Degenerate two-dimensional electron liquids are theoretically established to possess two vastly distinct collisional electron mean free paths, where even-parity deformations of the Fermi surface are hydrodynamic with a short collisional mean free path but odd-parity deformations remain near ballistic (known as the “tomographic” transport regime). Predicted signatures of this regime rely on the scaling of observables with temperature or device dimension, both of which are difficult to establish with certainty. Here, we consider magnetotransport in a minimal model of tomographic electrons and show that even a small magnetic field suppresses tomographic transport signatures and thus acts as a sensitive and unique probe of this regime. Fundamentally, the magnetic field breaks time-reversal invariance, which is a prerequisite for the odd-even parity effect in the collisional relaxation. We analyze in detail the scaling of the transverse conductivity, which has been linked to small-channel conductance of interaction-dominated electrons, and show that a tomographic scaling regime at intermediate wave numbers is quickly suppressed with magnetic field to a hydrodynamic or collisionless form. We confirm that the suppression occurs at relatively small magnetic fields when the cyclotron radius is comparable to the ballistic mean free path of the dominant odd-parity mode. This occurs at a much smaller magnetic field than the magnetic field strength required to suppress hydrodynamic electron transport, which suggests an experimental protocol to extract the odd-parity mean free path.

Magnetic interactions are thought to play a key role in the properties of many unconventional superconductors, including cuprates, iron pnictides, and square-planar nickelates. Superconductivity was also recently observed in the bilayer and trilayer Ruddlesden-Popper nickelates, the electronic structure of which is expected to differ from that of cuprates and square-planar nickelates. Here we study how electronic structure and magnetic interactions evolve with the number of layers, $n$, in thin film Ruddlesden-Popper nickelates ${\mathrm{Nd}}_{n+1}{\mathrm{Ni}}_n{\mathrm{O}}_{3n+1}$ with $n=1,3$, and 5 using resonant inelastic x-ray scattering (RIXS). The RIXS spectra are consistent with a high-spin $|3d^8\underline{L}\rangle$ electronic configuration, resembling that of ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{NiO}}_4$ and the parent perovskite, ${\mathrm{NdNiO}}_3$. The magnetic excitations soften to lower energy in the structurally self-doped, higher-$n$ films. Our observations confirm that structural tuning is an effective route for altering electronic properties, such as magnetic superexchange, in this prominent family of materials.

We investigate experimentally and theoretically the behavior of the confined dark exciton in an InAs/GaAs semiconductor quantum dot, under the application of an external magnetic field in a Voigt configuration. We show that by varying the magnitude and direction of the external field one can accurately control the dark-exciton fine-structure splitting. In addition, we show that the dark-exciton spin state is approximately polarized along the cubic crystallographic directions [100] or equivalents. By comparing our experimental results with a model for the exchange and Zeeman interactions, we find the conditions for nullifying the fine-structure splitting between the two eigenstates of the dark exciton, thereby stopping its qubit precession.

The recently discovered Kagome superconductor $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ (where $A$ refers to K, Rb, Cs) has stimulated widespread research interest due to its interplay of nontrivial topology and unconventional correlated physics including charge-density waves (CDW) and superconductivity. The essential prerequisite to understanding the microscopic mechanisms of this complex electronic landscape is to unveil the configuration and symmetry of the charge-density wave order. As to now, little consensus has been made on what symmetry is broken. Herein, we clarify the microscopic structure and symmetry breaking of the CDW phase in ${\mathrm{RbV}}_3{\mathrm{Sb}}_5$ and ${\mathrm{KV}}_3{\mathrm{Sb}}_5$ by ultrafast time-resolved reflectivity. Our approach is based on extracting coherent phonon spectra induced by three-dimensional CDW and comparing them to calculated phonon frequencies via density-functional theory. The combination of these experimental results and calculations provides compelling evidence that the CDW structure of both compounds prevailing up to $T_{\text{CDW}}$ is the $2\times 2\times 2$ staggered inverse Star-of-David pattern with interlayer $\pi$ phase shift, in which the sixfold rotational symmetry is broken. These observations thus corroborate sixfold rotational symmetry breaking throughout the CDW phase of ${\mathrm{RbV}}_3{\mathrm{Sb}}_5$ and ${\mathrm{KV}}_3{\mathrm{Sb}}_5$.

