We derive a Feynman-Hellmann theorem relating the second-order nucleon energy shift resulting from the introduction of periodic source terms of electromagnetic and isovector axial currents to the parity-odd nucleon structure function $F_3^N$. It is a crucial ingredient in the theoretical study of the $\gamma W$ and $\gamma Z$ box diagrams that are known to suffer from large hadronic uncertainties. We demonstrate that for a given $Q^2$ one only needs to compute a small number of energy shifts in order to obtain the required inputs for the box diagrams. Future lattice calculations based on this approach may shed new light on various topics in precision physics including the refined determination of the Cabibbo-Kobayashi-Maskawa matrix elements and the weak mixing angle.

A recent nuclear physics experiment [C. J. Chiara et al., Nature (London) 554, 216 (2018)] reports the first direct observation of nuclear excitation by electron capture (NEEC) in the depletion of the ${}^{93m}\mathrm{Mo}$ isomer. The experiment used a beam-based setup in which Mo highly charged ions with nuclei in the isomeric state ${}^{93m}\mathrm{Mo}$ at 2.4 MeV excitation energy were slowed down in a solid-state target. In this process, nuclear excitation to a higher triggering level led to isomer depletion. The reported excitation probability $P_{\text{exc}}=0.01$ was solely attributed to the so-far unobserved process of NEEC in lack of a different known channel of comparable efficiency. In this work, we investigate theoretically the beam-based setup and calculate excitation rates via NEEC using state-of-the-art atomic structure and ion stopping-power models. For all scenarios, our results disagree with the experimental data by approximately 9 orders of magnitude. This stands in conflict with the conclusion that NEEC was the excitation mechanism behind the observed depletion rate.

We investigate spin states of few electrons in a double quantum dot by coupling them to a magnetic field resilient NbTiN microwave resonator. The electric field of the resonator couples to the electric dipole moment of the charge states in the double dot. For a two-electron state the spin-triplet state has a vanishing electric dipole moment and can therefore be distinguished from the spin-singlet state. This way the charge dipole sensitivity of the resonator response is converted to a spin selectivity. We thereby investigate Pauli spin blockade known from transport experiments at finite source-drain bias. In addition we find an unconventional spin-blockade triggered by the absorption of resonator photons.

We introduce a second quantization scheme based on quasinormal modes, which are the dissipative modes of leaky optical cavities and plasmonic resonators with complex eigenfrequencies. The theory enables the construction of multiplasmon or multiphoton Fock states for arbitrary three-dimensional dissipative resonators and gives a solid understanding to the limits of phenomenological dissipative Jaynes-Cummings models. In the general case, we show how different quasinormal modes interfere through an off-diagonal mode coupling and demonstrate how these results affect cavity-modified spontaneous emission. To illustrate the practical application of the theory, we show examples using a gold nanorod dimer and a hybrid dielectric-metal cavity structure.

Collective transport through channels shows surprising properties under one-dimensional confinement: particles in a single file exhibit subdiffusive behavior, while liquid confinement causes distance-independent correlations between the particles. Such interactions in channels are well studied for passive Brownian motion, but driven transport remains largely unexplored. Here, we demonstrate gating of transport due to a speed-up effect for actively driven particle transport through microfluidic channels. We prove that particle velocity increases with particle density in the channel due to hydrodynamic interactions under electrophoretic and gravitational forces. Numerical models demonstrate that the observed speed-up of transport originates from a hydrodynamic pistonlike effect. Our discovery is fundamentally important for understanding protein channels and transport through porous materials and for designing novel sensors and filters.

The collective motion of microswimmers in suspensions induce patterns of vortices on scales that are much larger than the characteristic size of a microswimmer, attaining a state called bacterial turbulence. Hydrodynamic turbulence acts on even larger scales and is dominated by inertial transport of energy. Using an established modification of the Navier-Stokes equation that accounts for the small-scale forcing of hydrodynamic flow by microswimmers, we study the properties of a dense suspension of microswimmers in two dimensions, where the conservation of enstrophy can drive an inverse cascade through which energy is accumulated on the largest scales. We find that the dynamical and statistical properties of the flow show a sharp transition to the formation of vortices at the largest length scale. The results show that 2D bacterial and hydrodynamic turbulence are separated by a subcritical phase transition.

