As a prototype altermagnetic candidate, ${\mathrm{RuO}}_2$ has attracted abundant attention due to its exotic properties and potential applications in electronic devices. However, its altermagnetic nature is hotly debated primarily because theoretically the appearance of altermagnetism depends on the Hubbard $U$, a quantity that is difficult to measure experimentally. Here, we use high harmonic generation as a way to gauge the strength of the Hubbard $U$ in ${\mathrm{RuO}}_2$ through first-principles calculations. Without the Hubbard $U$, ${\mathrm{RuO}}_2$ is nonmagnetic and has the Kramers degeneracy. Our orientation-dependent harmonic yield exhibits a dumbbell-like shape. With the Hubbard $U$, altermagnetism appears and the Kramers degeneracy is lifted in ${\mathrm{RuO}}_2$. The orientation-dependent harmonic yield now has a clover shape. Microscopically, electronic excitations are altered by the interplay between the orbital ordering and the crystal field. By comparing with existing experiments, we conclude that the dumbbell-shape anisotropy in the orientation-dependent harmonic yield is likely to be intrinsic, indicating weak altermagnetism in ${\mathrm{RuO}}_2$. Our study represents another perspective for future research in this field.

We propose a mechanism and develop a theory for nonreciprocal Coulomb drag resistance. This effect arises in electron double-layer systems in the presence of an in-plane magnetic field in noncentrosymmetric conductors or in bilayers with spontaneously broken time-reversal symmetry and without Galilean invariance. We demonstrate the significance of this effect by examining the hydrodynamic regime of the electron liquid. The nonreciprocal component of the transresistance is shown to sensitively depend on the intrinsic conductivity, viscosity of the fluid, and the emergent nonreciprocity parameter.

We investigate the entanglement properties of a Kondo system undergoing a transition to a state with a quenched magnetic impurity, using the density matrix renormalization group method. We focus on a two-channel spin-1 Kondo impurity with single-ion anisotropy, where a quantum phase transition occurs between two topologically distinct local Fermi liquids. In the fully screened Kondo phase, realized at lower anisotropies, the entangled region surrounding the magnetic impurity mimics the Kondo screening cloud, although its length does not follow the conventional behavior. In contrast, beyond the transition, the system enters a non-Landau Fermi liquid phase with a markedly different entanglement structure: As the impurity is quenched and disentangled from the rest of the system due to the breakdown of the Kondo effect, the two conduction channels—coupled only through the impurity—develop a significant degree of entanglement with one another. Our findings demonstrate that a quenched magnetic impurity can passively and efficiently mediate entanglement between spatially separated conduction bands.

Anderson localization is typically associated with the quantum localization of a particle in the presence of time-independent disorder. However, there also exists a dynamical version of Anderson localization, in which classical diffusion of a particle in phase space in a classically chaotic system is suppressed by quantum interference, leading to localization in momentum space. Time-crystalline structures introduce a new type of Anderson localization: localization in the time domain in systems with temporal disorder. Here, we address whether localization can also occur in the domain conjugate to time, i.e., in the frequency domain, analogous to localization in momentum space in systems which are classically chaotic. We consider two cases: a simple system of a resonantly driven particle on a ring, and a more general system of a particle bouncing on an oscillating mirror. Our analysis shows that localization in the frequency domain can be observed in experimentally accessible platforms.

Recent interest in monolayer materials motivated a search for two-dimensional ferromagnets with sizable spin-orbit coupling. Magnetic anisotropy of exchange Hamiltonian, induced by spin-orbit coupling, may not only stabilize long-range order, but also in turn can be a source of frustration and accidental degeneracy, which is the case for the Kitaev-Heisenberg model. Here we present an extensive study of ground state and excitations of ferromagnetic anisotropic-exchange Kitaev-Heisenberg model on a triangular lattice using order-by-disorder and augmented spin-wave theory calculations. It is shown that while bond-dependent terms of the model do not affect the ground state classically, quantum fluctuations select preferred magnetization direction of the ferromagnetic state and significantly alter classical phase diagram. Anisotropic terms of the magnetic Hamiltonian also give rise to magnon-magnon interactions that lead to spontaneous decay and spectral renormalization, which we illustrate using nonlinear spin-wave theory.

