The effects of partial ionization ($n_i/n_n\lesssim 1\%$) on magnetic reconnection in the Hall regime have been studied systematically in the Magnetic Reconnection Experiment. It is shown that, when neutrals are added, the Hall quadrupole field pattern and thus electron flow are unchanged while the ion outflow speed is reduced due to ion-neutral drag. However, in contrast to theoretical predictions, the ion diffusion layer width does not change appreciably. Therefore, the total ion outflow flux and the normalized reconnection rate are reduced.

Using a 1-MA, 100 ns-rise-time pulsed power generator, radial foil configurations can produce strongly collimated plasma jets. The resulting jets have electron densities on the order of ${10}^{20}{\mathrm{cm}}^{-3}$, temperatures above 50 eV and plasma velocities on the order of $100\mathrm{km}/\mathrm{s}$, giving Reynolds numbers of the order of ${10}^3$, magnetic Reynolds and Péclet numbers on the order of 1. While Hall physics does not dominate jet dynamics due to the large particle density and flow inside, it strongly impacts flows in the jet periphery where plasma density is low. As a result, Hall physics affects indirectly the geometrical shape of the jet and its density profile. The comparison between experiments and numerical simulations demonstrates that the Hall term enhances the jet density when the plasma current flows away from the jet compared to the case where the plasma current flows towards it.

Using electron bunches generated by laser wakefield acceleration as a probe, the temporal evolution of magnetic fields generated by a $4\times {10}^{19}\mathrm{W}/{\mathrm{cm}}^2$ ultrashort (30 fs) laser pulse focused on solid density targets is studied experimentally. Magnetic field strengths of order $B_0\sim {10}^4\mathrm{T}$ are observed expanding at close to the speed of light from the interaction point of a high-contrast laser pulse with a $10\mathrm{\text{−}}\mu \mathrm{m}$-thick aluminum foil to a maximum diameter of $\sim 1\mathrm{mm}$. The field dynamics are shown to agree with particle-in-cell simulations.

We explore the far-from-equilibrium response of a holographic superfluid using the AdS/CFT correspondence. We establish the dynamical phase diagram corresponding to quantum quenches of the order parameter source field. We find three distinct regimes of behavior that are related to the spectrum of black hole quasinormal modes. These correspond to damped oscillations of the order parameter and to overdamped approaches to the superfluid and normal states. The presence of three regimes, which includes an emergent dynamical temperature scale, is argued to occur more generally in time-reversal-invariant systems that display continuous symmetry breaking.

The connectedness percolation threshold (${\eta }_c$) and critical coordination number ($Z_c$) of systems of penetrable spherocylinders characterized by a length polydispersity are studied by way of Monte Carlo simulations for several aspect ratio distributions. We find that (i) ${\eta }_c$ is a nearly universal function of the weight-averaged aspect ratio, with an approximate inverse dependence that extends to aspect ratios that are well below the slender rod limit and (ii) that percolation of impenetrable spherocylinders displays a similar quasiuniversal behavior. For systems with a sufficiently high degree of polydispersity, we find that $Z_c$ can become smaller than unity, in analogy with observations reported for generalized and complex networks.

We report on the observation of a glass transition in suspensions of very long and thin, highly charged colloidal rods (fd-virus particles). Structural particle arrest is found to occur at a low ionic strength due to caging of the charged rods in the potential setup by their neighbors through long-ranged electrostatic interactions. The relaxation time of density fluctuations as probed by dynamic light scattering is found to diverge within a small concentration range. The rod concentration where structural particle arrest occurs is well within the full chiral-nematic state, far beyond the two-phase isotropic-nematic coexistence region. The morphology of the suspensions thus consists of nematic domains with various orientations. We quantify the dynamics of the resulting texture with image-time correlation spectroscopy. Interestingly, the decay times of image correlation functions are found to diverge in a discontinuous fashion at the same concentration of charged rods where structural particle arrest is observed. At the glass-transition concentration, we thus find both structural arrest and freezing of the texture dynamics.