We report on an experimental study of spin and valley blockade in two-electron bilayer graphene (BLG) double quantum dots (DQDs) and explore the limits set by asymmetric orbitals and electron-electron interactions. The results obtained from magnetotransport measurements on two-electron BLG DQDs, where the resonant tunneling transport involves both orbital symmetric and antisymmetric two-particle states, show a rich level spectrum. We observe a magnetic field tunable spin and valley blockade, which is limited by the orbital splitting, the strength of the electron-electron interaction and the difference in the valley $g$-factors between the symmetric and antisymmetric two-particle orbital states. Our conclusions are supported by simulations based on rate equations, which allow the identification of prominent interdot transitions associated with the transition from single- to two-particle states observed in the experiment.

We investigate the nonequilibrium dynamics of periodically driven Tomonaga-Luttinger liquids (TLLs) coupled to a thermal bath using a Floquet-Lindblad approach. When the coupling to the bath satisfies detailed balance, we obtain a condition for parametric instabilities to be suppressed, symmetrically for both chiralities. Remarkably, by designing a purely chiral coupling to the bath, instead of instability suppression, we uncover a driven-dissipative phase transition between the former symmetric parametric instability and a new chiral parametric instability. In the latter, a single chirality of bosonic quasiparticles gets exponentially amplified, leading to a dynamical chiral imbalance within the TLL, reminiscent of the non-Hermitian skin effect.

Electron-phonon coupling in magic-angle twisted bilayer graphene is an important but difficult topic. We propose a projection scheme to understand and simplify this problem. Weighted by the coupling strength with the low-energy heavy electrons ($f$ orbitals), several active moiré optical phonons are singled out that strongly couple to the flat band. These modes exhibit unique envelope functions at the moiré scale, while at the atomic scale, they inherit monolayer oscillations akin to the Kekulé pattern. By including both these active moiré phonon modes and the $f$ orbitals, an effective extended Holstein model is constructed, based on which both the different correlated insulating phases and the superconducting phase can be further studied in detail.

Electron-magnon coupling reveals key insights into the interfacial properties between nonmagnetic metals and magnetic insulators, influencing charge transport and spin dynamics. Here, we present temperature-dependent Raman spectroscopy and magnetotransport measurements of few-layer graphene (FLG)/antiferromagnetic ${\mathrm{FePS}}_3$ heterostructures. The magnon mode in ${\mathrm{FePS}}_3$ softens below 40 K and effective magnon stiffness decreases with cooling. Magnetotransport measurements show that FLG exhibits negative magnetoresistance (MR) in the heterostructure at low fields ($\pm 0.2\text{T}$), persisting up to 100 K; beyond this, MR transitions to positive. Notably, as layer thickness decreases, the coupling strength at the interface reduces, suppressing negative MR. Additionally, magnetodielectric measurements in the FLG/${\mathrm{FePS}}_3$/FLG heterostructure show an upturn at temperatures significantly below ($T_{\text{N}}$), suggesting a role for the magnon mode in capacitance, as indicated by hybridization between magnon and phonon bands in pristine ${\mathrm{FePS}}_3$ via magnetoelastic coupling.

We show that dynamics of the magnetization in ferromagnets can pump orbital angular momentum, a phenomenon we refer to as orbital pumping. This is the reciprocal phenomenon to orbital torque that induces magnetization dynamics by the orbital angular momentum in nonequilibrium. The orbital pumping is analogous to the spin pumping established in spintronics, but it requires spin-orbit coupling for the orbital angular momentum to interact with magnetization. We develop a formalism that describes the generation of orbital angular momentum by magnetization dynamics within the adiabatic perturbation theory. Based on this, we perform first-principles calculations of orbital pumping in prototypical $3d$ ferromagnets, Fe, Co, and Ni. Results show that the ratio between orbital pumping and spin pumping ranges from 5% to 15%, being smallest in Fe and largest in Ni. This implies that ferromagnetic Ni is a good candidate for measuring the orbital pumping. Implications of our results on experiments are also discussed.