The understanding of the physics of the integrable $\mathrm{spin}\text{−}1/2$ XXZ chain has witnessed substantial progress due to the development and application of sophisticated analytical and numerical techniques. In particular, infinite-temperature magnetization transport has turned out to range from ballistic, over superdiffusive, to diffusive behavior in different parameter regimes of the anisotropy. Since integrability is rather the exception than the rule, a crucial question is the change in transport under integrability-breaking perturbations. This question includes the stability of superdiffusion at the isotropic point and the change in diffusion constants in the easy-axis regime. In our work, we study this change in diffusion constants using a variety of methods and cover both linear response theory in the closed system and the Lindblad equation in the open system, where we focus on periodic boundary conditions throughout. In the closed system, we compare results from the recursion method to calculations for finite systems and find evidence of a continuous change in diffusion constants over the full range of perturbation strengths. In the open system weakly coupled to baths, we find diffusion constants in quantitative agreement with the ones in the closed system in a range of nonweak perturbations but disagreement in the limit of weak perturbations. Using a simple model of this limit, we point out the possibility of a diverging diffusion constant in such an open system.

We report a systematic study of the normal-state and superconducting properties of single-crystal ${\mathrm{Pb}}_{1-\mathrm{x}}{\mathrm{Sn}}_{\mathrm{x}}{\mathrm{TaSe}}_2$ $(0\le x\le 0.23)$. Sn doping enhances the superconducting temperature $T_c$ up to 5.1 K while also significantly increasing impurity scattering in the crystals. For $x=0$ and 0.018, the specific-heat jump at $T_c$ exceeds the Bardeen-Cooper-Schrieffer (BCS) weak-coupling value of 1.43, indicating the realization of strong-coupling superconductivity in undoped and slightly-Sn-doped ${\mathrm{PbTaSe}}_2$. Substituting Pb with more Sn lowers the specific-heat jump at $T_c$ below the BCS value of 1.43, which cannot be explained by a single-gap model. Rather, the observed specific-heat data of moderately-Sn-doped ${\mathrm{PbTaSe}}_2$ $(x=0.08 \text{and} 0.15)$ are reproduced by a two-gap model. Our density functional theory calculations suggest that three-dimensional Fermi pockets appear due to a reduction of the spin-orbit gap with Sn doping, and the multiband effect arising from these emergent Fermi pockets enhances the effective electron-phonon coupling strength, leading to the increase in $T_c$ of ${\mathrm{Pb}}_{1-\mathrm{x}}{\mathrm{Sn}}_{\mathrm{x}}{\mathrm{TaSe}}_2$.

The transformer architecture, known for capturing long-range dependencies and intricate patterns, has extended beyond natural language processing. Recently, it has attracted significant attention in quantum information and condensed matter physics. In this work, we propose the transformer density operator ansatz for determining the steady states of dissipative quantum many-body systems. By vectorizing the density operator as a many-body state in a doubled Hilbert space, the transformer encodes the amplitude and phase of the state's coefficients, with its parameters serving as variational variables. Our design preserves translation invariance while leveraging attention mechanisms to capture diverse long-range correlations. We demonstrate the effectiveness of our approach by numerically calculating the steady states of dissipative Ising and Heisenberg spin chain models, showing that our method achieves excellent accuracy in predicting mixed steady states.

Thermal quenching has been employed to discover various metastable materials such as hard steels and metallic glasses. More recently, quenching-based phase control has been applied to correlated electron systems that exhibit metal-insulator, magnetic, or superconducting transitions. Despite the discovery of metastable electronic phases, however, the microscopic origin of metastability remains elusive, as the system's degrees of freedom—such as electrons—can vary even at low temperatures. Here, using Monte Carlo simulations, we demonstrate a thermally quenched metastable phase in the Ising model that does not conserve the total magnetization. When multiple types of interactions that stabilize different long-range orders are introduced, the ordering kinetics exhibits significant slowing down, and the characteristic timescale for ordering diverges as the temperature decreases. As a result, the system can reach low temperatures without undergoing ordering if the cooling rate is sufficiently high. Quantitative analysis of the divergent behavior suggests that the energy barrier for eliminating the local structure of competing orders is the origin of this metastability. Thus, the present simulation demonstrates that competing interactions play an essential role in achieving metastability.

