It has been predicted that the time-dependent current in a Fabry-Pérot cavity can turn negative even if the applied voltage pulses are always positive. It has also been suggested that the negative currents are related to interfering paths; however, an analytic description of this surprising phenomenon has so far been missing. Here, we make use of Floquet scattering theory to demonstrate that the negative currents indeed are caused by the interference of scattering paths through the cavity with different numbers of round trips. Our analytic result can be tested in future experiments, as it predicts exactly how the effect should be washed out by an increasing temperature. We show that a similar phenomenon is expected for the heat current, which may also turn negative.

We study inverse spin and orbital Hall effects in 19 transition metals using spin pumping driven by ferromagnetic resonance. Spin-to-charge conversion was measured in YIG/X(5), while orbital-to-charge conversion was probed in YIG/Pt(2)/X(5) heterostructures. Here, X represents the different transition metals. We observe that even metals with relatively weak spin-orbit coupling can exhibit strong orbital-to-charge conversion. Surprisingly, the orbital contribution overwhelmingly dominates over the spin response, clarifying the challenge of disentangling these effects. Our results largely agree with first-principles predictions for spin and orbital Hall conductivities but reveal discrepancies in select materials.

The random phase approximation (RPA) and the $GW$ approximation share the same total energy functional but RPA is defined on a restricted domain of Green's functions determined by a local Kohn-Sham (KS) potential. In this work, we perform self-consistent RPA calculations by optimizing the local KS potential through the optimized effective potential equation. We study a number of solids (C, Si, BN, LiF, MgO, ${\mathrm{TiO}}_2$) and find in all cases a lowering of the total energy with respect to non-self-consistent RPA. We then propose a variational approach to optimize parameter-dependent hybrid functionals based on the minimization of the RPA total energy with respect to the fraction of exact exchange used to generate the input KS orbitals. We show that this scheme leads to hybrid functionals with a KS band structure in close agreement with RPA and with lattice constants of similar accuracy as within RPA. Finally, we evaluate $G_0{W}_0$ gaps using RPA and hybrid KS potentials as starting points. Special attention is given to ${\mathrm{TiO}}_2$, which exhibits a strong starting-point dependence.

We compute magnon Hall conductivities in a minimal model of a two-dimensional altermagnet. To do so, we derive an analytic expression for the relevant quantum geometric tensor describing two-band bosonic Bogoliubov Hamiltonians, providing insight into the geometric, topological, and transport properties. The magnon thermal Hall and spin Nernst conductivities are shown to directly depend on the altermagnetic parameter, which may serve as an experimental probe of altermagnetism.

We theoretically investigate the magnetic excitations in the quantum anomalous Hall insulator phase of twisted bilayer ${\mathrm{MoTe}}_2$ at a hole filling factor of $\nu =1$, focusing on magnon and domain wall excitations. Using a generalized interacting Kane-Mele model, we obtain the quantum anomalous Hall insualtor ground state with spin polarization. The magnon spectrum is then computed via the Bethe-Salpeter equation, revealing two low-energy topological magnon bands with opposite Chern numbers. To further explore the magnon topology, we construct a tight-binding model for the magnon bands, which is analogous to the Haldane model. We also calculate the energy cost of domain walls that separate regions with opposite Chern numbers and bind chiral edge states. Finally, we propose an effective spin model that describes both magnon and domain wall excitations, incorporating Heisenberg spin interactions and Dzyaloshinskii-Moriya interactions. The coupling constants in this model are determined from the numerical results for magnons and domain walls. This model accounts for the Ising anisotropy of the system, captures the magnon topology, and allows for the estimation of the magnetic ordering temperature. Our findings provide a comprehensive analysis of magnetic excitations in twisted ${\mathrm{MoTe}}_2$ and offer new insights into collective excitations in moiré systems.

We address weak-coupling frictional sliding with phononic dissipation by means of analytic many-body techniques. Our model consists of a particle (the “slider”) moving through a two- or three-dimensional crystal and interacting weakly with its atoms, and therefore, exciting phonons. By means of linear response theory, we obtain explicit expressions for the friction force slowing down the slider as a function of its speed, and compare them to the friction obtained by simulations, demonstrating a remarkable accord.