High-flux polarized particle beams are of critical importance for the investigation of spin-dependent processes, such as in searches of physics beyond the standard model, as well as for scrutinizing the structure of solids and surfaces in material science. Here we demonstrate that kiloampere polarized electron beams can be produced via laser-wakefield acceleration from a gas target. A simple theoretical model for determining the electron beam polarization is presented and supported with self-consistent three-dimensional particle-in-cell simulations that incorporate the spin dynamics. By appropriately choosing the laser and gas parameters, we show that the depolarization of electrons induced by the laser-wakefield-acceleration process can be as low as 10%. Compared to currently available sources of polarized electron beams, the flux is increased by 4 orders of magnitude.

We perform comprehensive first-principles calculations and kinetic Monte Carlo simulations to explicitly elucidate the distinct roles of grain boundaries (GBs) in affecting hydrogen (H) diffusion in fcc nickel (Ni). We demonstrate the transition between slow and fast H diffusion along the GB with an abrupt change in H diffusivity. Low-angle GBs are shown to comprise isolated high-barrier regions to trap and inhibit H diffusion, with H diffusivity well prescribed by the classical trapping model, while high-angle GBs are shown to provide interconnected low-barrier channels to facilitate H transport. On the basis of the dislocation description of the GB and the Frank-Bilby model, the slow-fast diffusion transition is identified to result from dislocation core overlapping and is accurately predicted. The present Letter provides key mechanistic insights towards interpreting various experimental studies of H diffusion in metals, new critical knowledge for predictive modeling of H embrittlement, and better understanding of the kinetics of H and other interstitial impurities in microstructures.

By means of infrared spectroscopy, we determine the temperature-doping phase diagram of the Fano effect for the in-plane Fe-As stretching mode in ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$. The Fano parameter $1/q^2$, which is a measure of the phonon coupling to the electronic particle-hole continuum, shows a remarkable sensitivity to the magnetic and structural orderings at low temperatures. Most strikingly, at elevated temperatures in the paramagnetic tetragonal state we observe a linear correlation between $1/q^2$ and the superconducting critical temperature $T_c$. Based on theoretical calculations and symmetry considerations, we identify the relevant interband transitions that are coupled to the Fe-As mode. In particular, we show that a sizable $xy$ orbital component at the Fermi level is fundamental for the Fano effect and, thus, possibly also for the superconducting pairing.

We propose a topological characterization of Hamiltonians describing classical waves. Applying it to the magnetostatic surface spin waves that are important in spintronics applications, we settle the speculation over their topological origin. For a class of classical systems that includes spin waves driven by dipole-dipole interactions, we show that the topology is characterized by vortex lines in the Brillouin zone in such a way that the symplectic structure of Hamiltonian mechanics plays an essential role. We define winding numbers around these vortex lines and identify them to be the bulk topological invariants for a class of semimetals. Exploiting the bulk-edge correspondence appropriately reformulated for these classical waves, we predict that surface modes appear but not in a gap of the bulk frequency spectrum. This feature, consistent with the magnetostatic surface spin waves, indicates a broader realm of topological phases of matter beyond spectrally gapped ones.

Photoluminescence experiments from monolayer transition-metal dichalcogenides often show that the binding energy of trions is conspicuously similar to the energy of optical phonons. This enigmatic coincidence calls into question whether phonons are involved in the radiative recombination process. We address this problem, unraveling an intriguing optical transition mechanism. Its initial state is a localized charge (electron or hole) and delocalized exciton. The final state is the localized charge, phonon, and photon. In between, the intermediate state of the system is a virtual trion formed when the localized charge captures the exciton through emission of the phonon. We analyze the difference between radiative recombinations that involve real and virtual trions (i.e., with and without a phonon), providing useful ways to distinguish between the two in experiment.