Boron arsenide (BAs) is considered a scientifically intriguing semiconductor material because of its extraordinary transport properties. In this work, we use state-of-the-art ab initio computational methods to study the carrier mobilities and scattering mechanisms in BAs and other boron-V compounds (BN, BP, and BSb). BAs has both high electron and hole mobilities, and the computed values agree well with experimental data. Our analysis of the carrier-phonon interactions indicates that the predominant scattering mechanism at room temperature is the piezoelectric (PE) scattering for BN and the acoustic deformation potential scattering for BP, BAs, and BSb. At elevated temperatures, the optical deformation potential scattering has an enhanced effect on the hole mobility, whereas the Fröhlich interaction affects both electrons and holes considerably. Consequently, the high mobilities in BAs can be attributed to the small carrier effective masses, low polarity, small PE constant, large dielectric constant, and high energies of optical phonons. The interband scattering is found to provide important scattering channels for holes. The intervalley scattering is trivial for electrons in BN, but it plays a crucial role for electrons in BP, BAs, and BSb, with the $f$-process much more significant than the $g$-process. We also move beyond carrier-phonon scattering to include other scattering sources. The carrier mobility with ionized impurity (ii) scattering is computed under various ii concentrations, which becomes appreciable with the ii concentration above ${10}^{17}{\mathrm{cm}}^{-3}$ at room temperature. Based on the phonon-limited and ii-limited mobilities, the reliability of Matthiessen's rule is discussed. And the carrier mobility with surface scattering in films is calculated as a function of the thickness, which exhibits significant deviation from the bulk value, especially for BP and BAs. The {100} surfaces are found preferable for charge transport, consistent with the computed electronic mean free path spectra. Finally, we have also identified that the self-energy relaxation time approximation (SERTA) fails in predicting carrier mobilities in certain materials because of the significant forward scattering. This suggests that SERTA may be unreliable for systems where scattering induced by long-range potentials predominates, including PE scattering, Fröhlich interaction, and ii scattering. Moreover, although the momentum relaxation time approximation is usually accurate, it may overestimate carrier mobilities in some systems where Fröhlich interaction plays a substantial role.

One route to the control of quantum magnetism at ultrafast timescales is magnetophononics, the modulation of magnetic interactions by coherently driven lattice excitations. Theoretical studies of a gapped quantum magnet subject to continuous, single-frequency driving of one strongly coupled phonon mode find intriguing phenomena including mutually repelling phonon-bitriplon excitations and global renormalization of the spin excitation spectrum. Because experiments are performed with ultrashort pulses that contain a wide range of driving frequencies, we investigate phonon-bitriplon physics under pulsed laser driving. We use the equations of motion to compute the transient response of the driven and dissipative spin-phonon system, which we characterize using the phonon displacement, phonon number, and triplon occupations. In the Fourier transforms of each quantity we discover a low-frequency energetic oscillation between the lattice and spin sectors, which is an intrinsically nonequilibrium collective mode, and demonstrate its origin as a beating between mutually repelling composite excitations. We introduce a phonon-bitriplon approximation that captures all the physics of hybridization, collective mode formation, and difference-frequency excitation, and show that sum-frequency phenomena also leave clear signatures in the response. We model the appearance of such magnetophononic phenomena in the strongly coupled spin-chain compound ${\mathrm{CuGeO}}_3$, whose overlapping phonon and spin excitation spectra are well characterized, to deduce the criteria for their possible observation in quantum magnetic materials.