We report on the thermal conductivities of microcrystalline [6,6]-phenyl ${\mathrm{C}}_{61}$-butyric acid methyl ester (PCBM) thin films from 135 to 387 K as measured by time domain thermoreflectance. Thermal conductivities are independent of temperature above 180 K and less than $0.030\pm 0.003\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ at room temperature. The longitudinal sound speed is determined via picosecond acoustics and is found to be 30% lower than that in ${\mathrm{C}}_{60}/{\mathrm{C}}_{70}$ fullerite compacts. Using Einstein’s model of thermal conductivity, we find the Einstein characteristic frequency of microcrystalline PCBM is $2.88\times {10}^{12}\mathrm{rad}{\mathrm{s}}^{-1}$. By comparing our data to previous reports on ${\mathrm{C}}_{60}/{\mathrm{C}}_{70}$ fullerite compacts, we argue that the molecular tails on the fullerene moieties in our PCBM films are responsible for lowering both the apparent sound speeds and characteristic vibrational frequencies below those of fullerene films, thus yielding the exceptionally low observed thermal conductivities.

We use first principles calculations to investigate the adsorption and incorporation of nonmetal impurities (N, C) at the anatase ${\mathrm{TiO}}_2(001)\mathrm{\text{−}}(1\times 4)$ reconstructed surface. We analyze in detail the influence of the surface structure and local strain on the impurity binding sites and incorporation pathways and identify important intermediates that facilitate impurity incorporation. We find various subsurface interstitial binding sites and corresponding surface $\to$ subsurface penetration pathways on the reconstructed surface. This surface also favors the presence of subsurface oxygen vacancies, to which adsorbed species can migrate to form substitutional impurities. Most notably, we show that the nonexposed oxygen sites just below the surface have a key role in the incorporation of nitrogen and carbon in ${\mathrm{TiO}}_2(001)$.

We study inelastic decay of bosonic excitations in a Luttinger liquid. In a model with a linear excitation spectrum the decay rate diverges. We show that this difficulty is resolved when the interaction between constituent particles is strong, and the excitation spectrum is nonlinear. Although at low energies the nonlinearity is weak, it regularizes the divergence in the decay rate. We develop a theoretical description of the approach of the system to thermal equilibrium. The typical relaxation rate scales as the fifth power of temperature.

Excitonic effects are crucial to optical properties, and the exciton binding energy $E_b$ in technologically important semiconductors varies from merely a few meV to about 100 meV. This large variation, however, is not well understood. We investigate the relationship between the electronic band structures and exciton binding energies in semiconductors, employing first-principles calculations based on the density functional theory and the many-body perturbation theory using Green’s functions. Our results clearly show that $E_b$ increases as the localization of valence electrons increases due to the reduced electronic screening. Furthermore, $E_b$ increases in ionic semiconductors such as ZnO because, contrary to the simple two-level coupling model, it has both conduction and valence band edge states strongly localized on anion sites, leading to an enhanced electron-hole interaction. These trends are quantized by electronic structures obtained from the density functional theory; thus, our approach can be applied to understand the excitonic effects in complex semiconducting materials.

Gas molecule doping on the topological insulator ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ surface with existing Se vacancies is investigated using first-principles calculations. Consistent with experiments, ${\mathrm{NO}}_2$ and ${\mathrm{O}}_2$ are found to occupy the Se vacancy sites, remove vacancy-doped electrons, and restore the band structure of a perfect surface. In contrast, NO and ${\mathrm{H}}_2$ do not favor passivation of such vacancies. Interestingly we have revealed a ${\mathrm{NO}}_2$ dissociation process that can well explain the speculative introduced “photon-doping” effect reported by recent experiments. Experimental strategies to validate this mechanism are presented. The choice and the effect of different passivators are discussed. This step paves the way for the usage of such materials in device applications utilizing robust topological surface states.

The nonlinear conductance of semiconductor heterostructures and single molecule devices exhibiting Kondo physics has recently attracted attention. We address the observed sample dependence of the measured steady state transport coefficients by considering additional electronic contributions in the effective low-energy model underlying these experiments that are absent in particle-hole symmetric setups. A novel version of the superperturbation theory of Hafermann et al. in terms of dual fermions is developed, which correctly captures the low-temperature behavior. We compare our results with the measured transport coefficients.

We analyze the screening of a magnetic quantum dot with spin $\frac{1}{2}$ coupled to two helical liquids. Interestingly, we find two qualitatively different sets of Toulouse points, i.e., nontrivial parameters for which we can solve the two channel Kondo model exactly. This enables us to calculate the temperature and voltage dependent Kondo screening cloud, which develops oscillations for an applied spin voltage ${\mu }_s$. Such a spin voltage can be conveniently applied by a charge bias in a four-terminal helical liquid setup.

