We use multiscale techniques to determine the extent of local inhomogeneity and superconductivity in ${\mathrm{Ca}}_{0.86}{\mathrm{Pr}}_{0.14}{\mathrm{Fe}}_2{\mathrm{As}}_2$ single crystal. The inhomogeneity is manifested as a spatial variation of the praseodymium concentration, local density of states, and superconducting order parameter. We show that the high-$T_c$ superconductivity emerges from cloverlike defects associated with Pr dopants. The highest $T_c$ is observed in both the tetragonal and collapsed tetragonal phases, and its filamentary nature is a consequence of nonuniform Pr distribution that develops localized, isolated superconducting regions within the crystals.

The negatively charged nitrogen-vacancy (${\mathrm{NV}}^{-}$) center in diamond has realized new frontiers in quantum technology. Here, the optical and spin resonances of the ${\mathrm{NV}}^{-}$ center are observed under hydrostatic pressures up to 60 GPa. Our results motivate powerful new techniques to measure pressure and image high-pressure magnetic and electric phenomena. Additionally, molecular orbital analysis and semiclassical calculations provide insight into the effects of compression on the electronic orbitals of the ${\mathrm{NV}}^{-}$ center.

Unlike macroscopic multistable mechanical systems such as snap bracelets or elastic shells that must be physically manipulated into various conformations, microscopic systems can undergo spontaneous conformation switching between multistable states due to thermal fluctuations. Here we investigate the statistical mechanics of shape transitions in small elastic elliptical plates and shells driven by noise. By assuming that the effects of edges are small, which we justify exactly for plates and shells with a lenticular section, we decompose the shapes into a few geometric modes whose dynamics are easy to follow. We use Monte Carlo simulations to characterize the shape transitions between conformational minimal as a function of noise strength, and corroborate our results using a Fokker-Planck formalism to study the stationary distribution and the mean first passage time problem. Our results are applicable to objects such as graphene flakes or protein $\beta$ sheets, where fluctuations, geometry, and finite size effects are important.

We study the collective transport of paramagnetic colloids driven above a magnetic bubble lattice by an external rotating magnetic field. We measure a direct ratchet current which rises in integer and fractional steps with the field amplitude. The stepwise increase is caused by excluded volume interactions between the particles, which form composite clusters above the bubbles with mobile and immobile occupation sites. Transient energy minima located at the interstitials between the bubbles cause the colloids to hop from one composite cluster to the next with synchronous and period doubled modes of transport. The colloidal current may be polarized to make selective use of type up or type down interstitials.

Energies and spectroscopic factors of the first $7/2^{-}$, $3/2^{-}$, $1/2^{-}$, and $5/2^{-}$ states in the ${}^{35}{\mathrm{Si}}_{21}$ nucleus were determined by means of the ($d$, $p$) transfer reaction in inverse kinematics at GANIL using the MUST2 and EXOGAM detectors. By comparing the spectroscopic information on the ${}^{35}\mathrm{Si}$ and ${}^{37}\mathrm{S}$ isotones, a reduction of the $p_{3/2}\text{−}{p}_{1/2}$ spin-orbit splitting by about 25% is proposed, while the $f_{7/2}\text{−}{f}_{5/2}$ spin-orbit splitting seems to remain constant. These features, derived after having unfolded nuclear correlations using shell model calculations, have been attributed to the properties of the two-body spin-orbit interaction, the amplitude of which is derived for the first time in an atomic nucleus. The present results, remarkably well reproduced by using several realistic nucleon-nucleon forces, provide a unique touchstone for the modeling of the spin-orbit interaction in atomic nuclei.

Detection of electron motion by elastic scattering of short x-ray pulses from a coherent superposition of highly excited electronic states in rare gas atoms is investigated. The laser excitation of the electron wave packet introduces strong anisotropy which facilitates detection, and large differences in the radial distribution of the excited Rydberg and core electrons allow the dynamics to be detected using both soft and hard x rays.

We use coherent excitation of 3–16 atom ensembles to demonstrate collective Rabi flopping mediated by Rydberg blockade. Using calibrated atom number measurements, we quantitatively confirm the expected $\sqrt{N}$ Rabi frequency enhancement to within 4%. The resulting atom number distributions are consistent with an essentially perfect blockade. We then use collective Rabi $\pi$ pulses to produce $\mathcal{N}=1$, 2 atom number Fock states with fidelities of 62% and 48%, respectively. The $\mathcal{N}=2$ Fock state shows the collective Rabi frequency enhancement without corruption from atom number fluctuations.