The recent experimental studies of twisted bilayer graphene (TBG) raise a fundamental question: how do we understand Mott localization in a topological band? In this work, we offer a new perspective of Mott physics, which can be generalized to TBG directly in momentum space. In our theory, the Mott gap is understood as from an excitonlike hybridization $\mathrm{\Phi }\left(\mathbf{k}\right)c^{†}\left(\mathbf{k}\right)\psi \left(\mathbf{k}\right)$ between the physical electron $c$ and an ancilla fermion $\psi$. In the conventional Mott insulator of trivial band, the hybridization is $s$-wave with $\mathrm{\Phi }\left(\mathbf{k}\right)=\frac{U}{2}$, where $U$ is the on-site Hubbard interaction. On the other hand, the band topology in TBG enforces a topological Mott hybridization with $\mathrm{\Phi }\left(\mathbf{k}\right)\sim k_x\pm i{k}_y$ in a small region around $\mathbf{k}=0$. We dub this new Mott state as topological Mott localization because of the $p\pm ip$ order parameter analogous to the topological superconductor. At $\nu =0$, we find a topological Mott semimetal with a low energy effective theory resembling that of the untwisted bilayer graphene. For $\nu =\pm 1,\pm 2,\pm 3$, we show transitions from correlated insulators to Mott semimetals at smaller $U$. In the most intriguing density region $\nu =-2-x$, we propose a symmetric pseudogap metal at small $x$, which hosts a small Fermi surface and violates the perturbative Luttinger theorem. Interestingly, the quasiparticle is primarily formed by an ancilla fermion, which we interpret as a composite fermion formed by a hole bound to a particle-hole pair. Our theory offers a unified language to describe the Mott localization in both trivial and topological bands in momentum space, and we anticipate applications in other moiré systems with topological Wannier obstruction, such as the twisted transition-metal dichalcogenide (TMD) homobilayer.

Correctly characterizing the symmetry-protected topological (SPT) phases in open quantum systems is one of the most challenging problems in correlated quantum matter. Measurement-based quantum computation (MBQC) leverages the nontrivial edge states of SPT phases as logical qubits, making its computational power inherently linked to the topological properties of these phases. In this paper, we propose to use the gate fidelity which is a measure of the computational power of MBQC to identify the SPT phases in mixed-state settings. Specifically, we investigate the robustness of the Haldane phase by considering MBQC on the Affleck-Kennedy-Lieb-Tasaki state subject to different types of noises. To illustrate how our criterion works, we analytically and numerically calculated the gate fidelity to find that its behavior depends crucially on whether the noises satisfy a certain symmetry condition with respect to the on-site ${\mathbb{Z}}_2\times {\mathbb{Z}}_2$ symmetry. In particular, the fidelity for the identity gate, which is given by the sum of the nonlocal string order parameters, plays an important role. Furthermore, we demonstrate that stronger symmetry conditions are required to be able to perform other (e.g., the $Z$-rotation gate) gates with high fidelity. By examining which unitary gates can be implemented with MBQC on the decohered states, we can gain a useful insight into the richer structure of noisy SPT states that cannot be captured solely by the usual string order parameters.

Recently demonstrated intrinsic antiferromagnetic topological insulator ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ has attracted wide attention, due to its elevated temperature for the observation of quantum phenomena, including quantum anomalous Hall effect and axion insulator states. However, the ongoing debate regarding layer-number determination and the surface layer degradation or reconstruction revealed by prior studies poses significant challenges for the in-depth exploration of the intrinsic properties of ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$. Here, we present a systematic study of Raman spectroscopy in few-layer ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ samples, which were freshly cleaved and protected in an inert atmosphere throughout the experiments, ensuring adequate sample protection. We establish one-to-one correspondence between layer number and low-frequency breathing modes based on the uncontaminated samples. Comparison of the Raman spectra between ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ and ${\mathrm{Bi}}_2{\mathrm{Te}}_3$, as well as between samples near and far from regions sectioned by focused ion beam (FIB), demonstrates that surface-layer degradation from ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ to ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ can be effectively prevented by avoiding air exposure and damage from the FIB. Our results provide a reliable reference for identifying the layer number and crystal structure. In this study, we provide a valuable benchmark for assessing the sample quality and pave the way for further realization on exotic topological phases of ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$.

