We present first-principles results on the magnetoelastic coupling in $\alpha \text{−}{\mathrm{RuCl}}_3$ and uncover a striking dependence of the magnetic coupling constants on strain effects. Different magnetic interactions are found to respond very unequally to variations in the lattice, with the Kitaev interaction being the most sensitive. Exact diagonalization results on our magnetoelastic model reproduce recent measurements of the structural Grüneisen parameter and explain the origin of the negative magnetostriction of $\alpha \text{−}{\mathrm{RuCl}}_3$, disentangling contributions related to different anisotropic interactions and $g$ factors. Uniaxial strain perpendicular to the honeycomb planes is predicted to reorganize the relative coupling strengths, strongly enhancing the Kitaev interaction while simultaneously weakening the other anisotropic exchanges under compression. Uniaxial strain may therefore pose a fruitful route to experimentally tune $\alpha \text{−}{\mathrm{RuCl}}_3$ nearer to the Kitaev limit.

We demonstrate a first-principles method to study magnetotransport in materials by solving the Boltzmann transport equation (BTE) in the presence of an external magnetic field. Our approach employs ab initio electron-phonon interactions and takes spin-orbit coupling into account. We apply our method to various semiconductors (Si and GaAs) and two-dimensional (2D) materials (graphene) as representative case studies. The magnetoresistance, Hall mobility, and Hall factor in Si and GaAs are in very good agreement with experiments. In graphene, our method predicts a large magnetoresistance, consistent with experiments. Analysis of the steady-state electron occupations in graphene shows the dominant role of optical phonon scattering and the breaking of the relaxation time approximation. Our paper provides a detailed understanding of the microscopic mechanisms governing magnetotransport coefficients, establishing the BTE in a magnetic field as a broadly applicable first-principles tool to investigate transport in semiconductors and 2D materials.

We report on spin Hall magnetoresistance (SMR) and spin Seebeck effect (SSE) in a single crystal of the rare-earth antiferromagnet $\mathrm{Dy}\mathrm{Fe}{\mathrm{O}}_3$ with a thin Pt film contact. The angular shape and symmetry of the SMR at elevated temperatures reflect the antiferromagnetic order of the ${\mathrm{Fe}}^{3+}$ moments as governed by the Zeeman energy, the magnetocrystalline anisotropy, and the Dzyaloshinskii-Moriya interaction. We interpret the observed linear dependence of the signal on the magnetic field strength as evidence for field-induced order of the ${\mathrm{Dy}}^{3+}$ moments up to room temperature. At and below the Morin temperature of $50\mathrm{K}$, the SMR monitors the spin-reorientation phase transition of ${\mathrm{Fe}}^{3+}$ spins. Below $23\mathrm{K}$, additional features emerge that persist below $4\mathrm{K}$, the ordering temperature of the ${\mathrm{Dy}}^{3+}$ magnetic sublattice. We conclude that the combination of SMR and SSE is a simple and efficient tool to study spin reorientation phase transitions and sublattice magnetizations.

We report the magnetic imaging of ferromagnetic domains in the van der Waals single crystal ${\text{MnSb}}_{\text{2}}{\text{Te}}_{\text{4}}$ from two different sources using cryogenic magnetic force microscopy. The magnetic field dependence of the domains reveals very weak pinning of domain walls in ${\text{MnSb}}_{\text{2}}{\text{Te}}_{\text{4}}$, resulting in a negligibly small magnetic hysteresis loop. The temperature dependence of the domain contrast reveals a mean field like behavior, in good agreement with that of bulk magnetization measurements.

Single crystal inelastic neutron scattering is used to study spin-wave excitations in the fully polarized state of the frustrated quantum ferroantiferromagnet $\mathrm{BaCdVO}{\left({\mathrm{PO}}_4\right)}_2$ . The data analysis is based on a Heisenberg spin Hamiltonian that includes as many distinct nearest-neighbor and next-nearest-neighbor interactions as allowed by crystal symmetry. All eight such exchange constants are obtained in a simultaneous fit to over 150 scans across the dispersion manifold. This establishes a definitive quantitative model of this material. It turns out to be substantially different from the one assumed in numerous previous studies based on powder experiments.