The crystal symmetry plays a pivotal role in protecting the nontrivial electronic states in a topological phase. Manipulation of the crystal symmetry and hence the nontrivial topological states would serve as a fertile ground to explore exotic topological properties. Combining experimental and theoretical investigations, we demonstrate herein the flexible tuning of nontrivial topological states in the single phase of kagome magnet $\mathrm{GdM}{\mathrm{n}}_6\mathrm{G}{\mathrm{e}}_6$. Very large anomalous Hall conductivities caused by distinct Berry curvatures along different crystallographic directions are realized through selectively breaking the mirror symmetries in these directions by external magnetic field, which is fully supported by the first-principles calculations. Our results set an explicit example, demonstrating that the strong correlation between mirror symmetry and Berry curvature generates significant intrinsic anomalous Hall effect, as well as tunable topological properties in a single magnetic topological phase.

We study the nonunitary dynamics of a class of quantum circuits based on stochastically measuring check operators of subsystem quantum error-correcting codes, such as the Bacon-Shor code and its various generalizations. Our focus is on how properties of the underlying code are imprinted onto the measurement-only dynamics. We find that in a large class of codes with nonlocal stabilizer generators, at late times there is generically a nonlocal contribution to the subsystem entanglement entropy that scales with the subsystem size. The nonlocal stabilizer generators can also induce slow dynamics, since depending on the rate of competing measurements the associated degrees of freedom can take exponentially long (in system size) to purify (i.e., disentangle from the environment when starting from a mixed state) and to scramble (i.e., become entangled with the rest of the system when starting from a product state). Concretely, we consider circuits in which the nonlocal stabilizer generators of the underlying subsystem code take the form of subsystem symmetries. We present a systematic study of the phase diagrams and relevant time scales in two and three spatial dimensions for both Calderbank-Shor-Steane (CSS) and non-CSS codes, focusing in particular on the link between slow measurement-only dynamics and the geometry of the subsystem symmetry. A key finding of our paper is that slowly purifying or scrambling degrees of freedom appear to emerge only in codes for which subsystem symmetries are nonlocally generated, a strict subset of those whose symmetries are simply nonlocal. We comment on the link between our results on subsystem codes and the phenomenon of Hilbert-space fragmentation in light of their shared algebraic structure.

The continuous displacive phase transition in ${\mathrm{SrTiO}}_3$ near $T_c\approx 105$ K features a central elastic peak in neutron-scattering investigations at temperatures above $T_c$, i.e., before the corresponding soft phonon mode is overdamped upon cooling. The origin of this central peak is still not understood. Here, we report an inelastic x-ray scattering investigation of the cubic-to-tetragonal phase transition in ${\mathrm{SrTiO}}_3$. We compare quantitatively measurements of the soft phonon mode on two differently grown samples and discuss the findings regarding results from thermodynamic and transport probes such as specific heat and thermal conductivity. Furthermore, we use inelastic x-ray scattering to perform elastic scans with both high momentum and milli-electronvolt energy-resolution and, thus, are able to separate elastic intensities of the central peak from low-energy quasielastic phonon scattering. Our results indicate that the evolution of the soft mode is similar in both samples though the intensities of the central peak differ by a factor of four. Measurements revealing anisotropic correlation lengths on cooling towards $T_c$, indicate that local properties of the crystals to which collective lattice excitations are insensitive are likely at the origin of the central elastic line in ${\mathrm{SrTiO}}_3$.