Quasiparticle excitations in material solids often experience a fictitious gauge field, which can be a potential source of intriguing transport phenomena. Here, we show that low-energy excitations in insulating antiferromagnetic skyrmion crystals on the triangular lattice are effectively described by magnons with an SU(3) gauge field. The three-sublattice structure in the antiferromagnetic skyrmion crystals is inherited as three internal degrees of freedom for the magnons, which are coupled to their kinetic motion via the SU(3) gauge field that arises from the topologically nontrivial spin texture in real space. We show that the noncommutativity of the SU(3) gauge field breaks an effective time-reversal symmetry and contributes to a magnon thermal Hall effect. We further demonstrate the emergence of the finite thermal Hall conductivity in the antiferromagnetic skyrmion crystals by the linear spin-wave theory. The possible impact of different gauge structures on a thermal Hall conductivity is also discussed.

The Kitaev model, which involves bond-direction-dependent spin interactions on a honeycomb lattice, has attracted significant interest due to its exact solvability and potential uses in quantum computing. A key feature of this model is the half-quantized thermal Hall conductivity (HQTHC) under a magnetic field perpendicular to the lattice; however, HQTHC appears only at low temperatures. In the higher temperature range beyond the HQTHC regime, we theoretically suggest an extrinsic phonon contribution to the thermal Hall effect in the Kitaev model through skew scattering of chiral phonons by scalar spin chirality, previously examined in Mott insulators. We demonstrate the emergence of scalar spin chirality from fluctuating spins, estimate the resulting field strength and its symmetric form applied to chiral phonons, and obtain the associated thermal Hall conductivity in semiquantitative agreement with existing experiments. This work offers a fundamental understanding of how spin-phonon interactions influence strongly correlated systems.

Two-dimensional multiferroics with out-of-plane ferroelectricity hold significant promise for miniaturized magnetoelectric spin-orbit transistors, yet systems combining robust ferroelectricity and strong magnetoelectric coupling are exceedingly rare. Here, we demonstrate that epitaxial strain stabilizes out-of-plane ferroelectricity in exfoliated two-dimensional Ruddlesden-Popper derivatives. The hybrid improper ferroelectric $Pc$ phase transitions to a competing $P{2}_1$ phase with purely in-plane polarization upon switching, accompanied by a ${90}^{\circ }$ rotation of weak ferromagnetism. Crucially, the $Pc$ phase exhibits altermagnetism, while $P{2}_1$ displays full-Brillouin-zone band splitting, with persistent spin textures rotating ${90}^{\circ }$ at the phase boundary. This work establishes a pathway to engineer two-dimensional multiferroics that integrate vertical polarization, magnetoelectric coupling, and switchable spin textures—key features for next-generation spintronic devices.

Magnons possess the ability to transport spin angular momentum in insulating magnetic materials, a characteristic that sets them apart from traditional electronics where power consumption arises from the movement of electrons. However, the practical application of magnon devices demands room-temperature operation and low switching power of perpendicular magnetization. Here we demonstrate the low-power manipulation of perpendicular magnetization via magnon torques in SnTe/NiO/CoFeB devices at room temperature. Topological crystalline insulator SnTe exhibits a high spin Hall conductivity of ${\sigma }_s\approx 6.1\times {10}^4\left(\hslash /2e\right){\left(\mathrm{\Omega }\mathrm{m}\right)}^{-1}$, which facilitates the generation of magnon currents in an antiferromagnetic insulator NiO. The magnon currents traverse the 20-nm-thick NiO layer and subsequently exert magnon torque on the adjacent ferromagnetic layer, leading to magnetization switching. Notably, we achieve a 22-fold reduction in power consumption in SnTe/NiO/CoFeB heterostructures compared to ${\mathrm{Bi}}_2{\mathrm{Te}}_3/\mathrm{NiO}/\mathrm{CoFeB}$ control samples. Our findings establish the low-power perpendicular magnetization manipulation through magnon torques, significantly expanding the range of topological materials with practical applications.