Tetrahedra may be the ultimate frustrating, disordered glass forming units. Our experiments on tetrahedral dice indicate the densest (volume fraction $\varphi =0.76\pm .02$, compared with ${\varphi }_{\mathrm{sphere}}=0.64$), most disordered, experimental, random packing of any set of congruent convex objects to date. Analysis of MRI scans yield translational and orientational correlation functions which decay as soon as particles do not touch, much more rapidly than the $\sim 6$ diameters for sphere correlations to decay. Although there are only $6.3\pm .5$ touching neighbors on average, face-face and edge-face contacts provide enough additional constraints, $12\pm 1.6$ total, to roughly bring the structure to the isostatic limit for frictionless particles. Randomly jammed tetrahedra form a dense rigid highly uncorrelated material.

We show that a heavy quark moving sufficiently fast through a quark-gluon plasma may lose energy by Cherenkov-radiating mesons. We demonstrate that this takes place in all strongly coupled, large-$N_c$ plasmas with a gravity dual. The energy loss is exactly calculable in these models despite being an $\mathcal{O}(1/N_c)$ effect. We discuss implications for heavy-ion collision experiments.

Using four-point measurements of the Cluster spacecraft, the energy distribution was determined for magnetic field fluctuations in the solar wind directly in the three-dimensional wave-vector domain in the range $|k|\le 1.5\times {10}^{-3}\mathrm{rad}/\mathrm{km}$. The energy distribution exhibits anisotropic features characterized by a prominently extended structure perpendicular to the mean field preferring the ecliptic north direction and also by a moderately extended structure parallel to the mean field. From the three-dimensional energy distribution wave vector anisotropy is estimated with respect to directions parallel and perpendicular to the mean magnetic field, and the result suggests the dominance of quasi-two-dimensional turbulence toward smaller spatial scales.

We show, using Boltzmann transport calculations and analysis of experimental data, that hexagonal ${\mathrm{PdCoO}}_2$ and ${\mathrm{PtCoO}}_2$ have a highly unusual metallic transport. The in-plane transport is typical of a very good metal, with high conductivity and low positive thermopower. The $c$-axis transport is completely different, with 2 orders of magnitude lower, but still coherent, conductivity and remarkably a very large negative thermopower. This large anisotropy of the thermopower provides an opportunity for investigating transport in a highly unusual regime using bulk materials.

Applying adiabatic, cyclic two-parameter modulations we investigate quantum heat transfer across an anharmonic molecular junction contacted with two heat baths. We demonstrate that the pumped heat typically exhibits a Berry-phase effect in providing an additional geometric contribution to heat flux. Remarkably, a robust fractional quantized geometric phonon response is identified as well. The presence of this geometric phase contribution in turn causes a breakdown of the fluctuation theorem of the Gallavotti-Cohen type for quantum heat transfer. This can be restored only if (i) the geometric phase contribution vanishes and if (ii) the cyclic protocol preserves the detailed balance symmetry.

Long-lived $|m|=1$ positronium (Ps) atoms are produced in vacuum when high density bursts of positrons with net polarization $p_0$ are implanted into a porous silica film in a 2.3 T magnetic field. We observe a decrease in the $|m|=1$ population as the density of the incident positron beam is increased due to quenching interactions between oppositely polarized Ps atoms within the target. Saturation of this density dependent quenching indicates that the initial positron spin polarization $p_0=28\pm 1\%$, and demonstrates the long term (${10}^2\mathrm{s}$) survival of positron polarization in a Surko-type buffer gas trap. We conclude that, at high Ps densities, the minority spin component is essentially eliminated and the remaining Ps is almost entirely ($\sim 96\%$) polarized, as required for the formation of a Ps Bose-Einstein condensate.