An efficient algorithm is constructed for contracting two-dimensional tensor networks under periodic boundary conditions. The central ingredient is a novel renormalization step that scales linearly with system size, i.e., from $L\to L+1$. The numerical accuracy is comparable to state-of-the-art tensor network methods, while giving access to much more data points, and at a lower computational cost. Combining this contraction routine with the use of automatic differentiation, we arrive at an efficient algorithm for optimizing fully translation-invariant projected entangled-pair states on the torus. Our benchmarks show that this method yields finite-size energy results that are comparable to those from quantum Monte Carlo simulations. When combined with field-theoretical scaling techniques, our approach enables accurate estimates of critical properties for two-dimensional quantum lattice systems.

The texture in antiferromagnets hosting a topologically protected skyrmion can be viewed as two effectively coupled ferromagnetic skyrmions. Assuming rigid magnetic configurations, we show that this coupling accounts for the effective mass known for the antiferromagnetic skyrmion and that its dynamics can be viewed as due to a relative displacement of the two sublattice ferromagnetic skyrmions. The theory holds for different antiferromagnetic systems and includes effects from dissipation and external forces caused by electric currents. We verify our analytical results by micromagnetic simulations.

Nonlinear phenomena in condensed matter physics have been explored extensively in exotic materials, including materials with broken inversion symmetry, topological materials, metamaterials, and quantum wells. However, conventional nonmagnetic transition metals have largely been overlooked in this context because of their perceived relative simplicity. Here, we reveal the unexplored potential of nonmagnetic single-element metals by focusing on one type of nonlinear phenomenon: photocurrent generation induced by spin accumulation due to the spin Hall effect at the material edges. Distinctive behavior at the material edges, where the spatial symmetry is intrinsically broken, leads to observation of the nonlinear phenomenon. The resulting photocurrent is identified as a linear injection current and is associated with both time-reversal symmetry breaking and space-inversion symmetry breaking. This observation challenges the prevailing notion that nonlinear phenomena are exclusively observed in more exotic materials. We perform scanning photovoltage microscopy on Pt, W, Cu, and perpendicular magnetic anisotropy films to verify and characterize the observed linear injection current experimentally, thus confirming the crucial roles of spin accumulation and edge asymmetry in photocurrent generation. This finding provides a comprehensive understanding of linear injection currents realized during symmetry violation and is promising for development of spin- and symmetry-based techniques.

We present a comprehensive set of harmonic resistance measurements of the dampinglike (DL) and fieldlike (FL) torques in Pt/CoFeB, Pt/Co, W/CoFeB, W/Co, and YIG/Pt bilayers complemented by measurements of the DL torque using the magneto-optical Kerr effect and calibrated by nitrogen vacancy magnetometry on the same devices. The magnon creation-annihilation magnetoresistances depend strongly on temperature and on the magnetic and transport properties of each bilayer, affecting the estimate of both the DL and FL torque. The DL torque, the most important parameter for applications, is overestimated by a factor of 2 in W/CoFeB and by one order of magnitude in YIG/Pt when not accounting for the magnonic contribution to the planar Hall resistance. We further show that the magnonic contribution can be quantified by combining measurements of the nonlinear longitudinal and transverse magnetoresistances, thus providing a reliable method to measure the spin-orbit torques in different material systems.

Understanding quantum magnetism in two-dimensional systems represents a lively branch in modern condensed-matter physics. In the presence of competing superexchange couplings, magnetic order is frustrated and can be suppressed down to zero temperature. Still, capturing the correct nature of the exact ground state is a highly complicated task, since energy gaps in the spectrum may be very small and states with different physical properties may have competing energies. Here, we introduce a variational ansatz for two-dimensional frustrated magnets by leveraging the power of representation learning. The key idea is to use a particular deep neural network with real-valued parameters, a so-called transformer, to map physical spin configurations into a high-dimensional feature space. Within this abstract space, the determination of the ground-state properties is simplified and requires only a shallow output layer with complex-valued parameters. We illustrate the efficacy of this variational ansatz by studying the ground-state phase diagram of the Shastry-Sutherland model, which captures the low-temperature behavior of ${\mathrm{SrCu}}_2{\left({\mathrm{BO}}_3\right)}_2$ with its intriguing properties. With highly accurate numerical simulations, we provide strong evidence for the stabilization of a spin liquid between the plaquette and antiferromagnetic phases. In addition, a direct calculation of the triplet excitation at the $\mathrm{\Gamma }$ point provides compelling evidence for a gapless spin liquid. Our findings underscore the potential of neural-network quantum states as a valuable tool for probing uncharted phases of matter, and open up new possibilities for establishing the properties of many-body systems.

