Energy conservation dictates that an electron with elementary charge $e$ traversing a vacuum gap formed by electrodes maintained at potential difference $U$ volts acquires maximum energy of $eU$. In many experiments electrons with energies as high as $3eU$ have been observed. The experimental discovery of this effect was made over 50 years ago and is still a subject of significant controversy in applications related to x-ray generation from high voltage discharges. Here we explain these observations by a sequential increase of energy of runaway electrons released from the cathode due to the photoelectric absorption of bremsstrahlung radiation generated by the previous cycle of runaway electrons bombarding the anode.

The crystallographic restriction theorem constrains two-dimensional nematicity to display either Ising ($Z_2$) or three-state-Potts ($Z_3$) critical behaviors, both of which are dominated by amplitude fluctuations. Here, we use group theory and microscopic modeling to show that this constraint is circumvented in a 30°-twisted hexagonal bilayer due to its emergent quasicrystalline symmetries. We find a critical phase dominated by phase fluctuations of a $Z_6$ nematic order parameter and bounded by two Berezinskii-Kosterlitz-Thouless (BKT) transitions, which displays only quasi-long-range nematic order. The electronic spectrum in the critical phase displays a thermal pseudogaplike behavior, whose properties depend on the anomalous critical exponent. We also show that an out-of-plane magnetic field induces nematic phase fluctuations that suppress the two BKT transitions via a mechanism analogous to the Hall viscoelastic response of the lattice, giving rise to a putative nematic quantum critical point with emergent continuous symmetry. Finally, we demonstrate that even in the case of an untwisted bilayer, a critical phase emerges when the nematic order parameter changes sign between the two layers, establishing an odd-parity nematic state.

Attosecond observations of coherent electron dynamics in molecules and nanostructures can be achieved by combining conventional scanning tunneling microscopy (STM) with ultrashort femtosecond laser pulses. While experimental studies in the subcycle regime are under way, a robust strong-field theory description has remained elusive. Here we devise a model based on the strong-field approximation. Valid in all regimes, it provides a surprising analogy to the standard model of STM. We show that the intuitive three-step model of attosecond science directly emerges from our model and we also describe the optimal conditions for attosecond STM experiments.

Hypothetical axions provide a compelling explanation for dark matter and could be emitted from the hot solar interior. The CERN Axion Solar Telescope has been searching for solar axions via their back conversion to x-ray photons in a 9-T 10-m long magnet directed toward the Sun. We report on an extended run with the International Axion Observatory pathfinder detector, doubling the previous exposure time. The detector was operated with a xenon-based gas mixture for part of the new run, providing technical insights for future configurations. No counts were detected in the 95% signal-encircling region during the new run, while 0.75 were expected. The new data improve the axion-photon coupling limit to $5.8\times {10}^{-11}{\mathrm{GeV}}^{-1}$ at 95% CL (for $m_a\lesssim 0.02\mathrm{eV}$), the most restrictive experimental limit to date.

We present optimal Bayesian field-level cosmological constraints from nonlinear tracers of cosmic large-scale structure, specifically the amplitude ${\sigma }_8$ of linear matter fluctuations inferred from rest-frame simulated dark matter halos in a comoving volume of $8(h^{-1}\mathrm{Gpc}{)}^3$. Our constraint on ${\sigma }_8$ is entirely due to nonlinear information, and obtained by explicitly sampling the initial conditions along with tracer bias and noise parameters via a Lagrangian effective field theory-based forward model, leftfield. The comparison with a simulation-based inference of the power spectrum and bispectrum—likewise using the leftfield forward model—shows that, when including precisely the same modes of the same data up to $k_{\max }=0.10h{\mathrm{Mpc}}^{-1}$ ($0.12h{\mathrm{Mpc}}^{-1}$), the field-level approach yields a factor of 3.5 (5.2) improvement in the ${\sigma }_8$ constraint, going from 20.0% to 5.7% (17.0% to 3.3%). This study provides direct insights into cosmological information encoded in galaxy clustering beyond low-order $n$-point functions.