In the presence of time-reversal symmetry, quantum interference gives strong corrections to the electric conductivity of disordered systems. The self-interference of an electron wave function traveling time-reversed paths leads to effects such as weak localization and universal conductance fluctuations. Here, we investigate the effects of broken time-reversal symmetry in a driven artificial two-level system. Using a superconducting flux qubit, we implement scattering events as multiple Landau-Zener transitions by driving the qubit periodically back and forth through an avoided crossing. Interference between different qubit trajectories gives rise to a speckle pattern in the qubit transition rate, similar to the interference patterns created when coherent light is scattered off a disordered potential. Since the scattering events are imposed by the driving protocol, we can control the time-reversal symmetry of the system by making the drive waveform symmetric or asymmetric in time. We find that the fluctuations of the transition rate exhibit a sharp peak when the drive is time symmetric, similar to universal conductance fluctuations in electronic transport through mesoscopic systems.

We investigate electron dynamics at the graphene edge by studying the propagation of collective edge magnetoplasmon excitations. By timing the travel of narrow wave packets on picosecond time scales around exfoliated samples, we find chiral propagation with low attenuation at a velocity that is quantized on Hall plateaus. We extract the carrier drift contribution from the edge magnetoplasmon propagation and find it to be slightly less than the Fermi velocity, as expected for an abrupt edge. We also extract the characteristic length for Coulomb interaction at the edge and find it to be smaller than that for soft depletion-edge systems.

The application of spatially uniform light on conventional insulators can induce Floquet spectra with characteristics akin to those of topological insulators. We demonstrate that spatial modulation of light allows for remarkable control of the properties in these systems. We provide configurations to generate one-dimensional bulk modes, photoinduced currents, as well as fractionalized excitations. We show a close analogy to $p$-wave superconductors and use this analogy to explain our results.

We investigate two- and three-electron spin blockade in three vertical quantum dots (QDs) coupled in series. Two-electron spin blockade is found in a region where sequential tunneling through all QDs is forbidden but tunneling involving virtual hopping through an empty QD is allowed. It is observed only for the hole cycle with a distinct bias threshold for access to the triplet state. Three-electron spin blockade involving the quadruplet state is observed for nonequibilium conditions where sequential tunneling is allowed and the triplet state is accessible. Our results shine light on the importance of the nonequibilium conditions to obtain sufficient population of triplet and quadruplet states necessary for spin blockade.

We electrically control the coherent mixing of the optically bright spin states of excitons confined in $\mathrm{InAs}/\mathrm{GaAs}$ quantum dot molecules. By tunnel coupling two quantum dots, using a vertical electric field, the exciton fine structure splitting and eigenstate orientation relative to the crystal lattice are tuned. We model the electric field dependent anisotropic electron-hole exchange interaction accurately and propose that the controllable mixing of the spin states will enable electrically controlled quantum operations on exciton spin qubits.

A prevailing description of the stripe phase in underdoped cuprate superconductors is that the charge carriers (holes) phase segregate on a microscopic scale into hole-rich and hole-poor regions. We report resonant elastic x-ray scattering measurements of stripe-ordered ${\mathrm{La}}_{1.475}{\mathrm{Nd}}_{0.4}{\mathrm{Sr}}_{0.125}{\mathrm{CuO}}_4$ at the Cu $L$ and O $K$ absorption edges that identify an additional feature of stripe order. Analysis of the energy dependence of the scattering intensity reveals that the dominant signature of the stripe order is a spatial modulation in the energies of Cu $3d$ and O $2p$ states rather than the large modulation of the charge density (valence) envisioned in the common stripe paradigm. These energy shifts are interpreted as a spatial modulation of the electronic structure and may point to a valence-bond-solid interpretation of the stripe phase.

Quantum fluctuations in an anharmonic superconducting circuit enable frequency conversion of individual incoming photons. This effect, linear in the photon beam intensity, leads to ramifications for the standard input-output circuit theory. We consider an extreme case of anharmonicity in which photons scatter off a small set of weak links within a Josephson junction array. We show that this quantum impurity displays Kondo physics and evaluate the elastic and inelastic photon scattering cross sections. These cross sections reveal many-body properties of the Kondo problem that are hard to access in its traditional fermionic version.