Hopf insulators represent a unique class of topological insulators that exist exclusively in two-band systems and are inherently unstable upon the inclusion of additional bands. Meanwhile, recent studies have shown that non-Hermiticity gives rise to distinctive complex-energy gap structures, known as point gaps, and associated topological phases with no analogs in Hermitian systems. However, non-Hermitian counterparts of Hopf insulators have remained largely elusive. Here, we generally classify topological phases of two-band non-Hermitian systems based on the homotopy theory and uncover Hopf-type point-gap topology present only for two bands. Specifically, we reveal such Hopf-type point-gap topology for three-dimensional systems with chiral symmetry (class AIII) and four-dimensional systems with no symmetry (class A). Explicitly constructing prototypical models from the Hermitian Hopf insulator, we further demonstrate that these non-Hermitian topological phases lead to anomalous point-gapless boundary states spectrally detachable from the bulk bands.

Planar Josephson junctions (JJs) have emerged as a promising platform for the realization of topological superconductivity and Majorana zero modes. To obtain robust quasi one-dimensional (1D) topological superconducting states using planar JJs, limiting the number of 1D Andreev bound states' subbands that can be present, and increasing the size of the topological superconducting gap are two fundamental challenges. It has been suggested that both problems can be addressed by properly designing the interfaces between the JJ's normal region and the superconducting leads. We fabricated Josephson junctions with periodic hole structures on the superconducting contact leads on InAs heterostructures with epitaxial superconducting Al. By depleting the chemical potential inside the holes region with a top gate, we observed an enhancement of the supercurrent across the junction. The theoretical analysis shows that the enhancement of the JJ's critical current is achieved when the depletion of the holes is such to optimize the matching of quasiparticles' wave function at the normal/superconductor interface. These results show how the combination of carefully designed patterns for the Al coverage, and external gates, can be successfully used to tune the density and wave functions' profiles in the normal region of the JJ, and therefore open an avenue to tune some of the critical properties, such as number of subbands and size of the topological gap, that must be optimized to obtain robust quasi-1D superconducting states supporting Majorana bound states.

Recent experiments have revealed that superconductivity in rhombohedral tetralayer graphene can emerge from a valley-polarized and, hence, chiral normal state. The interplay of pairing and the reduced normal-state symmetries sparked widespread interest. In this Letter, we demonstrate within a microscopic theoretical formalism that this stabilizes a nonreciprocal superconducting state. Our results are based on a fully self-consistent framework for determining the superconducting order parameter from a Kohn-Luttinger mechanism. We show that the system displays a sizable intrinsic superconducting diode effect, i.e., without the need for applying external magnetic fields, which is also highly tunable by an external displacement field. Moreover, we find that the angular dependence of the critical current is highly sensitive to the Fermi surfaces of the normal state. Hence, our results suggest that the critical current could provide insights into the type of Fermi-surface topology from which superconductivity arises.

We develop a four-sublattice spin-wave theory for the $g$-wave altermagnet candidate hematite ($\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$), considering both its easy-axis phase below and its weak ferromagnetic phase above the Morin temperature. A key question is whether the defining altermagnetic feature—magnon spin splitting (also called chirality or polarization splitting) due to nonrelativistic time-reversal symmetry breaking—remains intact when relativistic corrections, which contribute to hematite's magnetic order, are included. Using a detailed symmetry analysis supported by density functional theory, we show that capturing the magnon splitting within a Heisenberg model requires exchange interactions extending at least to the 13th neighbor. We find an altermagnetic band splitting of approximately $2\mathrm{meV}$, which contrasts with the total bandwidth of about $100\mathrm{meV}$. The splitting scales as $k^4$ in the long-wavelength limit ($k$ is the crystal momentum) and exhibits complex direction dependence. While three of the four expected altermagnetic nodal surfaces align with crystallographic mirror planes, the fourth deviates from planarity, leading to twelve instead of six nodes in the $k_z=0$ plane. To evaluate the experimental observability of this splitting, we analyze relativistic corrections to the magnon spectrum in both magnetic phases. We show that spin-orbit coupling—manifesting as magnetocrystalline anisotropies and the Dzyaloshinskii-Moriya interaction (DMI)—does not obscure the key altermagnetic features. In the easy-axis phase, DMI introduces small spectral corrections on the order of $100\mu \mathrm{eV}$. In the easy-plane phase, while DMI induces the well-documented weak ferromagnetic moment due to spin canting, its effect on the magnon spectrum is negligibly small, on the order of $25\mu \mathrm{eV}$. The dominant relativistic effect arises from easy-plane anisotropy, which splits magnon modes at the Brillouin-zone center and suppresses their spin expectation value. However, this effect remains weaker than the altermagnetic splitting and rapidly diminishes away from the zone center. Our analysis suggests that nonrelativistic altermagnetic splitting dominates at energies above $\approx 30\mathrm{meV}$, where the magnon spin polarization nearly recovers its quantized value. These findings indicate that inelastic neutron scattering can directly probe altermagnetic magnon splitting in hematite. We also discuss implications for magnon transport, particularly magnonic contributions to the thermal Hall effect (which requires spin-orbit coupling) and to spin splitter effects (which do not). Notably, we predict a third-order nonlinear magnon spin splitter effect: when a temperature gradient is applied along a direction in the $ab$ plane that does not coincide with a mirror plane, a spin current emerges along the $c$ axis. This result suggests that the $g$-wave magnon spin splitting in hematite enables transverse heat-to-spin conversion without requiring an external magnetic field.