The Seebeck coefficient (thermopower) $S$ of the cuprate superconductor ${\mathrm{La}}_{1.6-x}{\mathrm{Nd}}_{0.4}{\mathrm{Sr}}_x\mathrm{Cu}{\mathrm{O}}_4$ was measured across its doping phase diagram (from $p=0.12$ to $p=0.25$), at various temperatures down to $T\simeq 2$ K, in the normal state accessed by suppressing superconductivity with a magnetic field up to $H=37.5$ T. The magnitude of $S/T$ in the $T=0$ limit is found to suddenly increase, by a factor $\simeq 5$, when the doping is reduced below $p^{★}=0.23$, the critical doping for the onset of the pseudogap phase. This confirms that the pseudogap phase causes a large reduction of the carrier density $n$, consistent with a drop from $n=1+p$ above $p^{★}$ to $n=p$ below $p^{★}$, as previously inferred from measurements of the Hall coefficient, resistivity, and thermal conductivity. When the doping is reduced below $p=0.19$, a qualitative change is observed whereby $S/T$ decreases as $T\to 0$, eventually to reach negative values at $T=0$. In prior work on other cuprates, negative values of $S/T$ at $T\to 0$ were shown to result from a reconstruction of the Fermi surface caused by charge density wave (CDW) order. We therefore identify $p_{\mathrm{CDW}}\simeq 0.19$ as the critical doping beyond which there is no CDW-induced Fermi surface reconstruction. The fact that $p_{\mathrm{CDW}}$ is well separated from $p^{★}$ reveals that there is a doping range below $p^{★}$ where the transport signatures of the pseudogap phase are unaffected by CDW correlations, as previously found in $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_y$ and ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x\mathrm{Cu}{\mathrm{O}}_4$.

We show that a Weyl superconductor can absorb light via a surface-to-bulk mechanism, which we dub the topological anomalous skin effect. This occurs even in the absence of disorder for a single-band superconductor, and is facilitated by the topological splitting of the Hilbert space into bulk and chiral surface Majorana states. In the clean limit, the effect manifests as a characteristic absorption peak due to surface-bulk transitions. We also consider the effects of bulk disorder, using the Keldysh response theory. For weak disorder, the bulk response is reminiscent of the Mattis-Bardeen result for $s$-wave superconductors, with strongly suppressed spectral weight below twice the pairing energy, despite the presence of gapless Weyl points. For stronger disorder, the bulk response becomes more Drude-like and the $p$-wave features disappear. We show that the surface-bulk signal survives when combined with the bulk in the presence of weak disorder. The topological anomalous skin effect can therefore serve as a fingerprint for Weyl superconductivity. We also compute the Meissner response in the slab geometry, incorporating the effect of the surface states.

We study how symmetry can enrich strong-randomness quantum critical points and phases, and lead to robust topological edge modes coexisting with critical bulk fluctuations. These are the disordered analogs of gapless topological phases. Using real-space and density matrix renormalization group approaches, we analyze the boundary and bulk critical behavior of such symmetry-enriched random quantum spin chains. We uncover a new class of symmetry-enriched infinite randomness fixed points: while local bulk properties are indistinguishable from conventional random singlet phases, nonlocal observables, and boundary critical behavior are controlled by a different renormalization group fixed point. We also illustrate how such new quantum critical points emerge naturally in Floquet systems.

