We study the magnetoresistance of two-dimensional bosonic Anderson insulators. We describe the change in spatial decay of localized excitations in response to a magnetic field, which is given by an interference sum over alternative tunneling trajectories. The excitations become more localized with increasing field (in sharp contrast to generic fermionic excitations which get weakly delocalized): the localization length $\xi (B)$ is found to change as ${\xi }^{-1}(B)-{\xi }^{-1}(0)\sim B^{4/5}$. The quantum interference problem maps onto the classical statistical mechanics of directed polymers in random media (DPRM). We explain the observed scaling using a simplified droplet model which incorporates the nontrivial DPRM exponents. Our results have implications for a variety of experiments on magnetic-field-tuned superconductor-to-insulator transitions observed in disordered films, granular superconductors, and Josephson junction arrays, as well as for cold atoms in artificial gauge fields.

A topological charge pump transfers charge in a quantized fashion. The quantization is stable against the detailed form of the pumping protocols and external noises and shares the same topological origin as the quantum Hall effect. We propose an experimental setup to realize topological charge pumping of cold fermionic atoms in a one-dimensional optical lattice. The quantization of the pumped charge is confirmed by first-principles simulations of the dynamics of uniform and trapped systems. Quantum effects are shown to be crucial for the topological protection of the charge quantization. Finite-temperature and nonadiabatic effects on the experimental observables are discussed. The realization of such a topological charge pump serves as a firm step toward exploring topological states and nonequilibrium dynamics using cold atoms.

We examine the nature of the transitions between the normal and superconducting branches in superconductor-graphene-superconductor Josephson junctions. We attribute the hysteresis between the switching (superconducting to normal) and retrapping (normal to superconducting) transitions to electron overheating. In particular, we demonstrate that the retrapping current corresponds to the critical current at an elevated temperature, where the heating is caused by the retrapping current itself. The superconducting gap in the leads suppresses the hot electron outflow, allowing us to further study electron thermalization by phonons at low temperatures ($T\lesssim 1\mathrm{K}$). The relationship between the applied power and the electron temperature was found to be $P\propto T^3$, which we argue is consistent with cooling due to electron-phonon interactions.

Using resistivity, heat-capacity, thermal-expansion, and susceptibility measurements we study the normal-state behavior of ${\mathrm{KFe}}_2{\mathrm{As}}_2$. Both the Sommerfeld coefficient ($\gamma \approx 103\mathrm{mJ}{\mathrm{mol}}^{-1}{\mathrm{K}}^{-2}$) and the Pauli susceptibility ($\chi \approx 4\times {10}^{-4}$) are strongly enhanced, which confirm the existence of heavy quasiparticles inferred from previous de Haas–van Alphen and angle-resolved photoemission spectroscopy experiments. We discuss this large enhancement using a Gutzwiller slave-boson mean-field calculation, which shows the proximity of ${\mathrm{KFe}}_2{\mathrm{As}}_2$ to an orbital-selective Mott transition. The temperature dependence of the magnetic susceptibility and the thermal expansion provide strong experimental evidence for the existence of a coherence-incoherence crossover, similar to what is found in heavy fermion and ruthenate compounds, due to Hund’s coupling between orbitals.

It has been suggested recently, based on subtle field-theoretical considerations, that the electromagnetic response of Weyl semimetals and the closely related Weyl insulators can be characterized by an axion term $\theta \mathit{E}·\mathit{B}$ with space and time dependent axion angle $\theta (\mathit{r},t)$. Here we construct a minimal lattice model of the Weyl medium and study its electromagnetic response by a combination of analytical and numerical techniques. We confirm the existence of the anomalous Hall effect expected on the basis of the field theory treatment. We find, contrary to the latter, that chiral magnetic effect (that is, ground state charge current induced by the applied magnetic field) is absent in both the semimetal and the insulator phase. We elucidate the reasons for this discrepancy.

We present an unrestricted Hartree-Fock computation of charge-ordering instabilities of two-dimensional metals with antiferromagnetic exchange interactions, allowing for arbitrary ordering wave vectors and internal wave functions of the particle-hole pair condensate. We find that the ordering has a dominant $d$ symmetry of rotations about lattice points for a range of ordering wave vectors, including those observed in recent experiments at low temperatures on ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_y$. This $d$ symmetry implies the charge ordering is primarily on the bonds of the Cu lattice, and we propose incommensurate bond order parameters for the underdoped cuprates. The field theory for the onset of Néel order in a metal has an emergent pseudospin symmetry which “rotates” $d$-wave Cooper pairs to particle-hole pairs [M. A. Metlitski and S. Sachdev, Phys. Rev. B 82, 075128 (2010)]; our results show that this symmetry has consequences even when the spin correlations are short ranged and incommensurate.

