Electron matter waves coherently diffract when passing through a periodic structure of light formed by two interfering light waves. In this so-called Kapitza-Dirac effect, the electron momentum changes due to absorption and emission of photons via stimulated Compton scattering. Until now, the effect has only been observed with low energy electrons due to the small momentum of a visible photon compared to the momentum of high energy electron leading to diffraction angles of ${10}^{-4}\mathrm{rad}$ or smaller. We report on the observation of the Kapitza-Dirac effect in a scanning electron microscope using high energy (20 and 30 keV) electrons with de Broglie wavelengths of 9 and 7 pm, respectively. The photon sidebands in the electron transverse momentum spectrum are detected in the convergent beam diffraction geometry using spatial filtering. As the coupling strength between the electrons and the light field increases, the sideband populations exhibit coherent, reversible oscillations among diffraction orders. The effect can serve as a coherent electron beam splitter or a phase plate in various types of electron microscopes.

The interaction between relativistic electron beams and intense laser fields has been extensively studied for generating high-energy radiation. However, achieving coherent radiation from such interactions needs to precisely control the phase matching of the radiating electrons, which has proven to be exceptionally challenging. In this Letter, we demonstrate that coherent attosecond radiation can be produced when a laser pulse interacts at a grazing angle with a relativistic electron beam. The electrons oscillate in the laser field and are modulated with a superluminal phase, and coherent ultrashort pulse trains are produced in the far field at the Cherenkov angle. This is verified by theoretical modeling and numerical simulations, including three-dimensional particle-in-cell (PIC) simulations and far-field time-domain radiation simulations. It is further demonstrated that the scheme exhibits robustness across a wide range of beam parameters, even for a large-size, rarefied electron beam, making it experimentally feasible. Based on our proposed scheme, high-repetition-rate, compact, and high-energy attosecond pulse sources are available.

Superconducting diodes that support nonreciprocal supercurrent flow in principle constitute attractive, nondissipative, circuit elements for superconducting electronics. But their realization faces fundamental challenges, as conventional Josephson tunnel junctions are inherently reciprocal. The so far existing approaches to break the reciprocity typically involve magnetism or spin-orbit coupling, which often increase the device complexity and limit the reproducibility. Here, we demonstrate an alternative dynamical route to supercurrent nonreciprocity based on parametric driving. By applying a frequency-modulated supercurrent amplitude, we show that effective higher-order, nonharmonic terms are generated in the current-phase relation. Leveraging strong mathematical similarities with the Kapitza pendulum, we show that these terms dynamically break the reciprocity. This establishes the concept of a Kapitza supercurrent diode and demonstrates that nonreciprocal superconducting transport can be engineered by a nonequilibrium driving the conventional Josephson tunnel junctions. We propose two implementations of such a Kapitza supercurrent diode—via gate-controlled superconducting interferometers or flux-driven double-loop SQUIDs—to achieve nonreciprocal supercurrent transport within experimentally accessible frequencies $\omega /2\pi \sim 1$ to 10 GHz.

In classical fluid dynamics, the von Kármán vortex street behind a bluff body is the canonical example used to study vortex formation and dynamics. This is governed by a local Hopf bifurcation, which drives the continuous and asynchronous release of vortical structures in the wake. In superfluid systems, however, the quantization of vorticity fundamentally alters this picture. Several aspects of quantum vortex shedding remain unresolved, including the absence of a unified description spanning the pre- and post-threshold regimes. We show that the onset of vortex emission is governed by a global saddle node on invariant circle bifurcation. This mechanism produces a discrete transition characterized by a macroscopic bottleneck in phase space and a universal inverse-square-root divergence of the emission period near the critical velocity. These features are robust across obstacles of different shapes, revealing a universal mechanism underlying vortex shedding in quantum fluids.

Nanometer-scale modulations can spontaneously emerge in complex materials when multiple degrees of freedom interact. Here we demonstrate that ferroelectric ${\mathrm{Sr}}_{1-x}{\mathrm{Ca}}_x{\mathrm{TiO}}_3$ lies in close proximity to a finite-$q$ lattice instability associated with a tendency toward structural modulation. Using inelastic neutron and x-ray scattering, we show that upon cooling, dipolar fluctuations strongly couple to and soften the $c_{44}$ transverse acoustic mode. We identify the wave vector at which this softening is maximal, thereby defining the characteristic length scale of the instability. Calcium substitution enhances both the amplitude and the wavevector of the softening by strengthening the ferroelectric and antiferrodistortive instabilities. Our results demonstrate that the flexoelectric phonon coupling promotes a tendency toward a dynamically modulated state that cooperates with, rather than competes against, the other lattice instabilities in ${\mathrm{SrTiO}}_3$.

