces are essential for their use in quantum applications. Here we investigate the impact of ABSs on the supercurrent through a Josephson junction containing a quantum dot (QD). Additional normal-metal tunneling probes on both sides of the junction unveil the ABSs residing at the semiconductor-superconductor interfaces. Such knowledge provides an ingredient missing in previous studies, improving the connection between theory and experimental data. By varying the ABS energies using electrostatic gates, we show control of the switching current, with the ability to alter it by more than an order of magnitude. Finally, the large degree of ABS tunability allows us to realize a three-site Andreev molecule in which the central QD is screened by both ABSs. This system is studied simultaneously using both supercurrent and spectroscopy.

The advent of moiré platforms for engineered quantum matter has led to discoveries of integer and fractional quantum anomalous Hall effects, with predictions for correlation-driven topological states based on electron crystallization. Here, we report an array of trivial and topological insulators formed in a moiré lattice of rhomobohedral pentalayer graphene (R5G). At a doping of one electron per moiré unit cell ($\nu =1$), we see a correlated insulator with a Chern number that can be tuned between $C=0$ and $+1$ by an electric displacement field. This is accompanied by a series of additional Chern insulators with $C=+1$ originating from fractional fillings of the moiré lattice—$\nu =1/4$, $1/3$, and $2/3$—associated with the formation of moiré-driven topological electronic crystals. At $\nu =2/3$ the system exhibits an integer quantum anomalous Hall effect at zero magnetic field, but further develops hints of an incipient $C=2/3$ fractional Chern insulator in a modest field. Our results establish moiré R5G as a fertile platform for studying the competition and potential intertwining of integer and fractional Chern insulators.

Because of the lack of a net magnetic moment, antiferromagnets possess a unique robustness to external magnetic fields and are thus predicted to play an important role in future magnetic technologies. However, this robustness also makes them quite difficult to control, and the development of novel methods to manipulate these systems with external stimuli is a fundamental goal of antiferromagnetic spintronics. In this work, we report evidence for a metastable reorientation of the order parameter in an antiferromagnetic semiconductor triggered by an ultrafast quench of the equilibrium order via photoexcitation above the band gap. The metastable state forms less than 10 ps after the excitation pulse, and persists for longer than 150 ps before decaying to the ground state via thermal fluctuations. Importantly, this transition cannot be induced thermodynamically, and requires the system to be driven out of equilibrium. Broadly speaking, this phenomenology is ultimately the result of large magnetoelastic coupling in combination with a relatively low symmetry of the magnetic ground state. Since neither of these properties are particularly uncommon in magnetic materials, the observations presented here imply a generic path toward novel device technology enabled by ultrafast dynamics in antiferromagnets.

Spin-flip dark excitons are optical-dipole-forbidden quasiparticles with remarkable potential in optoelectronics, especially when they are realized within cleavable van der Waals materials. Despite this potential, dark excitons have not yet been definitively identified in ferromagnetic van der Waals materials. Here, we report two dark excitons in a model ferromagnetic material ${\mathrm{CrI}}_3$ using high-resolution resonant inelastic x-ray scattering and show that they feature narrower linewidths compared to the bright excitons previously reported in this material. These excitons are shown to have spin-flip character, to disperse as a function of momentum, and to change through the ferromagnetic transition temperature. Given the versatility of van der Waals materials, these excitons hold promise for new types of magneto-optical functionality.

Viscous electron flow exhibits exotic signatures such as superballistic conduction. In order to observe hydrodynamics effects, a 2D device where the current flow is as inhomogeneous as possible is desirable. To this end, we build three antidot graphene superlattices with different hole diameters. We measure their electrical properties at various temperatures and under the effect of a perpendicular magnetic field. We find an enhanced superballistic effect, suggesting the effectiveness of the geometry at bending the electron flow. In addition, superballistic conduction, which is related to a transition from a noncollective to a collective regime of transport, behaves nonmonotonically with the magnetic field. We also analyze the device resistance as a function of the size of the antidot superlattice to find characteristic scaling laws describing the different transport regimes. We prove that the antidot superlattice is a convenient geometry for realizing hydrodynamic flow and provide valuable explanations for the technologically relevant effects of superballistic conduction and scaling laws.

Centrosymmetric compounds which host three-dimensional topological spin structures comprise a distinct subclass of materials in which multiple-$\mathbf{q}$ magnetic order is stabilized by anisotropy and bond frustration in contrast to the more common path of antisymmetric exchange interactions. Here we investigate static and dynamic magnetic properties of the cubic perovskite ${\mathrm{SrFeO}}_3$—a rare example of a centrosymmetric material hosting two types of topological spin textures: skyrmionlike and hedgehog-lattice phases. Our detailed magnetization and dilatometry measurements describe the domain selection processes and phase transitions in ${\mathrm{SrFeO}}_3$. Spin excitations are investigated using inelastic neutron scattering for all three zero-field phases. In the higher-temperature ordered phases, high-energy magnons increasingly lose coherence, so that spin fluctuations are dominated by a distinct quasielastic component at low energies. We anticipate that this could be generic to symmetric helimagnets in which the chiral symmetry is spontaneously broken by the magnetic order.

The discovery of two-dimensional superconductivity in ${\mathrm{LaAlO}}_3/{\mathrm{KTaO}}_3$ (111) and (110) interfaces has raised significant interest in this system. In this paper, we report the first successful fabrication of a direct current superconducting quantum interference device (dc-SQUID) in the KTO system. The key device elements, superconducting weak links, are created by conductive atomic force microscope lithography, which can reversibly control the conductivity at the LAO/KTO (110) interface with nanoscale resolution. The periodic modulation of the SQUID critical current $I_{\mathrm{c}}(B)$ with magnetic field corresponds well with our theoretical modeling, which reveals a large kinetic inductance of the superconducting two-dimensional electron gas in KTO. The kinetic inductance of the SQUID is tunable by electrical gating from the back, due to the large dielectric constant of KTO. The demonstration of weak links and SQUIDs in KTO broadens the scope for exploring the underlying physics of KTO superconductivity, including the role of spin-orbit coupling, pairing symmetry, and inhomogeneity. It also promotes KTO as a versatile platform for a growing family of quantum devices, which could be applicable in the realm of quantum computing and information.