We present a theory that studies the state composition of a bound exciton in magnetic field. Using a basis set made of products of free electron and hole wave functions in Landau gauge, we derive a secular equation which shows the relation between Landau levels (LLs) of the electron and hole when a bound exciton is formed. Focusing on excitons in the light cone, we establish a scattering selection rule for the interaction of an electron in LL $n_{\text{e}}$ with a hole in LL $n_{\text{h}}$. We solve the resulting secular equation and identify a simple pairing law, $n_{\text{e}}=n_{\text{h}}+l$, which informs us on the construction of a bound exciton state with magnetic quantum number $l$, and on the interaction of the exciton magnetic moment with magnetic field. We obtain good agreement between theory results and recent measurements of the diamagnetic shifts of exciton states in ${\mathrm{WSe}}_2$ monolayers.

We develop an excited-state real-space renormalization group (RSRG-X) formalism to describe the dynamics of conserved densities in randomly interacting spin-$\frac{1}{2}$ systems. Our formalism is suitable for systems with $\text{U}\left(1\right)$ and ${\mathbb{Z}}_2$ symmetries, and we apply it to chains of randomly positioned spins with dipolar $XX+YY$ interactions, as arise in Rydberg quantum simulators and other platforms. The formalism generates a sequence of effective Hamiltonians that provide approximate descriptions for dynamics on successively smaller energy scales. These effective Hamiltonians involve “superspins”: two-level collective degrees of freedom constructed from (anti)aligned microscopic spins. Conserved densities can then be understood as relaxing via coherent collective spin flips. For the well-studied simpler case of randomly interacting nearest-neighbor $XX+YY$ chains, the superspins reduce to single spins. Our formalism also leads to a numerical method capable of simulating the dynamics up to an otherwise inaccessible combination of large system size and late time. Focusing on disorder-averaged infinite-temperature autocorrelation functions, in particular the spin survival probability $\overline{S_p}\left(t\right)$, we demonstrate quantitative agreement between our algorithm and exact diagonalization (ED) at low but nonzero frequencies. Such agreement holds for chains with nearest-neighbor, next-nearest-neighbor, and long-range dipolar interactions. Our results indicate decay of $\overline{S_p}\left(t\right)$ slower than any power law and feature no significant deviation from the $\sim 1/{ln}^2\left(t\right)$ asymptote expected from the infinite-randomness fixed-point of the nearest-neighbor model. We also apply the RSRG-X formalism to two-dimensional long-range systems of moderate size and find slow late-time decay of $\overline{S_p}\left(t\right)$.

