We study the interaction of surface acoustic waves (SAWs) with a contact-based vibrational resonance of $1\mu \mathrm{m}$ silica microspheres forming a two-dimensional granular crystal adhered to a substrate. The laser-induced transient grating technique is used to excite SAWs and measure their dispersion. The measured dispersion curves exhibit “avoided crossing” behavior due to the hybridization of the SAWs with the microsphere resonance. We compare the measured dispersion curves with those predicted by our analytical model and find excellent agreement. The approach presented can be used to study the contact mechanics and adhesion of micro- and nanoparticles as well as the dynamics of microscale granular crystals.

The stacking orders in layered hexagonal boron nitride bulk and bilayers are studied using high-level ab initio theory [local second-order Møller-Plesset perturbation theory (LMP2)]. Our results show that both electrostatic and London dispersion interactions are responsible for interlayer distance and stacking order, with $A{A}^{\prime }$ being the most stable one. The minimum energy sliding path includes only the $A{A}^{\prime }$ high-symmetry stacking, and the energy barrier is 3.4 meV per atom for the bilayer. State-of-the-art density functionals with and without London dispersion correction fail to correctly describe the interlayer energies with the exception of a Perdew-Burke-Ernzerhof functional intended for solid state and surface systems that agrees very well with our LMP2 results and experiment.

We investigate the transport properties of a correlated metal within dynamical mean-field theory. Canonical Fermi liquid behavior emerges only below a very low temperature scale $T_{\mathrm{FL}}$. Surprisingly the quasiparticle scattering rate follows a quadratic temperature dependence up to much higher temperatures and crosses over to saturated behavior around a temperature scale $T_{\mathrm{sat}}$. We identify these quasiparticles as constituents of the hidden Fermi liquid. The non-Fermi-liquid transport above $T_{\mathrm{FL}}$, in particular the linear-in-$T$ resistivity, is shown to be a result of a strongly temperature dependent band dispersion. We derive simple expressions for the resistivity, Hall angle, thermoelectric power and Nernst coefficient in terms of a temperature dependent renormalized band structure and the quasiparticle scattering rate. We discuss possible tests of the dynamical mean-field theory picture of transport using ac measurements.

We derive an exchange energy functional of generalized gradient form with a corresponding potential that changes discontinuously at integer particle numbers. The functional is semilocal, yet incorporates key features that are connected to the derivative discontinuity of Kohn-Sham density-functional theory. We validate our construction for several paradigm systems and explain how it addresses central well-known deficiencies of antecedent semilocal methods, i.e., the description of charge transfer, properly localized orbitals, and band gaps. We find, e.g., an improved shell structure for atoms, eigenvalues that more closely correspond to ionization energies, and an improved description of band structure where localized states are lowered in energy.

We study the anomalous Hall effect due to noncoplanar magnetism on a pyrochlore structure. We focus on the frustration-induced spatial inhomogeneity of different magnetic low-temperature regimes, between which one can efficiently tune using an external magnetic field. We incorporate nonmagnetic scattering on a phenomenological level so that we can distinguish between the effects of short-range correlations and short-range coherence. We obtain a Hall conductivity (${\sigma }_H$) as a function of field strength and direction which compares well to the experimental data of ${\mathrm{Pr}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$. In particular, we show that the observed peak in ${\sigma }_H$ for $\mathbf{H}\parallel [111]$ signals the crossover from zero-field spin ice to kagome ice.

The microscopic structure of spin-orbit fields for the technologically important $\mathrm{Fe}/\mathrm{GaAs}$ interface is uncovered from first principles. A symmetry based method allows us to obtain the spin-orbit fields—both their magnitude and orientation—for a generic Bloch state, from the electronic band structure for any in-plane magnetization orientation. It is demonstrated that the spin-orbit fields depend not only on the electric field across the interface, but also surprisingly strongly on the Fe magnetization orientation, opening prospects for their magnetic control. These results give important clues in searching for spin-orbit transport and optical phenomena in ferromagnet/nonmagnet heterostructures.

We study Coulomb interactions in the finite bias response of Mach-Zehnder interferometers, which exploit copropagating edge states in the integer quantum Hall effect. Here, interactions are particularly important since the coherent coupling of edge channels is due to a resonant mechanism that is spoiled by inelastic processes. We find that interactions yield a saturation, as a function of bias voltage, of the period-averaged interferometer current, which gives rise to unusual features, such as negative differential conductance, enhancement of the visibility of the current, and nonbounded or even diverging visibility of the differential conductance.

