Two conducting quantum systems coupled only via interactions can exhibit the phenomenon of Coulomb drag, in which a current passed through one layer can pull a current along in the other. However, in systems with particle-hole symmetry—for instance, the half filled Hubbard model or graphene near the Dirac point—the Coulomb drag effect vanishes to leading order in the interaction. Its thermal analog, whereby a thermal current in one layer pulls a thermal current in the other, does not vanish and is indeed the dominant form of drag in particle-hole symmetric systems. By studying a quantum quench, we show that thermal drag, unlike charge drag, displays a non-Fermi’s golden rule growth at short times due to a logarithmic scattering singularity generic to one dimension. Exploiting the integrability of the Hubbard model, we obtain the long-time limit of the quench for weak interactions. Finally, we comment on thermal drag effects in higher dimensional systems.

Breaking bonds selectively in molecules is vital in many chemistry reactions and custom nanoscale device fabrications. The scanning tunneling microscope (STM) has proved to be an ideal tool to initiate and view bond-selective chemistry at the single-molecule level, offering opportunities for the further study of the dynamics in single molecules on metal surfaces. We demonstrate $\mathrm{H}─\mathrm{HS}$ and $\mathrm{H}─\mathrm{S}$ bond breaking on Au(111) induced by tunneling electrons using low-temperature STM. An experimental study combined with theoretical calculations shows that the dissociation pathway is facilitated by vibrational excitations. Furthermore, the dissociation probabilities of the two different dissociation processes are bias dependent due to different inelastic-tunneling probabilities, and they are also closely linked to the lifetime of inelastic-tunneling electrons. Combined with time-dependent ab initio nonadiabatic molecular dynamics simulations, the dynamics of the injected electron and the phonon-excitation-induced molecule dissociation can be understood at the atomic scale, demonstrating the potential application of STM for the investigation of excited-state dynamics of single molecules on surfaces.

Recent nuclear magnetic resonance studies [A. Pustogow et al., Nature 574, 72 (2019)] have challenged the prevalent chiral triplet pairing scenario proposed for ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$. To provide guidance from microscopic theory as to which other pair states might be compatible with the new data, we perform a detailed theoretical study of spin fluctuation mediated pairing for this compound. We map out the phase diagram as a function of spin-orbit coupling, interaction parameters, and band structure properties over physically reasonable ranges, comparing when possible with photoemission and inelastic neutron scattering data information. We find that even-parity pseudospin singlet solutions dominate large regions of the phase diagram, but in certain regimes spin-orbit coupling favors a near-nodal odd-parity triplet superconducting state, which is either helical or chiral depending on the proximity of the $\gamma$ band to the van Hove points. A surprising near degeneracy of the nodal $s^{\prime }$ and $d_{x^2-y^2}$ wave solutions leads to the possibility of a near-nodal time-reversal symmetry broken $s^{\prime }+i{d}_{x^2-y^2}$ pair state. Predictions for the temperature dependence of the Knight shift for fields in and out of plane are presented for all states.

A quantitative investigation of the current-induced torque in antiferromagnets represents a great challenge due to the lack of an independent method for controlling Néel vectors. By utilizing an antiferromagnetic insulator with the Dzyaloshinskii-Moriya interaction $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$, we show that the Néel vector can be controlled with a moderate external field, which is further utilized to calibrate the current-induced magnetic dynamics. We find that the current-induced magnetoresistance change in antiferromagnets can be complicated by resistive switching that does not have a magnetic origin. By excluding nonmagnetic switching and comparing the current-induced dynamics with the field-induced one, we determine the nature and magnitude of current-induced effects in $\mathrm{Pt}/\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ bilayer films.

Nanowire antennas embedding a single quantum dot (QD) have recently emerged as versatile platforms to realize bright sources of quantum light. In this theoretical work, we show that the thermally driven, low-frequency vibrations of the nanowire have a major impact on the QD light emission spectrum. Even at liquid helium temperatures, these prevent the emission of indistinguishable photons. To overcome this intrinsic limitation, we propose three designs that restore photon indistinguishability thanks to a specific engineering of the mechanical properties of the nanowire. We anticipate that such a mechanical optimization will also play a key role in the development of other high-performance light-matter interfaces based on nanostructures.

We report on a new mode of self-propulsion exhibited by compact drops of active liquids on a substrate which, remarkably, is tractionless, i.e., which imparts no mechanical stress locally on the surface. We show, both analytically and by numerical simulation, that the equations of motion for an active nematic drop possess a simple self-propelling solution, with no traction on the solid surface and in which the direction of motion is controlled by the winding of the nematic director field across the drop height. The physics underlying this mode of motion has the same origins as that giving rise to the zero viscosity observed in bacterial suspensions. This topologically protected tractionless self-propusion provides a robust physical mechanism for efficient cell migration in crowded environments like tissues.