High resolution polar Kerr effect measurements were performed on ${\mathrm{La}}_{1.875}{\mathrm{Ba}}_{0.125}{\mathrm{CuO}}_4$ single crystals revealing that a finite Kerr signal is measured below an onset temperature $T_K$ that coincides with the charge ordering transition temperature $T_{CO}$. We further show that the sign of the Kerr signal cannot be trained with the magnetic field, is found to be the same on opposite sides of the same crystal, and is odd with respect to strain in the diagonal direction of the unit cell. These observations are consistent with a chiral “gyrotropic” order above $T_c$ for ${\mathrm{La}}_{1.875}{\mathrm{Ba}}_{0.125}{\mathrm{CuO}}_4$; similarities to other cuprates suggest that it is a universal property in the pseudogap regime.

Heavy-tailed distributions of meme popularity occur naturally in a model of meme diffusion on social networks. Competition between multiple memes for the limited resource of user attention is identified as the mechanism that poises the system at criticality. The popularity growth of each meme is described by a critical branching process, and asymptotic analysis predicts power-law distributions of popularity with very heavy tails (exponent $\alpha <2$, unlike preferential-attachment models), similar to those seen in empirical data.

The problem of identifying measurement scenarios capable of revealing state-independent contextuality in a given Hilbert space dimension is considered. We begin by showing that for any given dimension $d$ and any measurement scenario consisting of projective measurements, (i) the measure of contextuality of a quantum state is entirely determined by its spectrum, so that pure and maximally mixed states represent the two extremes of contextual behavior, and that (ii) state-independent contextuality is equivalent to the contextuality of the maximally mixed state up to a global unitary transformation. We then derive a necessary and sufficient condition for a measurement scenario represented by an orthogonality graph to reveal state-independent contextuality. This condition is given in terms of the fractional chromatic number of the graph ${\chi }_f(G)$ and is shown to identify all state-independent contextual measurement scenarios including those that go beyond the original Kochen-Specker paradigm.

We precisely test scale invariance and examine local thermal equilibrium in the hydrodynamic expansion of a Fermi gas of atoms as a function of interaction strength. After release from an anisotropic optical trap, we observe that a resonantly interacting gas obeys scale-invariant hydrodynamics, where the mean square cloud size $⟨{\mathbf{r}}^2⟩=⟨x^2+y^2+z^2⟩$ expands ballistically (like a noninteracting gas) and the energy-averaged bulk viscosity is consistent with zero, $0.00(0.04)\mathrm{\hslash }n$, with $n$ the density. In contrast, the aspect ratios of the cloud exhibit anisotropic “elliptic” flow with an energy-dependent shear viscosity. Tuning away from resonance, we observe conformal symmetry breaking, where $⟨{\mathbf{r}}^2⟩$ deviates from ballistic flow.

Galaxy bias, the unknown relationship between the clustering of galaxies and the underlying dark matter density field is a major hurdle for cosmological inference from large-scale structure. While traditional analyses focus on the absolute clustering amplitude of high-density regions mapped out by galaxy surveys, we propose a relative measurement that compares those to the underdense regions, cosmic voids. On the basis of realistic mock catalogs we demonstrate that cross correlating galaxies and voids opens up the possibility to calibrate galaxy bias and to define a static ruler thanks to the observable geometric nature of voids. We illustrate how the clustering of voids is related to mass compensation and show that volume-exclusion significantly reduces the degree of stochasticity in their spatial distribution. Extracting the spherically averaged distribution of galaxies inside voids from their cross correlations reveals a remarkable concordance with the mass-density profile of voids.

A search is presented for dark matter pair production in association with a $W$ or $Z$ boson in $pp$ collisions representing $20.3{\mathrm{fb}}^{-1}$ of integrated luminosity at $\sqrt{s}=8\mathrm{TeV}$ using data recorded with the ATLAS detector at the Large Hadron Collider. Events with a hadronic jet with the jet mass consistent with a $W$ or $Z$ boson, and with large missing transverse momentum are analyzed. The data are consistent with the standard model expectations. Limits are set on the mass scale in effective field theories that describe the interaction of dark matter and standard model particles, and on the cross section of Higgs production and decay to invisible particles. In addition, cross section limits on the anomalous production of $W$ or $Z$ bosons with large missing transverse momentum are set in two fiducial regions.