The pursuit of low-loss, directional energy transport and heat localization is critical in frontier fields such as hybrid quantum computing, thermophysics, and materials science. Localized phonon modes, with spatially confined vibrational energy and suppressed diffusive scattering, provide promising routes toward this goal. However, conventional approaches to phonon localization necessitate material disorder or nonlinear interactions, limiting their potential for controllable, on-demand energy routing and concentration in pristine, scalable materials. In this Letter, we demonstrate disorder-free, solitonlike phonon transport in atomic moiré superlattices (AMS), arising purely from engineered moiré interlayer interactions and characterized by non-Anderson localization and phononic flat bands. The vibrational energy is robustly confined at AA-stacking regions of 1D-AMS, and evolves into solitonlike propagation along moiré-induced channels in translational 2D-AMS. Furthermore, we show that coaxial double-wall moiré nanotubes leverage anisotropic interwall coupling to achieve simultaneous azimuthal localization and axial channelized transport, enabling programmable, low-dispersion phononic signal transmission with minimal crosstalk. By bridging concepts from classical wave physics and quantum moiré materials science, this Letter reveals a method for controlling phononic energy routing and local entropy landscapes in designed atomic structures. This moiré phononics approach holds great potential in thermal logic circuits, low-loss phonon-based communication networks, and phononic information processing in van der Waals structures.

The full three-dimensional spin texture of the Bi(110) surface states has been investigated by means of spin- and angle-resolved photoemission spectroscopy. The observed complex in-plane spin texture is in basic agreement with the prior reports and first-principles calculations, and the previously unreported out-of-plane spin polarization ($P_Z$) is observed to be substantial. More significantly, the $P_Z$ texture exhibits an unexpected breaking of spin degeneracy at the Brillouin zone boundary, in contradiction to standard symmetry-based expectations. Our first-principles calculations reveal that the anticipated vanishing of $P_Z$ arises from the quantum interference between two surface wave functions propagating with opposite out-of-plane spin polarizations. These findings strongly suggest that the photoemission process selectively probes one of them, exposing the hidden $P_Z$ polarization of the surface wave function; i.e., the apparent spin vanishing is not due to an absence of polarization, but rather results from interference at the wave function level.

The spatial resolution of core-loss electron energy-loss spectroscopy (EELS) is limited by the delocalization of inelastic scattering from long-range Coulomb interactions, especially for light-element edges at low energy losses. Here we demonstrate atomic-resolution imaging of the Si $L$ edge in Si [110] using dark-field annular EELS that selectively detects high-angle inelastic scattering. Although the intrinsic excitation delocalization remains unchanged, momentum-transfer–selective detection suppresses long-range contributions, enabling the 1.36 Å Si dumbbell to be resolved.

Altermagnetism has emerged as an unconventional form of collinear magnetism with spatial rotational symmetries, that give rise to strongly spin-split bands despite an underlying fully compensated antiferromagnetic order. Here, we develop a theory for the angle resolved photoemission spectroscopy response of altermagnetic Mott insulators. Crucially, the spectrum does not simply reflect the noninteracting band structure, but instead a magnetic polaron is formed at low energies, that can be interpreted as a spinon-holon bound state. We develop a spinon-holon parton theory and predict a renormalized bandwidth that we confirm by tensor network simulations. We analyze the characteristic spin-split spectrum and identify a spin-dependent spectral weight of the magnetic polaron, resulting from the altermagnetic symmetry. Our Letter paves the way for a systematic study of doping effects and correlation phenomena in altermagnetic Mott insulators.

Many strongly correlated states, such as those arising in the fractional quantum Hall effect and spin liquids, are described by wave functions obtained by dividing particles into multiple clusters, constructing a readily evaluable wave function in each cluster, and (anti)symmetrizing across these clusters. We introduce a method to compute quantities such as energies and correlators, using Monte Carlo simulations for these states. Our framework overcomes the factorial scaling of explicit (anti)symmetrization, allowing for studies of systems beyond the reach of exact diagonalization.

Seeking symmetry where it is not manifest, physicists have explored hidden symmetries throughout classical to quantum systems. A framework of generalized geometric symmetries, dubbed “latent symmetries,” whose existence can be elegantly discerned entirely algebraically, has fueled this endeavor anew. However, latent symmetries become obvious only in a suitably chosen effective Hamiltonian. In this Letter, we drastically advance the understanding of latent symmetries by establishing a correspondence between latent and geometric symmetries. We show that any system with latent symmetry can be similarity transformed into an equivalent system with explicit geometric symmetry and that the converse holds under mild conditions. In other words, latent symmetries are geometric symmetries in disguise. On a practical level, this latent-geometry correspondence implies that phenomena associated with spatial symmetries can be further engineered in latently symmetric systems. We demonstrate this by experimentally realizing higher-order topological insulators (HOTI) protected by latent symmetry groups. Our Letter provides a unified perspective on latent and geometric symmetry, and it paves the way for utilizing latent symmetries in designing and engineering exotic matter in classical waves, solid-state materials, and quantum systems.