Many systems are topologically trivial in the bulk but still have nontrivial wave functions locally in the Brillouin zone. For example, in a small-gap Dirac material the Berry curvature is strongly peaked but cancels over the full Brillouin zone, while in semimetals and in nodal superconductors there may be a lower-dimensional winding topology associated to the nodes. Experimentally, it is difficult to directly observe such topology. We consider general bulk Hamiltonians with nodes and chiral symmetry, extending to them the method developed in Dutreix et al. [Nature (London) 574, 219 (2019)], which in particular detected the winding around Dirac cones in graphene using charge modulations around an impurity. We apply our method to nodal superconductors in two dimensions (2D), in the presence of a (non)magnetic impurity, measured by standard or spin-polarized scanning tunneling microscope (STM) tip. We derive general conditions on the impurity scattering and on the STM tip, expressed in terms of their preference among the two chiralities, for when the measurement near the impurity captures the winding difference between any chosen pair of (Bogoliubon) Dirac cones. We emphasize the robustness of observing vortices in momentum space, in contrast to dislocations in real space, in STM data. Testing the conditions on the topological nodal superconductor proposed for monolayer ${\mathrm{NbSe}}_2$ under an in-plane magnetic field, we find that spin-polarized STM on a magnetic impurity can detect the winding of each of the 12 nodes. We conclude that a judicious choice of impurity can be a powerful tool to determine topological quantities in 2D superconducting systems as well as any nodal chiral system.

Recent experiments with quantum simulators using ultracold atoms and superconducting qubits have demonstrated the potential of controlled dissipation as a versatile tool for realizing correlated many-body states. However, determining the dynamics of dissipative quantum many-body systems remains a significant analytical and numerical challenge. In this paper, we focus on a dissipative impurity problem as a testbed for new methodological developments. We introduce an efficient nonperturbative framework that combines the superposition of Gaussian states variational ansatz with the quantum trajectory approach to simulate open systems featuring a dissipative impurity. Applying this method to a spinful impurity subject to two-body losses and embedded in a bath of noninteracting fermions, we explore the full crossover from weak to strong dissipation regimes. The nonperturbative nature of the SGS ansatz allows us to thoroughly examine this crossover, providing comprehensive insights into the behavior of the system. In the strong dissipation regime, our approach reproduces the finding that localized two-body losses can induce the Kondo effect [Stefanini et al., arXiv:2406.03527], characterized by a slowdown of spin relaxation and an enhancement of charge conductance. Furthermore, we reveal an exotic negative conductance phenomenon at zero potential bias—a counterintuitive single-body effect resulting from intermediate dissipation and finite bandwidth. Finally, we investigate the formation of ferromagnetic domains and propose an extension to realize a higher-spin Kondo model using localized dissipation.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) possess a large direct band gap, which has been experimentally observed to shrink with increasing charge carrier density (doping). The effect has been the subject of theoretical study in recent years using various approaches and approximations. In this work, we develop the theory of band gap renormalization based on the Feynman diagrammatic technique in the imaginary frequency formalism. We consider dynamical screening from conduction band electrons using the random phase approximation (RPA), as well as screening from a metallic gate. While our theory is general for any 2D semiconductor, to be specific we consider ${\mathrm{MoS}}_2$ and ${\mathrm{WSe}}_2$ and compare with available experimental data. In both cases, we calculate large band gap renormalization that reaches several hundred meV at relatively low carrier density. This is in good agreement with experimental data.