The magnetic properties of a ${\mathrm{Co}}_2\mathrm{FeAl}/(\mathrm{Ga},\mathrm{Mn})\mathrm{As}$ bilayer epitaxied on GaAs (001) are studied both experimentally and theoretically. Unlike the common antiferromagnetic interfacial interaction existing in most ferromagnet-magnetic semiconductor bilayers, a ferromagnetic interfacial interaction in the ${\mathrm{Co}}_2\mathrm{FeAl}/(\mathrm{Ga},\mathrm{Mn})\mathrm{As}$ bilayer is observed from measurements of magnetic hysteresis and x-ray magnetic circular dichroism. The Mn ions in a 1.36 nm thick (Ga,Mn)As layer remain spin polarized up to 400 K due to the magnetic proximity effect. The minor loops of the ${\mathrm{Co}}_2\mathrm{FeAl}/(\mathrm{Ga},\mathrm{Mn})\mathrm{As}$ bilayer shift with a small ferromagnetic interaction field of $+24\mathrm{Oe}$ and $-23\mathrm{Oe}$ at 15 K. The observed ferromagnetic interfacial coupling is supported by ab initio density functional calculations. These findings may provide a viable pathway for designing room-temperature semiconductor spintronic devices through magnetic proximity effect.

We demonstrate in situ 90° electric field-induced uniform magnetization rotation in single domain submicron ferromagnetic islands grown on a ferroelectric single crystal using x-ray photoemission electron microscopy. The experimental findings are well correlated with micromagnetic simulations, showing that the reorientation occurs by the strain-induced magnetoelectric interaction between the ferromagnetic nanostructures and the ferroelectric crystal. Specifically, the ferroelectric domain structure plays a key role in determining the response of the structure to the applied electric field, resulting in three strain-induced regimes of magnetization behavior for the single domain islands.

The stability of the well-known Walker propagating domain wall (DW) solution of the Landau-Lifshitz-Gilbert equation is analytically investigated. Surprisingly, a propagating DW is always dressed with spin waves so that the Walker rigid-body propagating DW mode does not occur in reality. In the low field region only stern spin waves are emitted while both stern and bow waves are generated under high fields. In a high enough field, but below the Walker breakdown field, the Walker solution could be convective or absolute unstable if the transverse magnetic anisotropy is larger than a critical value, corresponding to a significant modification of the DW profile and DW propagating speed.

We have investigated the Co-$3d$ spin-orbital state in a thin film of perovskite ${\mathrm{LaCoO}}_3$ to clarify the origin of strain induced spontaneous magnetization ($T_C=94\mathrm{K}$) by means of x-ray diffraction, optical spectroscopy, and magnetization measurements. A lattice distortion with the propagation vector ($1/4$ $-\text{}\text{}1/4$ $1/4$) and an anomalous activation of optical phonons coupled to Co-$3d$ orbital are observed below 126 K. Combined with the azimuthal angle analysis of superlattice reflection, we propose that the ordering of Co-$3d$ orbital promoted by an epitaxial strain produces a unique ferrimagnetic structure.

We demonstrate that the proximity-induced exchange field $H_{\mathrm{ex}}$ in ferromagnetic-paramagnetic bilayers can be modulated with an electric field. An electrostatic gate arrangement is used to tune the magnitude of $H_{\mathrm{ex}}$ in the Al component of $\mathrm{EuS}/\mathrm{Al}$ bilayers. In samples with $H_{\mathrm{ex}}\sim 2\mathrm{T}$, we were able to produce modulations of $\pm 10\mathrm{mT}$ with the application of perpendicular electric fields of the order of $\pm {10}^6\mathrm{V}/\mathrm{cm}$. We discuss several possible mechanisms accounting for the electric field’s influence on the interfacial coupling between the Al layer and the ferromagnetic insulator EuS, along with the prospects of producing a superconducting field-effect transistor.

We present a model of the attosecond resonant dynamics of valence electron photoemission and MMM Auger electron emission in copper dichloride. At ${\tau }_h\gg 1/{\omega }_{\mathrm{IR}}$ (where ${\tau }_h$ is the core-hole relaxation time and ${\omega }_{\mathrm{IR}}$ is the energy of the infrared probe field), high-order Fano sideband resonances on both sides of the original (zeroth-order) resonance are found in the energy domain on the time scale of ${\tau }_h$. This is confirmed by a coherent $\pi$ rotation of the relative phase between the photoelectron and Auger electron at each resonance, due to the subfemtosecond quantum correlation. We also find a core-hole clock, the asymmetry factor $q$ of the first-order Fano sideband resonance, which can directly trace the core-hole relaxation in the time domain.

We propose a scheme to efficiently couple a single quantum dot electron spin to an optical nano-cavity, which enables us to simultaneously benefit from a cavity as an efficient photonic interface, as well as to perform high fidelity (nearly 100%) spin initialization and manipulation achievable in bulk semiconductors. Moreover, the presence of the cavity speeds up the spin initialization process beyond the GHz range.