We derive the radial action of a spinning probe particle in Kerr spacetime from the worldline formalism in the first-order form, focusing on linear in spin effects. We then develop a novel covariant Dirac bracket formalism to compute the impulse and the spin kick directly from the radial action, generalizing some conjectural results in the literature and providing ready-to-use expressions for amplitude calculations with generic spin orientations. This allows, for the first time, to find new covariant expressions for scattering observables in the probe limit up to $\mathcal{O}(G^6{s}_1{s}_2^4)$. Finally, we use the action-angle representation to compute the fundamental frequencies for generic bound orbits, including the intrinsic spin precession, the periastron advance, and the precession of the orbital plane.

Using the $\mathrm{B}\rho$-defined isochronous mass spectrometry technique, we report the first determination of the ${}^{23}\mathrm{Si}$, ${}^{26}\mathrm{P}$, ${}^{27}\mathrm{S}$, and ${}^{31}\mathrm{Ar}$ masses and improve the precision of the ${}^{28}\mathrm{S}$ mass by a factor of 11. Our measurements confirm that these isotopes are bound and fix the location of the proton dripline in P, S, and Ar. We find that the mirror energy differences of the mirror-nuclei pairs ${}^{26}\mathrm{P}\text{−}{}^{26}\mathrm{Na}$, ${}^{27}\mathrm{P}\text{−}{}^{27}\mathrm{Mg}$, ${}^{27}\mathrm{S}\text{−}{}^{27}\mathrm{Na}$, ${}^{28}\mathrm{S}\text{−}{}^{28}\mathrm{Mg}$, and ${}^{31}\mathrm{Ar}\text{−}{}^{31}\mathrm{Al}$ deviate significantly from the values predicted assuming mirror symmetry. In addition, we observe similar anomalies in the excited states, but not in the ground states, of the mirror-nuclei pairs ${}^{22}\mathrm{Al}\text{−}{}^{22}\mathrm{F}$ and ${}^{23}\mathrm{Al}\text{−}{}^{23}\mathrm{Ne}$. Using ab initio VS-IMSRG and mean field calculations, we show that such a mirror-symmetry breaking phenomenon can be explained by the extended charge distributions of weakly bound, proton-rich nuclei. When observed, this phenomenon serves as a unique signature that can be valuable for identifying proton-halo candidates.

The formation of dark states is an important concept in quantum sciences, but its compatibility with strong interparticle interactions—for example, in a quantum degenerate gas—is hardly explored. Here, we realize a dark state in one of the spins of a two-component, resonantly interacting Fermi gas using a $\mathrm{\Lambda }$ system within the $D_2$ transitions of ${}^6\mathrm{Li}$ at high magnetic field. The dark state is created in a micrometer-sized region within a one-dimensional channel connecting two superfluid reservoirs. The particle transport between the reservoirs is used as a probe. We observe that atoms are transported in the dark state and the superfluid-assisted fast current is preserved. If the dark state resonant condition is not met, the transport is suppressed by the spontaneous emission. We also uncover an asymmetry in the transport timescale across the two-photon resonance, which is absent in the noninteracting regime and diminished at higher temperatures. This work raises questions on the interplay of dark states with interparticle interactions and opens up perspectives for optical manipulation of fermionic pairing.

The formation and subsequent self-organization of a spiral electron density modulation initialized in a plasma produced by optical-field ionization of various gas species is studied. Our analytical model predicts that the spiral density modulation results from space-dependent drift velocities of the ionized electrons due to the spatial and temporal intensity distributions of the circularly polarized ionizing laser. The spiral topology of the electron density has been validated by three-dimensional particle-in-cell simulations. Simulations further show that the spiral density modulation subsequently self-organizes to form numerous microfilaments, i.e., thin strands that arise because of a modified streaming instability.