The phase-dependent bound states (Andreev levels) of a Josephson junction can cross at the Fermi level if the superconducting ground state switches between even and odd fermion parity. The level crossing is topologically protected, in the absence of time-reversal and spin-rotation symmetry, irrespective of whether the superconductor itself is topologically trivial or not. We develop a statistical theory of these topological transitions in an $N$-mode quantum-dot Josephson junction by associating the Andreev level crossings with the real eigenvalues of a random non-Hermitian matrix. The number of topological transitions in a $2\pi$ phase interval scales as $\sqrt{N}$, and their spacing distribution is a hybrid of the Wigner and Poisson distributions of random-matrix theory.

Single phase and strained ${\mathrm{LuMnO}}_3$ thin films are discovered to display coexisting ferromagnetic and antiferromagnetic orders. A large moment ferromagnetism ($\approx 1{\mu }_{\mathrm{B}}$), which is absent in bulk samples, is shown to display a magnetic moment distribution that is peaked at the highly strained substrate-film interface. We further show that the strain-induced ferromagnetism and the antiferromagnetic order are coupled via an exchange field, therefore demonstrating strained rare-earth manganite thin films as promising candidate systems for new multifunctional devices.

We construct a class of projected entangled pair states which is exactly the resonating valence bond wave functions endowed with both short range and long range valence bonds. With an energetically preferred resonating valence bond pattern, the wave function is simplified to live in a one-parameter variational space. We tune this variational parameter to minimize the energy for the frustrated spin-$1/2$ $J_1-J_2$ antiferromagnetic Heisenberg model on the square lattice. Taking a cylindrical geometry, we are able to construct four topological sectors with an even or odd number of fluxes penetrating the cylinder and an even or odd number of spinons on the boundary. The energy splitting in different topological sectors is exponentially small with the cylinder perimeter. We find a power law decay of the dimer correlation function on a torus, and a $\ln L$ correction to the entanglement entropy, indicating a gapless spin-liquid phase at the optimum parameter.

We present the electrical readout of time trajectories obtained from an isolated nuclear spin. The device, a ${\mathrm{TbPc}}_2$ single-molecule magnet spin transistor, detects the four different nuclear spin states of the ${\mathrm{Tb}}^{3+}$ ion with fidelities better than 69%, allowing us to measure individual relaxation times ($T_1$) of several tens of seconds. A good agreement with quantum Monte Carlo simulations suggests that the relaxation times are limited by the current tunneling through the transistor, which opens up the possibility to tune $T_1$ electrically by means of bias and gate voltages.

Both electric- and magnetic-dipole active spin excitations, i.e., electromagnons, which mediate the dynamical magnetoelectric effect, have been investigated for a multiferroic perovskite of manganite by optical spectroscopy at terahertz frequencies. Upon the magnetoelectric resonance at 1 meV in the multiferroic phase with the $bc$-plane spin cycloidal order, a gigantic dynamical magnetoelectric effect has been observed as a nonreciprocal directional dichroism or birefringence. The light $k$-vector-dependent difference ($\Delta \kappa ={\kappa }_{+}-{\kappa }_{-}$) of the extinction coefficient (${\kappa }_{\pm }$) is as large as $\Delta \kappa \sim 1$ or $2\Delta \kappa /({\kappa }_{+}+{\kappa }_{-})\sim 0.7$ at the lowest-lying electromagnon energy. We clarified the mutual coupling of the $E^{\omega }\parallel a$-polarized electromagnons of the different origins, leading to the enhancement of the magnetoelectric resonance.

The viscosity of lipid bilayer membranes plays an important role in determining the diffusion constant of embedded proteins and the dynamics of membrane deformations, yet it has historically proven very difficult to measure. Here we introduce a new method based on quantification of the large-scale circulation patterns induced inside vesicles adhered to a solid surface and subjected to simple shear flow in a microfluidic device. Particle image velocimetry based on spinning disk confocal imaging of tracer particles inside and outside of the vesicle and tracking of phase-separated membrane domains are used to reconstruct the full three-dimensional flow pattern induced by the shear. These measurements show excellent agreement with the predictions of a recent theoretical analysis, and allow direct determination of the membrane viscosity.

Scattering is an important phenomenon which is observed in systems ranging from the micro- to macroscale. In the context of nuclear reaction theory, the Heidelberg approach was proposed and later demonstrated to be applicable to many chaotic scattering systems. To model the universal properties, stochasticity is introduced to the scattering matrix on the level of the Hamiltonian by using random matrices. A long-standing problem was the computation of the distribution of the off-diagonal scattering-matrix elements. We report here an exact solution to this problem and present analytical results for systems with preserved and with violated time-reversal invariance. Our derivation is based on a new variant of the supersymmetry method. We also validate our results with scattering data obtained from experiments with microwave billiards.