Zirconium pentatelluride was recently reported to be a 3D Dirac semimetal, with a single conical band, located at the center of the Brillouin zone. The cone’s lack of protection by the lattice symmetry immediately sparked vast discussions about the size and topological or trivial nature of a possible gap opening. Here, we report on a combined optical and transport study of ${\mathrm{ZrTe}}_5$, which reveals an alternative view of electronic bands in this material. We conclude that the dispersion is approximately linear only in the $a$-$c$ plane, while remaining relatively flat and parabolic in the third direction (along the $b$ axis). Therefore, the electronic states in ${\mathrm{ZrTe}}_5$ cannot be described using the model of 3D Dirac massless electrons, even when staying at energies well above the band gap $2\mathrm{\Delta }=6\mathrm{meV}$ found in our experiments at low temperatures.

We experimentally demonstrate the nonreciprocal generation of spin current ($J_s$) in a surface-oxidized copper film. The efficiency of conversion is at least 320 times larger than the inverse conversion. This nonreciprocity is due to a novel type of $J_s$ generation, which relies on the transfer of angular momentum from the velocity field of free electrons. A gradient in the electrical mobility in the film produces vorticity in the in-plane drift velocity of the free electrons. The inverse process can hardly occur when $J_s$ is collinear with the gradient in the electrical mobility.

Spin-orbit coupling is relatively weak for electrons in bulk silicon, but enhanced interactions are reported in nanostructures such as the quantum dots used for spin qubits. These interactions have been attributed to various dissimilar interface effects, including disorder or broken crystal symmetries. In this Letter, we use a double-quantum-dot qubit to probe these interactions by comparing the spins of separated singlet-triplet electron pairs. We observe both intravalley and intervalley mechanisms, each dominant for [110] and [100] magnetic field orientations, respectively, that are consistent with a broken crystal symmetry model. We also observe a third spin-flip mechanism caused by tunneling between the quantum dots. This improved understanding is important for qubit uniformity, spin control and decoherence, and two-qubit gates.

Interfaces play an important role in modifying the dynamics of polymers confined to the nanoscale. We demonstrate that the distance over which an interface suppresses molecular mobility in poly(styrene) thin films can be systematically increased by tens of nanometers by controlling the chain of conformation, i.e., the height of the loops in irreversibly adsorbed nanolayers. These effects arise from topological interaction between adsorbed and neighboring unadsorbed chains, respectively, which increase their motional coupling to facilitate the propagation of suppressed dynamics originating at the interface, thus highlighting the ability to manipulate interfacial effects by local conformation of chains in adsorbed nanolayers.

The collective properties of colloidal suspensions, including their rheology, reflect an interplay between colloidal and hydrodynamic forces. The surface characteristics of the particles play a crucial role, in particular, for applications in which interparticle distances become small, i.e., at high concentrations or in aggregates. In this Letter, we directly investigate this interplay via the linear viscoelastic response of the suspensions in the high-frequency regime, using particles with controlled surface topographies, ranging from smooth to hairy and rough particles. We focus directly on the stresses at the particle level and reveal a strong impact of the surface topography on the short-range interactions, both dissipative and elastic. As the particle topography becomes more complex, the local stresses depend on how the topography is generated. The findings in this Letter, in particular, show how changes in topography can both screen or enhance the dissipation, which can be used to engineer the properties of dense or aggregated suspensions.

We show that inversion-asymmetric tethered membranes exhibit a new double-spiral phase with long range orientational order not present in symmetric membranes. We calculate the universal algebraic spiral shapes of these membranes in this phase. Asymmetry can trigger the crumpling of these membranes as well. In vitro experiments on lipid membranes, red blood cell membrane extracts, and on graphene coated on one side, could test these predictions.