The dispersion relation $\epsilon \left(k\right)$ of the elementary excitations of superfluid ${}^4\mathrm{He}$ has been measured at very low temperatures, from saturated vapor pressure up to solidification, using a high-flux time-of-flight neutron-scattering spectrometer equipped with a high-spatial-resolution detector (${10}^5$ “pixels”). A complete determination of $\epsilon \left(k\right)$ is achieved, from very low wave vectors up to the end of Pitaeskii's plateau. The results compare favorably in the whole the wave-vector range with the predictions of the dynamic many-body theory. At low wave vectors, bridging the gap between ultrasonic data and former neutron measurements, the evolution with the pressure from anomalous to normal dispersion, as well as the peculiar wave-vector dependence of the phase and group velocities, are accurately characterized. The thermodynamic properties have been calculated analytically, developing Landau's model, using the measured dispersion curve. A good agreement is found below 0.85 K between direct heat capacity measurements and the calculated specific heat, if thermodynamically consistent power series expansions are used. The thermodynamic properties have also been calculated numerically; in this case, the results are applicable with excellent accuracy up to 1.3 K, a temperature above which the dispersion relation itself becomes temperature dependent.

We study stabilizer quantum error correcting codes (QECC) generated under hybrid dynamics of local Clifford unitaries and local Pauli measurements in one dimension. Building upon (1) a general formula relating the error susceptibility of a subregion to its entanglement properties, and (2) a previously established mapping between entanglement entropies and domain wall free energies of an underlying spin model, we propose a statistical mechanical description of the QECC in terms of “entanglement domain walls.” Free energies of such domain walls generically feature a leading volume law term coming from its “surface energy,” and a subvolume law correction coming from thermodynamic entropies of its transverse fluctuations. These are most easily accounted for by capillary-wave theory of liquid-gas interfaces, which we use as an illustrative tool. We show that the information-theoretic decoupling criterion corresponds to a geometric decoupling of domain walls, which further leads to the identification of the “contiguous code distance” of the QECC as the crossover length scale at which the energy and entropy of the domain wall are comparable. The contiguous code distance thus diverges with the system size as the subleading entropic term of the free energy, protecting a finite code rate against local undetectable errors. We support these correspondences with numerical evidence, where we find capillary-wave theory describes many qualitative features of the QECC; we also discuss when and why it fails to do so.

An emerging frontier in condensed matter physics involves novel electromagnetic responses, such as the anomalous Hall effect (AHE), in ferromagnetic Weyl semimetals (FM-WSMs). Candidate FM-WSMs have been limited to materials that preserve inversion symmetry and generate Weyl crossings by breaking the time-reversal symmetry. These materials share three common features: a centrosymmetric lattice, a collinear FM ordering, and a large AHE observed when the field is parallel to the magnetic easy axis. Here we present CeAlSi as a FM-WSM in which the Weyl nodes are stabilized by breaking the inversion symmetry, but their positions are tuned by breaking the time-reversal symmetry. Unlike the other FM-WSMs, CeAlSi has a noncentrosymmetric lattice, a noncollinear FM ordering, and a novel AHE that is anisotropic between the easy and hard magnetic axes. It also exhibits large FM domains that are promising for exploring both device applications and the interplay between the Weyl nodes and FM domain walls.

Most material properties of great physical interest are directly related to nuclear dynamics, e.g., the ionic thermal conductivity, Raman/IR vibrational spectra, inelastic x-ray, and neutron scattering. A theory able to compute from first principles these properties, accounting for the anharmonicity and quantum fluctuations in the nuclear energy landscape that can be implemented in systems with hundreds of atoms is missing. Here, we derive an approximate theory for the quantum time evolution of lattice vibrations at finite temperature. This theory introduces the time dynamics in the self-consistent harmonic approximation (SCHA) and shares with the static case the same computational cost. It is nonempirical, as pure states evolve according to the Dirac least action principle and the dynamics of the thermal ensemble conserves both energy and entropy. The static SCHA is recovered as a stationary solution of the dynamical equations. We apply perturbation theory around the static SCHA solution and derive an algorithm to compute efficiently quantum dynamical response functions. Thanks to this new algorithm, we have access to the response function of any general external time-dependent perturbation, enabling the simulation of phonon spectra without following any perturbative expansion of the nuclear potential or empirical methods. We benchmark the algorithm on the IR and Raman spectroscopy of high-pressure hydrogen phase III, with a simulation cell of 96 atoms. Our work also explores the nonlinear regime of the dynamical nuclear motion, providing a paradigm to simulate the interaction with intense or multiple probes, as in pump-probe spectroscopy, or chemical reactions involving light atoms, as the proton transfer in biomolecules.