We prove the equivalence of an arbitrary single-mode Gaussian quantum channel and a newly defined fiducial channel preceded by a phase shift and followed by a Gaussian unitary operation. This equivalence implies that the energy-constrained classical capacity of any single-mode Gaussian channel can be calculated based on this fiducial channel, which is furthermore simply realizable with a beam splitter, two identical single-mode squeezers, and a two-mode squeezer. In a large domain of parameters, we also provide an analytical expression for the Gaussian classical capacity, exploiting its additivity, and prove that the classical capacity cannot exceed it by more than $1/\ln 2$ bits.

We identify a class of condensate states in the group field theory (GFT) formulation of quantum gravity that can be interpreted as macroscopic homogeneous spatial geometries. We then extract the dynamics of such condensate states directly from the fundamental quantum GFT dynamics, following the procedure used in ordinary quantum fluids. The effective dynamics is a nonlinear and nonlocal extension of quantum cosmology. We also show that any GFT model with a kinetic term of Laplacian type gives rise, in a semiclassical (WKB) approximation and in the isotropic case, to a modified Friedmann equation. This is the first concrete, general procedure for extracting an effective cosmological dynamics directly from a fundamental theory of quantum geometry.

Determining the dark matter (DM) mass is of paramount importance for understanding dark matter. We present a novel parametrization of the DM speed distribution which will allow the DM mass to be accurately measured using data from weakly interacting massive particle (WIMP) direct detection experiments. Specifically, we parametrize the natural logarithm of the speed distribution as a polynomial in the speed $v$. We demonstrate, using mock data from upcoming experiments, that by fitting the WIMP mass and interaction cross section, along with the polynomial coefficients, we can accurately reconstruct both the WIMP mass and speed distribution. This new method is the first demonstration that an accurate, unbiased reconstruction of the WIMP mass is possible without prior assumptions about the distribution function. We anticipate that this technique will be invaluable in the analysis of future experimental data.

We report measurements of $B\to {\chi }_{c1}\gamma K$ and ${\chi }_{c2}\gamma K$ decays using $772\times {10}^6$ $B\overline{B}$ events collected at the ${\rm Y}(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. Evidence of a new resonance in the ${\chi }_{c1}\gamma$ final state is found with a statistical significance of $3.8\sigma$. This state has a mass of $3823.1\pm 1.8(\mathrm{stat})\pm 0.7(\mathrm{syst})\mathrm{MeV}/c^2$, a value that is consistent with theoretical expectations for the previously unseen $1{}^{3}D_2$ $c\overline{c}$ meson. We find no other narrow resonance and set upper limits on the branching fractions of the $X(3872)\to {\chi }_{c1}\gamma$ and ${\chi }_{c2}\gamma$ decays.

The jet fragmentation function is measured with direct photon-hadron correlations in $p+p$ and $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$. The $p_T$ of the photon is an excellent approximation to the initial $p_T$ of the jet and the ratio $z_T=p_T^h/p_T^{\gamma }$ is used as a proxy for the jet fragmentation function. A statistical subtraction is used to extract the direct photon-hadron yields in $\mathrm{Au}+\mathrm{Au}$ collisions while a photon isolation cut is applied in $p+p$. $I_{\mathrm{AA}}$, the ratio of hadron yield opposite the photon in $\mathrm{Au}+\mathrm{Au}$ to that in $p+p$, indicates modification of the jet fragmentation function. Suppression, most likely due to energy loss in the medium, is seen at high $z_T$. The associated hadron yield at low $z_T$ is enhanced at large angles. Such a trend is expected from redistribution of the lost energy into increased production of low-momentum particles.

We introduce a field transformation method for wave manipulation based on completely reciprocal and passive materials. While coordinate transformations in transformation optics (TO) change the size and shape of an object, field transformations give us direct control on the impedance and polarization signature of an object. Using our approach, a new type of perfect conductor can be realized to completely convert between transverse electric and transverse magnetic polarizations at any incidence angles and a perfect magnetic conductor of arbitrary shape can be mimicked by using anisotropic materials. The approach can be further combined with TO to enhance existing TO devices. For example, a dielectric cylinder can become completely transparent for both polarizations using bianisotropic materials.