We study the full distribution of the zero-temperature Hall conductivity in a lattice model of the integer quantum Hall effect across disorder realizations. Near the plateau transition, the distributions develop heavy power-law tails with exponent $\alpha \approx 2.2-2.5$, implying a finite mean but divergent variance. The tail persists across system sizes, correlation lengths of the disorder potential, and fillings indicating that Hall conductivity is not self-averaging at criticality. The exponent is qualitatively compatible with predictions from random-matrix models of Berry curvature fluctuations, hinting at a universal statistical structure of the quantum Hall critical regime.

Previous studies of the $S=1/2$ triangular-lattice $J_1\text{−}{J}_2$ Heisenberg antiferromagnet have inferred the existence of a nonmagnetic ground-state phase for an intermediate range of $J_2$, but disagree concerning whether it is a gapped ${\mathbb{Z}}_2$ quantum spin liquid (QSL), a gapless (Dirac) QSL, or a weakly symmetry-broken phase. Using an improved dynamical density-matrix renormalization group method, we investigate the relevant intermediate $J_2$ regime for cylinders with circumferences from 6 to 9. Depending on the initial state and boundary conditions, we find two distinct variational states. The higher energy state is consistent with a Dirac QSL. In the lower-energy state, both the static and dynamical properties are qualitatively similar to the magnetically ordered state at $J_2=0$, suggestive of either a weakly magnetically ordered non-QSL or a gapped QSL proximate to a continuous transition to such an ordered state.

Unidirectional spin Hall magnetoresistance (USMR), arising from the interaction between nonequilibrium spin accumulation and magnetization, has been proposed as a simple two-terminal method for electrically detecting magnetic states in heavy-metal/ferromagnet bilayers. However, conventional spin polarization along the $y$ direction restricts USMR to responding only to the transverse component of in-plane magnetization when a charge current is applied along the $x$ direction. As a result, the electrical readout of longitudinal magnetic states via USMR has remained elusive. Here, we overcome this limitation by demonstrating an unconventional USMR induced by an $x$-polarized spin current in (001)-oriented epitaxial IrMn/FeNi bilayers, which enables electrical detection of the longitudinal magnetic state via USMR. By applying charge currents along different crystal orientations, we show that the $x$-polarized spin current exhibits a fourfold symmetry, consistent with the crystal mirror symmetry of (001)-oriented IrMn. This unconventional spin current further generates a large spin-orbit torque efficiency, an order of magnitude higher than that reported in antiferromagnetic ${\mathrm{Mn}}_3\mathrm{Pt}$ and ${\mathrm{MnPd}}_3$ systems. Our Letter not only highlights the crucial role of intrinsic crystal symmetry in controlling spin polarization, but also extends the applicability of USMR to the electrical detection of longitudinal magnetization in two-terminal magnetic memory and spin-logic devices.

Hyperbolic polaritons exhibit nonclosed hyperbolic isofrequency contours and hold great promise for integrated photonics due to their ultralarge momentum and directional propagation characteristics. However, their direct far-field observation remains challenging due to the large momentum mismatch. Here, we report a far-field, momentum-space imaging technique for hyperbolic exciton polaritons in the van der Waals magnet CrSBr. By employing one-dimensional gold gratings in conjunction with polarization- and angle-resolved reflectance spectroscopy, we achieve panoramic detection of multiple hyperbolic polariton mode families with momentum up to 5.6 times the vacuum wave vector. The experimental dispersion relations and isofrequency contours agree excellently with theoretical calculations, validating the method. Furthermore, leveraging the strong exciton-magnon coupling inherent to CrSBr, we demonstrate active tuning of the energy dispersion of the hyperbolic modes by an external magnetic field. Our Letter provides a comprehensive platform for investigating hyperbolic polaritons and establishes CrSBr hyperbolic polaritons as a magnetically tunable system for tailored light-matter interactions in the near-infrared range.

Tunable Josephson harmonics open new avenues for qubit design. We demonstrate a superconducting circuit element consisting of a tunnel junction in series with a superconducting quantum interference device (SQUID) loop, yielding a Josephson potential whose harmonic content is strongly tunable by magnetic flux. Through spectroscopy of the first four qubit transitions, together with an effective single-mode model renormalized by the internal mode, we resolve a second harmonic with an amplitude up to $\sim 10\%$ of the fundamental. We identify a flux sweet spot where the ground-state dispersive shift vanishes, achieved by balancing the dispersive couplings to the internal and qubit modes. This highly tunable element provides a route toward protected qubits and customizable nonlinear microwave devices.