Two-dimensional materials with interacting Dirac excitations can host quantum multicritical behavior near the phase boundaries of the semimetallic and two-ordered phases. We study such behavior in Gross–Neveu–Yukawa field theories where $N_f$ flavors of Dirac fermions are coupled to two order-parameter fields with $\mathrm{SO}\left(N_A\right)$ and $\mathrm{SO}\left(N_B\right)$ symmetry, respectively. To that end, we employ the perturbative renormalization group up to three-loop order in $4-\epsilon$ spacetime dimensions. We distinguish two key scenarios: (i) The two orders are compatible as characterized by anticommuting mass terms, and (ii) the orders are incompatible. For the first case, we explore the stability of a quantum multicritical point with emergent $\mathrm{SO}(N_A+N_B)$ symmetry. We find that the stability is controlled by increasing the number of Dirac fermion flavors. Moreover, we extract the series expansion of the leading critical exponents for the chiral $\mathrm{SO}\left(4\right)$ and $\mathrm{SO}\left(5\right)$ models up to third order in $\epsilon$. Notably, we find a tendency towards rapidly growing expansion coefficients at higher orders, rendering an extrapolation to $\epsilon =1$ difficult. For the second scenario, we study a model with $\mathrm{SO}\left(4\right)\simeq \mathrm{SO}\left(3\right)\times \mathrm{SO}\left(3\right)$ symmetry, which was recently suggested to describe criticality of antiferromagnetism and superconductivity in Dirac systems. However, it was also argued that a physically admissible renormalization-group fixed point only exists for $N_f$ above a critical number $N_c^{>}$. We determine the corresponding series expansion at three-loop order as $N_c^{>}\approx 16.83-7.14\epsilon -7.12{\epsilon }^2$. This suggests that the physical choice of $N_f=2$ may be a borderline case, where true criticality and pseudocriticality, as induced by fixed-point annihilation, are extremely challenging to distinguish.

Twisted graphene moiré superlattice has been demonstrated as an exotic platform for investigating correlated states and nontrivial topology. Among the moiré family, twisted double bilayer graphene (TDBG) is a tunable flat band system expected to show stacking-dependent topological properties. However, electron correlations and the band topology are usually intertwined in the flat band limit, rendering the unique topological property due to stacking still elusive. Focusing on a large-angle TDBG with weak electron correlations, we probe the Landau level (LL) spectra in two differently stacked TDBG, i.e., ABBA- and ABAB-TDBG, to unveil their distinct topological properties. For ABBA-TDBG, we observe nontrivial topology at zero electric field $D=0$, evident from both the emergence of Chern bands from half-fillings $v=\pm 2$ and a giant orbital magnetic momentum. For ABAB-TDBG, by contrast, we find that the moiré band is topologically trivial at $D=0$, supported by the absence of LLs from half-fillings and the persistence of the gap at CNP above the critical magnetic fields. In addition, we also observe an evolution of the trivial-to-nontrivial topological transition at $D\ne 0$, confirmed by the emerged Landau fans originating from quarter-filling $v=1$. Our result demonstrates, for the first time, the unique stacking-dependent topology in TDBG, offering a promising avenue for future investigations on topological states, as well as correlated states, in dispersive band limits.

In the submonolayer regime, fullerene molecules tend to form compact islands on dense-packed metal surfaces. On Cu(111), these islands exhibit various types of corrugation patterns caused by a compressive epitaxial stress and an atomic-scale reconstruction of the metal surface. Following a strategy we introduced in a previous study [Phys. Rev. Lett. 131, 186201 (2023)], we use scanning tunneling microscopy to study molecule-molecule correlations in stripelike corrugation patterns that develop in ${\mathrm{C}}_{60}$ islands. Overall, these correlations are well described by a frustrated on-lattice Ising spin Hamiltonian that includes spin-spin interactions up to the third-nearest neighbor. Careful comparison between the measured and computed correlations allows us to distinguish molecular islands that exhibit a corrugation pattern typical of an equilibrated spin configuration from those that appear nonequilibrated. The statistical analysis performed both in real and reciprocal space confirms the relevance of using a frustrated spin Hamiltonian to characterize nontrivial molecular arrangements on surfaces.

Magnetic topological insulators (MTIs) host topologically protected edge states, but the role these edge states play in electronic transport remains unclear. Using scanning superconducting quantum interference device (SQUID) microscopy, we performed local measurements of the current distribution in a quantum anomalous Hall (QAH) insulator at large bias currents, where the quantization of the conductivity tensor breaks down. We find that bulk currents in the channel interior coexist with edge currents at the sample boundary. While the position of the edge current changes with the reversal of the magnetic field, it does not depend on the current direction. To understand our observations, we introduce a model that includes contributions from both the sample magnetization and currents driven by chemical potential gradients. To parametrize our model, we use local measurements of the chemical potential induced changes in the sample magnetization. Our model reveals that the observed edge currents can be understood as changes in the magnetization generated by the electrochemical potential distribution in the sample under bias. Our work underscores the complexity of electronic transport in MTIs and highlights both the value and challenges of using magnetic imaging to disentangle various contributions to the electronic transport signatures.