Cracks develop various surface patterns as they propagate in three-dimensional (3D) materials. Symmetry-breaking topological defects in nominally tensile (mode-I) fracture emerge in the slow (noninertial) regime, taking the form of surface steps. We show that the same phase-field framework that recently shed basic light on dynamic (inertial) tensile fracture in three dimensions, also gives rise to crack surface steps. Step formation is shown to involve two essential physical ingredients: finite-strength quenched disorder and a small, mesoscopic antiplane shear (mode-III) loading component (on top of the dominant tensile, mode-I loading component). We quantify the nonlinear interplay between disorder (both its strength and spatial correlation length) and mesoscopic mode $\mathrm{I}+\mathrm{III}$ mixity in controlling step formation. Finally, we show that surface steps grow out of the small-scale, background surface roughness and are composed of two overlapping crack segments connected by a bridging crack, in agreement with experiments.

The miniaturization of integrated circuits is facing an obstruction due to the escalating electrical resistivity of conventional copper interconnects. The underlying reason for this problem was unveiled by Fuchs and Sondheimer, who showed that thinner wires are more resistive because current- carrying electrons encounter the rough surfaces of the wire more frequently therein. Here, we present a generalization of the Fuchs-Sondheimer theory to Dirac and Weyl materials, which are candidates for next-generation interconnects. We predict a nonlinear longitudinal electric current originating from the combined action of the Berry curvature and nonspecular surface scattering.

We study how sharp signatures of fractionalization emerge in nonlinear spectroscopy experiments on spin liquids with separated energy scales. Our model is that of dipolar-octupolar rare earth pyrochlore materials, prime candidates for realizing quantum spin ice. This family of three-dimensional quantum spin liquids exhibits fractionalization of spin degrees of freedom into spinons charged under an emergent $U(1)$ gauge field. We show that the technique of two-dimensional coherent spectroscopy can identify clear signatures of fractionalized spinon dynamics in dipolar-octupolar quantum spin ices. However, at intermediate temperatures, spinon dynamics are heavily constrained in the presence of an incoherent spin background, leading to a broad two-dimensional coherent spectroscopy response. At lower temperatures, a sharp signal emerges as the system enters a coherent spin liquid state. This lower temperature signal can in turn distinguish between zero-flux and $\pi$-flux forms of quantum spin ice.

We study a mechanism to induce superconductivity in atomically thin semiconductors where excitons mediate an effective attraction between electrons. Our model includes interaction effects beyond the paradigm of phonon-mediated superconductivity and connects to the well-established limits of Bose and Fermi polarons. By accounting for the strong-coupling physics of trions, we find that the effective electron-exciton interaction develops a strong frequency and momentum dependence accompanied by the system undergoing an emerging BCS-BEC crossover from weakly bound $s$-wave Cooper pairs to a superfluid of bipolarons. Even at strong-coupling the bipolarons remain relatively light, resulting in critical temperatures of up to 10% of the Fermi temperature. This renders heterostructures of two-dimensional materials a promising candidate to realize superconductivity at high critical temperatures set by electron doping and trion binding energies.

We propose and experimentally demonstrate a novel protocol for transferring quantum states between superconducting cavities. This approach utilizes continuous two-mode squeezing interactions to generate entanglement without the exchange of any carrier photons. In contrast to the discrete operations of entanglement and Bell-state measurement in quantum teleportation, our scheme is symmetric and continuous. We experimentally realize coherent and bidirectional transfer of arbitrary quantum states, including bosonic quantum error correction codes. Our results offer new insights into the quantum state transfer and quantum teleportation. In particular, our demonstration validates a new approach to realize quantum transducers and might find applications in a wide range of physical platforms.

A dense neutrino gas exhibiting angular crossings in the electron lepton number is unstable and develops fast flavor conversions. Instead of assuming an unstable configuration from the onset, we imagine that the system is externally driven toward instability. We use the simplest model of two neutrino beams initially of different flavor that either suddenly appear or one or both slowly build up. Flavor conversions commence well before the putative unstable state is fully attained, and the final outcome depends on how the system is driven. The system generally sticks to the closest state that is linearly stable, a conclusion that we prove for the first time using quasilinear theory. Our results suggest that in an astrophysical setting, one should focus less on flavor instabilities in the neutrino radiation field and more on the external dynamics that leads to the formation of the unstable state.