Tin-phthalocyanine molecules adsorbed on Ag(111) were contacted with the tip of a cryogenic scanning tunneling microscope. Orders-of-magnitude variations of the single-molecule junction conductance were achieved by controllably dehydrogenating the molecule and by modifying the atomic structure of the surface electrode. Nonequilibrium Green’s function calculations reproduce the trend of the conductance and visualize the current flow through the junction, which is guided through molecule-electrode chemical bonds.

We have revealed the underlying mechanism of the martensitic phase transition (MPT) in a new class of ferromagnetic shape memory alloys, ${\mathrm{Ni}}_2{\mathrm{Mn}}_{1+x}{\mathrm{Sn}}_{1-x}$, by the combination of bulk-sensitive hard-x-ray photoelectron spectroscopy and a first-principles density-functional calculation. The Ni $3d$ $e_g$ state in the cubic phase systematically shifts towards the Fermi energy with an increase in the number of Mn atoms substituted in the Sn sites. An abrupt decrease of the intensity of the Ni $3d$ $e_g$ states upon MPT for $x=0.36–0.42$ has been observed in the vicinity of the Fermi level. The energy shift of the Ni $3d$ minority-spin $e_g$ state in the cubic phase originates from hybridization with the antiferromagnetically coupled Mn in the Sn site. Below the MPT temperature, the Ni $3d$ state splits into two levels located below and above the Fermi energy in order to achieve an energetically stable state.

The antiferromagnetic to valence-bond-solid phase transition in the two-dimensional $J\mathrm{\text{−}}Q$ model (an $S=1/2$ Heisenberg model with four-spin interactions) is studied using large-scale quantum Monte Carlo simulations. The results support a continuous transition of the ground state, in agreement with the theory of “deconfined” quantum criticality. There are, however, large corrections to scaling, of logarithmic or very slowly decaying power-law form, which had not been anticipated. This suggests that either the $\mathrm{SU}(N)$ symmetric noncompact $C{P}^{N-1}$ field theory for deconfined quantum criticality has to be revised or that the theory for $N=2$ (as in the system studied here) differs significantly from $N\to \infty$ (where the field theory is analytically tractable).

It has been widely believed that, except in very extreme situations, the influence of gravity on quantum fields should amount to just small, subdominant contributions. This view seemed to be endorsed by the seminal results obtained over the last decades in the context of renormalization of quantum fields in curved spacetimes. Here, however, we argue that this belief is false by showing that there exist well-behaved spacetime evolutions where the vacuum energy density of free quantum fields is forced, by the very same background spacetime, to become dominant over any classical energy-density component. By estimating the time scale for the vacuum energy density to become dominant, and therefore for backreaction on the background spacetime to become important, we argue that this (infrared) vacuum dominance may bear unexpected astrophysical and cosmological implications.

We analyze the Bose-Hubbard model with a three-body hard-core constraint by mapping the system to a theory of two coupled bosonic degrees of freedom. We find striking features that could be observable in experiments, including a quantum Ising critical point on the transition from atomic to dimer superfluidity at unit filling, and a continuous supersolid phase for strongly bound dimers.

We report experimental observations and numerical simulations of the formation, dynamics, and lifetimes of single and multiply charged quantized vortex dipoles in highly oblate dilute-gas Bose-Einstein condensates (BECs). We nucleate pairs of vortices of opposite charge (vortex dipoles) by forcing superfluid flow around a repulsive Gaussian obstacle within the BEC. By controlling the flow velocity we determine the critical velocity for the nucleation of a single vortex dipole, with excellent agreement between experimental and numerical results. We present measurements of vortex dipole dynamics, finding that the vortex cores of opposite charge can exist for many seconds and that annihilation is inhibited in our trap geometry. For sufficiently rapid flow velocities, clusters of like-charge vortices aggregate into long-lived multiply charged dipolar flow structures.

We apply the large-deviation method to study trajectories in dissipative quantum systems. We show that in the long time limit the statistics of quantum jumps can be understood from thermodynamic arguments in terms of dynamical phases and transitions between them in trajectory space. We illustrate our approach with three simple examples: a driven 2-level system where we find a particular scale invariance point in the ensemble of trajectories of emitted photons; a blinking 3-level system, where we argue that intermittency in the photon count is related to a crossover between distinct dynamical phases; and a micromaser, where we find an actual first-order phase transition in the ensemble of trajectories.