The ab initio computational method known as Hubbard-corrected density functional theory ($\mathrm{DFT}+U$) captures well ground electronic structures of a set of solids that are poorly described by standard DFT alone. Since lattice dynamical properties are closely linked to electronic structures, the Hubbard-corrected density functional perturbation theory ($\mathrm{DFPT}+U$) can calculate them at the same level of accuracy. To investigate the effects of $U$ on electron-phonon (el-ph) interactions, we implemented $\mathrm{DFPT}+U$ with a Hartree-Fock-based pseudohybrid functional formalism to determine $U$ self-consistently and applied our method to compute optical and transport properties of transparent conductive oxides of CdO and ZnO. For CdO, we find that opening a band gap due to $U$ restores the long-range Fröhlich interaction and that its calculated mobility and absorption spectrum are in excellent agreement with experiments. For ZnO where a band gap already appears at the DFT level, $\mathrm{DFPT}+U$ brings the results into much closer alignment with experiment, thus demonstrating improved accuracy of our method in dealing with el-ph interactions in these technologically important materials.

Following significant progress in the visualization and characterization of hybrid superconducting-semiconducting systems, greatly propelled by reports of Majorana zero modes in nanowire devices, considerable attention has been devoted to investigating the electronic structure at the buried superconducting-semiconducting interface and the nature of the induced superconducting correlations. The properties of that interface and the structure of the electronic wave functions that occupy it determine the functionality and the topological nature of the induced superconducting state. Here, we introduce a hybrid platform for proximity-inducing superconductivity in ${\mathrm{InAs}}_{0.6}{\mathrm{Sb}}_{0.4}$ nanowires, leveraging a unique architecture and material combination. By dispersing these nanowires over a superconducting indium film we exploit indium's high critical temperature of 3.7 K and the anticipated high spin-orbit and Zeeman couplings of ${\mathrm{InAs}}_{0.6}{\mathrm{Sb}}_{0.4}$. This design preserves the pristine top facet of the nanowires, making it highly compatible with scanning tunneling spectroscopy. Using this architecture we demonstrate that the mechanical contact supports Cooper-pair transparency as high as 90%, comparable with epitaxial interfaces. The anisotropic angular response to an applied magnetic field shows the quasi-two-dimensional nature of the parent superconductivity in the indium film and the quasi-one-dimensional nature of the induced superconductivity in the nanowires. Our platform offers robust and advantageous foundations for studying the emergence of topological superconductivity and the interplay of superconductivity and magnetism using atomic-scale spectroscopic tools.

High field magnetic resonance of nuclear spins is normally carried out at or near the Larmor frequency ${\mathrm{\Omega }}_0$. Here, we report an observation of nonsecular nuclear spin resonances far off ${\mathrm{\Omega }}_0$. These originate from nonsecular terms of the nuclear spin-spin interaction and provide means for the control of coupled two level systems. In systems of equivalent nuclear spins, nonsecular resonances were predicted to lead to anomalous, fast relaxation of the nuclear spin polarization. We report experimental results that are in excellent agreement with the predicted behavior.

Inspired by the discovery of a variety of correlated insulators in the moiré universe, controlled by interactions projected to a set of isolated bands with a narrow bandwidth, we examine here a partial sum rule associated with the inverse frequency-weighted optical conductivity restricted to low energies. Unlike standard sum rules that extend out to infinite frequencies, which include contributions from all interband transitions, we focus here on transitions associated only with the projected degrees of freedom. We analyze the partial sum rule in a nonperturbative but solvable limit for a variety of correlation-induced insulators. This includes (i) magic-angle twisted bilayer graphene at integer filling with projected Coulomb interactions, starting from the chiral flat-band limit and including realistic perturbations, (ii) fractional fillings of Chern-bands which support generalized Laughlin-like states, starting from a Landau level and including a periodic potential and magnetic-field, respectively, drawing connections to twisted ${\mathrm{MoTe}}_2$, and (iii) integer filling in toy models of nontopological flat bands with a tunable quantum geometry in the presence of repulsive interactions. The partial sum rule in all these examples is implicitly constrained by the form of the band quantum geometry via the low-lying excitation spectrum but is not related to it explicitly. For interacting Slater-determinant insulators, the partial sum rule is related to a “many-body projected quantum geometry”, obtained from the interaction-renormalized electronic bands. We also point out an intriguing connection between the partial sum rule and the quantum Fisher information associated with the projected many-body position operator. Finding alternative experimental routes that enable a direct measurement of the quantum geometry in correlated insulators remains an exciting and uncharted territory.