Despite their importance in many biological, ecological, and physical processes, microorganismal fluid flows under tight confinement have not been investigated experimentally. Strong screening of Stokelets in this geometry suggests that the flow fields of different microorganisms should be universally dominated by the 2D source dipole from the swimmer’s finite-size body. Confinement therefore is poised to collapse differences across microorganisms, which are instead well established in bulk. We combine experiments and theoretical modeling to show that, in general, this is not correct. Our results demonstrate that potentially minute details like microswimmer spinning and the physical arrangement of the propulsion appendages have in fact a leading role in setting qualitative topological properties of the hydrodynamic flow fields of microswimmers under confinement. This is well captured by an effective 2D model, even under relatively weak confinement. These results imply that active confined hydrodynamics is much richer than in bulk and depends in a subtle manner on the size, shape, and propulsion mechanisms of the active components.

The studies of mechanical resonators in the quantum regime not only provide insight into the fundamental nature of quantum mechanics of massive objects, but also introduce promising platforms for novel hybrid quantum technologies. Here we demonstrate a configurable interaction between a superconducting qubit and many acoustic modes in the quantum regime. Specifically, we show how consecutive Landau-Zener-Stückelberg (LZS) tunneling type of transitions, which take place when a system is tuned through an avoided crossing of the coupled energy levels, interfere in a multimode system. The work progresses experimental LZS interference to cover a new class of systems where the coupled levels are those of a quantum two-level system interacting with a multitude of mechanical oscillators. The work opens up applications in controlling multiple acoustic modes via parametric modulation.

We study heating rates in strongly interacting quantum lattice systems in the thermodynamic limit. Using a numerical linked cluster expansion, we calculate the energy as a function of the driving time and find a robust exponential regime. The heating rates are shown to be in excellent agreement with Fermi’s golden rule. We discuss the relationship between heating rates and, within the eigenstate thermalization hypothesis, the smooth function that characterizes the off-diagonal matrix elements of the drive operator in the eigenbasis of the static Hamiltonian. We show that such a function, in nonintegrable and (remarkably) integrable Hamiltonians, can be probed experimentally by studying heating rates as functions of the drive frequency.

I consider quantum electrodynamics with many electrons in $2+1$ space-time dimensions at finite temperature. The relevant dimensionless interaction parameter for this theory is the fine structure constant divided by the temperature. The theory is solvable at any value of the coupling, in particular for very weak (high temperature) and infinitely strong coupling (corresponding to the zero temperature limit). Concentrating on the photon, each of its physical degrees of freedom at infinite coupling only contributes half of the free-theory value to the entropy. These fractional degrees of freedom are reminiscent of what has been observed in other strongly coupled systems (such as $\mathcal{N}=4$ supersymmetric Yang-Mills theory), and bear similarity to the fractional quantum Hall effect, potentially suggesting connections between these phenomena. The results found for ($2+1$)-dimensional QED are fully consistent with the expectations from particle-vortex duality.

A search for the lepton-flavor violating decays $B^{+}\to K^{+}{\mu }^{\pm }{e}^{\mp }$ is performed using a sample of proton-proton collision data, collected with the LHCb experiment at center-of-mass energies of 7 and 8 TeV and corresponding to an integrated luminosity of $3{\mathrm{fb}}^{-1}$. No significant signal is observed, and upper limits on the branching fractions are set as $\mathcal{B}(B^{+}\to K^{+}{\mu }^{-}{e}^{+})<7.0(9.5)\times {10}^{-9}$ and $\mathcal{B}(B^{+}\to K^{+}{\mu }^{+}{e}^{-})<6.4(8.8)\times {10}^{-9}$ at 90% (95)% confidence level. The results improve the current best limits on these decays by more than one order of magnitude.

Nitrogen molecules are promoted to excited neutral states during femtosecond laser pulse filamentary propagation in atmosphere, leading to a characteristic UV fluorescence. Using a laser-induced fluorescence depletion technique, we measure the formation dynamics of these excited neutral nitrogen molecules with femtosecond time resolution. We find that the excited neutral molecules are formed in an unexpected ultrafast timescale of $\sim 4\mathrm{ps}$ at 1 bar and $\sim 120\mathrm{ps}$ at 30 mbar pressure. From this observation we deduce that the excitation of neutral ${\mathrm{N}}_2$ occurs via multiple collisions with hot free electrons. Numerical simulations based on rate equations reproduce well this ultrafast formation time and its dependence on gas pressure, and thus support this interpretation.