We determine the magnetization of quantum chromodynamics for several temperatures around and above the transition between the hadronic and the quark-gluon phases of strongly interacting matter. We obtain a paramagnetic response that increases in strength with the temperature. We argue that due to this paramagnetism, chunks of quark-gluon plasma produced in noncentral heavy ion collisions should become squeezed perpendicular to the magnetic field. This anisotropy will then contribute to the elliptic flow $v_2$ observed in such collisions, in addition to the pressure gradient that is usually taken into account. We present a simple estimate for the magnitude of this new effect and a rough comparison to the effect due to the initial collision geometry. We conclude that the paramagnetic effect might have a significant impact on the value of $v_2$.

In this Letter, ab initio molecular dynamics simulations based on time-dependent density functional theory for the electrons and Ehrenfest dynamics for the nuclei are reported that detail the interaction of a vibrating HCl molecule with an Al(111) substrate. The mechanism responsible for the strong electron-hole-pair (EHP)–vibrational coupling in case of highly vibrationally excited molecules is traced back to a large eigenenergy shift of the $s{p}_z^{*}$-like antibonding HCl lowest unoccupied molecular orbital with the bond length. As a consequence of this mechanism, the electronic excitation spectra turn out to be highly asymmetric. The simulations suggest an explanation of how to reconcile a strong EHP-vibrational coupling in case of highly vibrationally excited molecules with the small, but clearly evident, electronic contribution to the $v=0\to v=1$ vibrational excitation observed experimentally during the scattering of HCl molecules at a hot Au surface by Ran et al. [Phys. Rev. Lett. 98, 237601 (2007)].

We investigate transverse symmetry-breaking instabilities emerging from the optomechanical coupling between light and the translational degrees of freedom of a collisionless, damping-free gas of cold, two-level atoms. We develop a kinetic theory that can also be mapped on to the case of an electron plasma under ponderomotive forces. A general criterion for the existence and spatial scale of transverse instabilities is identified; in particular, we demonstrate that monotonically decreasing velocity distribution functions are always unstable.

In this Letter we propose to use subwavelength diffraction gratings as very good semitransparent mirrors for electromagnetic waves to design open cavities. To do so, we replace part of the walls of a cavity by such a grating. We numerically and analytically link the grating characteristics to the spectral properties of the realized open cavity. Then we demonstrate that the eigenmodes of the cavity can be transmitted perfectly through the grating to the exterior, thereby turning a point source inside the cavity into a very directive source. We investigate the effect of disorder, which leads to isotropic radiation patterns, and perform experiments in the microwave domain in order to support our claims. Finally, we present an example of application of the concept in fundamental physics, by measuring from outside the eigenmodes of a disordered microwave cavity.

Microresonator-based optical frequency combs have been a topic of extensive research during the last few years. Several theoretical models for the comb generation have been proposed; however, they do not comprehensively address experimental results that show a variety of independent comb generation mechanisms. Here, we present frequency-domain experiments that illuminate the transition of microcombs into phase-locked states, which show characteristics of injection locking between ensembles of comb modes. In addition, we demonstrate the existence of equidistant optical frequency combs that are phase stable but have nondeterministic phase relationships between individual comb modes.

We investigate the first example of self-consistent impact ionization fronts propagating at relativistic speeds and involving interacting, high-energy electrons. These fronts, which we name relativistic runaway ionization fronts, show remarkable features such as a bulk speed within less than one percent of the speed of light and the stochastic selection of high-energy electrons for further acceleration, which leads to a power-law distribution of particle energies. A simplified model explains this selection in terms of the overrun of Coulomb-scattered electrons. Appearing as the electromagnetic interaction between electrons saturates the exponential growth of a relativistic runaway electron avalanche, relativistic runaway ionization fronts may occur in conjunction with terrestrial gamma-ray flashes and thus explain recent observations of long, power-law tails in the terrestrial gamma-ray flash energy spectrum.

We show that the phase of a Bose-Einstein condensate wave function of ultracold atoms in an optical lattice potential in two dimensions can be detected. The time-of-flight images, obtained in a free expansion of initially trapped atoms, are related to the initial distribution of atomic momenta but the information on the phase is lost. However, the initial atomic cloud is bounded and this information, in addition to the time-of-flight images, is sufficient in order to employ the phase retrieval algorithms. We analyze the phase retrieval methods for model wave functions in a case of a Bose-Einstein condensate in a triangular optical lattice in the presence of artificial gauge fields.