We study the late time relaxation dynamics of a pure $U\left(1\right)$ lattice gauge theory in the form of a dimer model on a bilayer geometry. To this end, we first develop a proper notion of hydrodynamic transport in such a system by constructing a global conservation law that can be attributed to the presence of topological solitons. The correlation functions of local objects charged under this conservation law can then be used to study the universal properties of the dynamics at late times, applicable to both quantum and classical systems. Performing the time evolution via classically simulable automata circuits unveils a rich phenomenology of the system's nonequilibrium properties: For a large class of relevant initial states, local charges are effectively restricted to move along one-dimensional “tubes” within the quasi-two-dimensional system, displaying fracton-like mobility constraints. The timescale on which these tubes are stable diverges with increasing systems size, yielding a novel mechanism for nonergodic behavior in the thermodynamic limit. We further explore the role of geometry by studying the system in a quasi-one-dimensional limit, where the Hilbert space is strongly fragmented due to the emergence of an extensive number of conserved quantities. This provides an instance of a recently introduced concept of “statistically localized integrals of motion,” whose universal anomalous hydrodynamics we determine by a mapping to a problem of classical tracer diffusion. We conclude by discussing how our approach might generalize to study transport in other lattice gauge theories.

High-$T_{\mathrm{c}}$ cuprate superconductors host spin, charge, and lattice instabilities. In particular, in the antiferromagnetic glass phase, over a large doping range, lanthanum-based cuprates display a glass-like spin freezing with antiferromagnetic correlations. Previously, sound velocity anomalies in ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ (LSCO) for hole doping $p=x\ge 0.145$ were reported and interpreted as arising from a coupling of the lattice to the magnetic glass [M. Frachet, I. Vinograd et al., Nat. Phys. 16, 1064 (2020)]. Here we report both sound velocity and attenuation in LSCO $p=0.12$, i.e., at a doping level for which the spin freezing temperature is the highest. Using high magnetic fields and comparing with nuclear magnetic resonance measurements, we confirm that the anomalies in the low temperature ultrasound properties of LSCO are produced by a coupling between the lattice and the spin glass. Moreover, we show that both sound velocity and attenuation can be simultaneously accounted for by a simple phenomenological model originally developed for canonical spin glasses. Our results point towards a strong competition between superconductivity and spin freezing, tuned by the magnetic field. A comparison of different acoustic modes suggests that the slow spin fluctuations have a nematic character.

We theoretically investigate the switching dynamics of stochastic nanomagnets and highlight the mechanism describing their relaxation time. We reveal the distinct switching mechanisms in perpendicular and in-plane easy-axis nanomagnets, and report that the relaxation time in in-plane nanomagnets varies by a few orders of magnitude only by changing the effective perpendicular anisotropy field, even though it does not contribute to the thermal stability factor. We introduce the entropy of nanomagnets into Brown's theory [Phys. Rev. 130, 1677 (1963)], and reveal that the system with faster precession as well as larger damping and smaller magnetic moment shows shorter relaxation time, and explains the different time scale of relaxation times in perpendicular- and in-plane easy-axis nanomagnets. We also show that the attempt frequency changes in both perpendicular and in-plane nanomagnets depending on the magnitude of anisotropy and show that there is a lower limit of relaxation time in perpendicular systems. This work gives physical insights of thermally activated dynamics of nanomagnets as well as its material/device design guidelines for achieving shorter relaxation times.