We construct a light-guiding model of the retina outside the fovea, in which an array of glial (Muller) cells permeates the depth of the retina down to the photoreceptors. Based on measured refractive indices, we propagate light to obtain a significant increase of the intensity at the photoreceptors. For pupils up to 6 mm width, the coupling between neighboring cells is only a few percent. Low cross talk over the whole visible spectrum also explains the insensitivity to chromatic aberrations of the eye. The retina is revealed as an optimal structure designed for improving the sharpness of images.

Dark matter with Sommerfeld-enhanced annihilation has been proposed to explain observed cosmic ray positron excesses in the 10 GeV to TeV energy range. We show that the required enhancement implies thermal relic densities that are too small to be all of dark matter. We also show that the dark matter is sufficiently self-interacting that observations of elliptical galactic dark matter halos exclude large Sommerfeld enhancement for light force carriers. Resonant Sommerfeld enhancement does not modify these conclusions, and the astrophysical boosts required to resolve these discrepancies are disfavored, especially when significant self-interactions suppress halo substructure.

When a droplet of water impacts a hydrophobic surface, the drop is often observed to bounce. However, for about 10 years it has been known that the addition of very small quantities ($\sim 100\mathrm{ppm}$) of a flexible polymer such as poly-(ethylene oxide) can completely prevent rebound. This effect has for some time been explained in terms of the stretching of polymer chains by a velocity gradient in the fluid, resulting in a transient increase in the so-called “extensional viscosity.” Here we show, by measuring the fluid velocity inside the impacting drop, that the extensional viscosity plays no role in the antirebound phenomenon. Using fluorescently labeled $\lambda$ DNA we demonstrate that the observed effect is due to the stretching of polymer molecules as the droplet edge sweeps the substrate, retarding the movement of the receding contact line.

We demonstrate for the first time natural phase matching for optical frequency doubling in a high-$Q$ whispering-gallery-mode resonator made of lithium niobate. A conversion efficiency of 9% is achieved at $30\mu \mathrm{W}$ in-coupled continuous wave pump power. The observed saturation pump power of 3.2 mW is almost 2 orders of magnitude lower than the state-of-the-art value. This suggests an application of our frequency doubler as a source of nonclassical light requiring only a low-power pump, which easily can be quantum noise limited. Our theoretical analysis of the three-wave mixing in a whispering-gallery-mode resonator provides the relative conversion efficiencies for frequency doubling in various modes.

The discovery of a new chemical element with atomic number $Z=117$ is reported. The isotopes ${}^{293}117$ and ${}^{294}117$ were produced in fusion reactions between ${}^{48}\mathrm{Ca}$ and ${}^{249}\mathrm{Bk}$. Decay chains involving 11 new nuclei were identified by means of the Dubna gas-filled recoil separator. The measured decay properties show a strong rise of stability for heavier isotopes with $Z\ge 111$, validating the concept of the long sought island of enhanced stability for superheavy nuclei.

We present lattice results for the ground state energies of tritium, helium-3, helium-4, lithium-6, and carbon-12 nuclei. Our analysis includes isospin breaking, Coulomb effects, and interactions up to next-to-next-to-leading order in chiral effective field theory.

We study the structure of a pairing order parameter for spin-$1/2$ fermions with attractive interactions in a square lattice under a uniform magnetic field. Because the magnetic translation symmetry gives a unique degeneracy in the single-particle spectrum, the pair wave function has both zero and finite-momentum components coexisting, and their relative phases are determined by a self-consistent mean-field theory. We present a microscopic calculation that can determine the vortex lattice structure in the superfluid phase for different flux densities. Phase transition from a Hofstadter insulator to a superfluid phase is also discussed.