We report the experimental observation of collective multimode vacuum Rabi splitting in free space. In contrast to optical cavities, the atoms couple to a continuum of modes, and the optical thickness of the cloud provides a measure of this coupling. The splitting, also referred as normal mode splitting, is monitored through the Rabi oscillations in the scattered intensity, and the results are fully explained by a linear-dispersion theory.

We show how to derive a consistent quantum theory of radiation reaction of a nonrelativistic point-dipole quantum oscillator by including the dynamical fluctuations of the position of the dipole. The proposed nonlinear theory displays neither runaway solutions nor acausal behavior without requiring additional assumptions. Furthermore, we show that quantum (zero-point) fluctuations of the electromagnetic field are necessary to satisfy the second law of thermodynamics. Our results are obtained by developing a nonperturbative technique involving a weak-coupling approximation at the level of the effective action.

High-harmonic generation (HHG) of laser radiation has led to attosecond pulse formation which offers unprecedented temporal resolution in observing and controlling electron and nuclear dynamics. But the energy of attosecond pulses remains quite small, especially for photon energies exceeding 100 eV, which limits their practical applications. We propose a method for amplification of attosecond pulses in the active medium of a plasma-based x-ray laser dressed by a replica of the laser field used for HHG. The experimental implementation is suggested in hydrogenlike $\mathrm{C}5+$ x-ray laser at 3.4 nm wavelength in the “water window” range.

The optical Hall effect manifests itself as angular momentum separation induced by the photonic spin-orbit interaction. Such a celebrated Hall effect, at the mercy of the angular momentum conservation law, has attracted tremendous interest owing to its science and potential applications in precision measurements, material characterizations, and photonic devices, as well as quantum optics. However, to date, the Hall effect only expresses angular momentum separation of the spin term (spin-spin separation) or the orbital term (orbit-orbit separation), whereas the spin-orbit angular momentum separation, named as the spin-orbit Hall effect, remains unexplored. Here we demonstrate for the first time that this spin-orbit effect could appear when the polarization state of the light beam evolves adiabatically from the equator toward the poles of the higher-order Poincaré sphere, rather than the conventional Poincaré sphere. In this scenario, the intrinsic spin and orbital components of the light beam become separated, leading to equal nonzero spin and orbital angular momenta in magnitude but with the opposite sign. We further show that the spin-orbit Hall effect can be controlled via crystal birefringence and hence holds promise for applications; e.g., it is shown that the separated orbital angular momentum could be utilized in particle manipulations.

This Letter reports the successful experimental demonstration of amplification of subterahertz radiation in a klystron with photonic crystal cavities. The klystron has six cavities, with each cavity having a series of oversized photonic crystal cells made up of a $5\times 3$ array of square posts. The center post is removed from each cell to form a highly oversized ($0.8\mathrm{mm}\sim \lambda /4$) beam tunnel, with power coupling from cell to cell through the tunnel. The pulsed electron beam is operated at 23.5 kV, 330 mA in a 0.5 T solenoidal field. At 93.7 GHz, a small-signal gain of 26 dB and a saturated output power of 30 W are obtained. Experimental results are in very good agreement with the predictions of a particle-in-cell code. The successful achievement of high gain operation of a photonic crystal klystron amplifier is promising for the future extension of klystron operation well into the terahertz frequency region.

Recent scanning tunneling microscopy (STM) experiments reported single-molecule fluorescence induced by tunneling currents in the nanoplasmonic cavity formed by the STM tip and the substrate. The electric field of the cavity mode couples with the current-induced charge fluctuations of the molecule, allowing the excitation of photons. We investigate theoretically this system for the experimentally relevant limit of large damping rate $\kappa$ for the cavity mode and arbitrary coupling strength to a single-electronic level. We find that for bias voltages close to the first inelastic threshold of photon emission, the emitted light displays antibunching behavior with vanishing second-order photon correlation function. At the same time, the current and the intensity of emitted light display Franck-Condon steps at multiples of the cavity frequency ${\omega }_c$ with a width controlled by $\kappa$ rather than the temperature $T$. For large bias voltages, we predict strong photon bunching of the order of $\kappa /\mathrm{\Gamma }$ where $\mathrm{\Gamma }$ is the electronic tunneling rate. Our theory thus predicts that strong coupling to a single level allows current-driven nonclassical light emission.