We resolve the domain-wall structure of the model antiferromagnet ${\text{Cr}}_2{\text{O}}_3$ using nanoscale scanning diamond magnetometry and second-harmonic-generation microscopy. We find that the ${180}^{\circ }$ domain walls are predominantly Bloch-like, and can coexist with Néel walls in crystals with significant in-plane anisotropy. In the latter case, Néel walls that run perpendicular to a magnetic easy axis acquire a well-defined chirality. We further report quantitative measurement of the domain-wall width and surface magnetization. Our results provide fundamental input and an experimental methodology for the understanding of domain walls in pure, intrinsic antiferromagnets, which is relevant to achieve electrical control of domain-wall motion in antiferromagnetic compounds.

We introduce a two-dimensional network model that realizes a higher-order topological phase (HOTP). We find that in the HOTP the bulk and boundaries of the system are gapped, and a total of 16 corner states are protected by the combination of a fourfold rotation, a phase-rotation, and a particle-hole symmetry. In addition, the model exhibits a strong topological phase at a point of maximal coupling. This behavior is in opposition to conventional network models, which are gapless at this point. By introducing the appropriate topological invariants, we show how a point group symmetry can protect a topological phase in a network. Our work provides the basis for the realization of HOTP in alternative experimental platforms implementing the network model.

Signatures of both the quantum anomalous Hall effect and axion electrodynamics have been recently observed to exist in thin films of ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$, a stoichiometric antiferromagnetic topological insulator. Direct evidence of the bulk topological magnetoelectric response in an axion insulator requires an energy gap at its topological surface state (TSS). However, independent spectroscopic experiments revealed that such a surface gap is much smaller than previously thought. Here we utilize angle resolved photoemission spectroscopy and density functional theory calculations to demonstrate that a sizable TSS gap unexpectedly exists in Sb-doped ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ where the bulk system remains topologically nontrivial. This gap is found to be insensitive to the bulk antiferromagnetic-paramagnetic transition, while it enlarges along with increasing Sb concentration, enabling simultaneous tunability of the Fermi level and the TSS gap size (up to $>100$ meV). Our work shows that Sb dopants in ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ can not only control the Fermi level but also induce a tunable surface gap, providing a potential platform to observe the key features of the high-temperature axion-insulator phase.

We have investigated the electronic structure of the metallic oxide ${\mathrm{ReO}}_3$ using bulk-sensitive angle-resolved soft-x-ray and angle-integrated hard-x-ray photoelectron spectroscopy. We observed clear dispersions of the Re $5d$- and O $2p$-derived bands as well as the momentum splitting of the Fermi surface due to the Re $5d$ spin-orbit interaction. We found that density-functional-based band structure methods can provide an accurate description of the observed electronic states. To achieve the accuracy it was necessary to utilize hybrid functionals for the calculations.

With the breakdown of coherent Kondo scattering with rising temperature, the Fermi surface (FS) in rare-earth Kondo lattices is expected to transition from the large FS counting the $4f$ moments, which form entangled quasiparticles with strongly enhanced effective masses with the conduction states, to the small FS with decoupled and localized $4f$ moments. A direct observation of this transition with temperature has, however, remained elusive, because conventional probes of the FS in Kondo lattices require high magnetic fields, which might reconstruct the FS or cannot work reliably at elevated temperatures. Using high-resolution Compton scattering, we overcome these limitations and show that the FS topology in the prototypical Kondo lattice $\mathrm{Yb}{\mathrm{Rh}}_2{\mathrm{Si}}_2$ undergoes pronounced changes between 14 and 300 K in zero magnetic field. In particular, we present clear evidence for a largely restored small FS at room temperature. Our results suggest the relevant energy scale of the complex Kondo crossover phenomenon in $\mathrm{Yb}{\mathrm{Rh}}_2{\mathrm{Si}}_2$.