The ultrafast dynamics of excited carriers in graphene is closely linked to the Dirac spectrum and plays a central role for many electronic and optoelectronic applications. Harvesting energy from excited electron-hole pairs, for instance, is only possible if these pairs can be separated before they lose energy to vibrations, merely heating the lattice. Until now, the hot carrier dynamics in graphene could only be accessed indirectly. Here, we present a dynamical view on the Dirac cone by time- and angle-resolved photoemission spectroscopy. This allows us to show the quasi-instant thermalization of the electron gas to a temperature of $\approx 2000\mathrm{K}$, to determine the time-resolved carrier density, and to disentangle the subsequent decay into excitations of optical phonons and acoustic phonons (directly and via supercollisions).

We prepare packings of frictional tetrahedra with volume fractions $\varphi$ ranging from 0.469 to 0.622 using three different experimental protocols under isobaric conditions. Analysis via x-ray microtomography reveals that the contact number $Z$ grows with $\varphi$, but does depend on the preparation protocol. While there exist four different types of contacts in tetrahedra packings, our analysis shows that the edge-to-face contacts contribute about 50% of the total increase in $Z$. The number of constraints per particle $C$ increases also with $\varphi$ and even the loosest packings are strongly hyperstatic, i.e., mechanically overdetermined with $C$ approximately twice the degrees of freedom each particle possesses.

We investigate the impact of droplets of dense suspensions onto a solid substrate. We show that a global hydrodynamic balance is unable to predict the splash onset and propose to replace it by an energy balance at the level of the particles in the suspension. We experimentally verify that the resulting, particle-based Weber number gives a reliable, particle size and density dependent splash onset criterion. We further show that the same argument also explains why, in bimodal systems, smaller particles are more likely to escape than larger ones.

The adsorption of a colloidal particle at a fluid interface is studied theoretically and numerically, documenting distinctly different relaxation regimes. The adsorption of a perfectly smooth particle is characterized by a fast exponential relaxation to thermodynamic equilibrium where the interfacial free energy reaches the global minimum. The short relaxation time is given by the ratio of viscous damping to capillary forces. Physical and/or chemical heterogeneities, however, can result in multiple minima of the free energy giving rise to metastability. In the presence of metastable states we observe a crossover to a slow logarithmic relaxation reminiscent of physical aging in glassy systems; sufficiently close to equilibrium the slow relaxation becomes exponential. The long relaxation time is determined by the Kramers escape rate from metastable states. Derived analytical expressions yield quantitative agreement with molecular dynamics simulations and recent experimental observations. This work provides new insights on the adsorption of colloidal particles with surface microstructure.

We report on the observation of two neutrino-induced events which have an estimated deposited energy in the IceCube detector of $1.04\pm 0.16$ and $1.14\pm 0.17\mathrm{PeV}$, respectively, the highest neutrino energies observed so far. These events are consistent with fully contained particle showers induced by neutral-current ${\nu }_{e,\mu ,\tau }$ (${\overline{\nu }}_{e,\mu ,\tau }$) or charged-current ${\nu }_e$ (${\overline{\nu }}_e$) interactions within the IceCube detector. The events were discovered in a search for ultrahigh energy neutrinos using data corresponding to 615.9 days effective live time. The expected number of atmospheric background is $0.082\pm 0.004(\mathrm{stat}{)}_{-0.057}^{+0.041}(\mathrm{syst})$. The probability of observing two or more candidate events under the atmospheric background-only hypothesis is $2.9\times {10}^{-3}$ ($2.8\sigma$) taking into account the uncertainty on the expected number of background events. These two events could be a first indication of an astrophysical neutrino flux; the moderate significance, however, does not permit a definitive conclusion at this time.

Nonautonomous oscillatory systems with stable amplitudes and time-varying frequencies have often been treated as stochastic, inappropriately. We therefore formulate them as a new class and discuss how they generate complex behavior. We show how to extract the underlying dynamics, and we demonstrate that it is simple and deterministic, thus paving the way for a diversity of new systems to be recognized as deterministic. They include complex and nonautonomous oscillatory systems in nature, both individually and in ensembles and networks.

We have calculated the thermal conductivities ($\kappa$) of cubic III-V boron compounds using a predictive first principles approach. Boron arsenide is found to have a remarkable room temperature $\kappa$ over $2000\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$; this is comparable to those in diamond and graphite, which are the highest bulk values known. We trace this behavior in boron arsenide to an interplay of certain basic vibrational properties that lie outside of the conventional guidelines in searching for high $\kappa$ materials, and to relatively weak phonon-isotope scattering. We also find that cubic boron nitride and boron antimonide will have high $\kappa$ with isotopic purification. This work provides new insight into the nature of thermal transport at a quantitative level and predicts a new ultrahigh $\kappa$ material of potential interest for passive cooling applications.