We demonstrate time reversal of nuclear spin dynamics in highly magnetized dilute liquid ${}^3\mathrm{He}\mathrm{\text{−}}{}^4\mathrm{He}$ mixtures through effective inversion of long-range dipolar interactions. These experiments, which involve using magic sandwich NMR pulse sequences to generate spin echoes, probe the spatiotemporal development of turbulent spin dynamics and promise to serve as a versatile tool for the study and control of dynamic magnetization instabilities. We also show that a repeated magic sandwich pulse sequence can be used to dynamically stabilize modes of nuclear precession that are otherwise intrinsically unstable. To date, we have extended the effective precession lifetimes of our magnetized samples by more than three orders of magnitude.

A new terahertz (THz) source in air based on the bifilamentation of femtosecond laser pulses is reported. This THz radiation is 1 order of magnitude more intense than the transition-Cherenkov THz emission from femtosecond laser filaments reported recently and shows different angular and polarization properties. We attribute it to the emission from a bimodal transmission line created by two plasma filaments.

Inelastic neutron scattering measurements on thermoelectric ${\mathrm{Zn}}_4{\mathrm{Sb}}_3$ reveal a dominant soft local phonon mode at 5.3(1) meV. The form factor of this local mode is characteristic for dumbbells vibrating preferably along the dumbbell axis and can be related to a vibration of Sb dimers along the $c$ axis. The Lorentzian width of the mode corresponds to short phonon lifetimes of 0.39(2) ps and yields an estimate of the thermal conductivity that agrees quantitatively with recent steady state measurements. Heat capacity measurements are consistent with an Einstein mode model describing the local Sb-dimer rattling mode with an Einstein temperature of 62(1) K. Our study suggests that soft localized phonon modes in crystalline solids are not restricted to cagelike structures and are likely to be a universal feature of good thermoelectric materials.

Cyclic voltammetry is a fundamental experimental method for characterizing electrochemical surfaces. Despite its wide use, a way to quantitatively and directly relate cyclic voltammetry to ab initio calculations has been lacking. We derive the cyclic voltammogram for H on Pt(111) and Pt(100), based solely on density functional theory calculations and standard molecular tables. By relating the gas phase adsorption energy to the electrochemical electrode potential, we provide a direct link between surface science and electrochemistry.

Reversible computing requires preservation of all information throughout the entire computational process; this implies that all errors that appear as a result of the interaction of the information-carrying system with uncontrolled degrees of freedom must be corrected. But this can only be done at the expense of an increase in the entropy of the environment corresponding to the dissipation, in the form of heat, of the “noisy” part of the system’s energy. This Letter gives an expression of that energy in terms of the effective noise temperature, and analyzes the relationship between the energy dissipation rate and the rate of computation. Finally, a generalized Clausius principle based on the concept of effective temperature is presented.

Direct evidence for the presence of an inertial energy cascade, the most characteristic signature of hydromagnetic turbulence (MHD), is observed in the solar wind by the Ulysses spacecraft. After a brief rederivation of the equivalent of Yaglom’s law for MHD turbulence, a linear relation is indeed observed for the scaling of mixed third-order structure functions involving Elsässer variables. This experimental result firmly establishes the turbulent character of low-frequency velocity and magnetic field fluctuations in the solar wind plasma.

We present a new class of boron sheets, composed of triangular and hexagonal motifs, that are more stable than structures considered to date and thus are likely to be the precursors of boron nanotubes. We describe a simple and clear picture of electronic bonding in boron sheets and highlight the importance of three-center bonding and its competition with two-center bonding, which can also explain the stability of recently discovered boron fullerenes. Our findings call for reconsideration of the literature on boron sheets, nanotubes, and clusters.

An inertial mass of a vortex can be calculated by driving it around in a circle with a steadily revolving pinning potential. We show that in the low-frequency limit this gives precisely the same formula that was used by Baym and Chandler, but find that the result is not unique and depends on the force field used to cause the acceleration. We apply this method to the Gross-Pitaevskii model, and derive a simple formula for the vortex mass. We study both the long-range and short-range properties of the solution. We agree with earlier results that the nonzero compressibility leads to a divergent mass. From the short-range behavior of the solution we find that the mass is sensitive to the form of the pinning potential, and diverges logarithmically when the radius of this potential tends to zero.

Silicon carbide particles exhibit both electric and magnetic optical resonances, allowing unexplored dielectric metamaterial designs. Experimental extinction spectra and Mie theory calculations of single microscale rod-shaped particles reveal three observable midinfrared resonant modes. Two of the modes are degenerate, with a frequency that can be tuned according to a resonance condition derived within the Letter. The existence of both electric and magnetic resonances may enable a novel negative refractive index metamaterial design.

We show that photon nonlinearities in an electromagnetically induced transparency can be at least 1 order of magnitude larger than predicted in all previous approaches. As an application we demonstrate that in this regime they give rise to very strong photon-photon interactions which are strong enough to make an experimental realization of a photonic Mott insulator state feasible in arrays of coupled ultrahigh-$Q$ microcavities.

We report the first successful extraction of accumulated ultracold neutrons (UCN) from a converter of superfluid helium, in which they were produced by downscattering neutrons of a cold beam from the Munich research reactor. Windowless UCN extraction is performed in vertical direction through a mechanical cold valve. This prototype of a versatile UCN source is comprised of a novel cryostat designed to keep the source portable and to allow for rapid cooldown. We measured time constants for UCN storage and extraction into a detector at room temperature, with the converter held at various temperatures between 0.7 and 1.3 K. The UCN production rate inferred from the count rate of extracted UCN is close to the theoretical expectation.

We present spatially resolved radio-frequency spectroscopy of a trapped Fermi gas with resonant interactions and observe a spectral gap at low temperatures. The spatial distribution of the spectral response of the trapped gas is obtained using in situ phase-contrast imaging and 3D image reconstruction. At the lowest temperature, the homogeneous rf spectrum shows an asymmetric excitation line shape with a peak at $0.48(4){\epsilon }_F$ with respect to the free atomic line, where ${\epsilon }_F$ is the local Fermi energy.

The viscosity is measured for a Fermi liquid, a dilute ${}^3\mathrm{He}\mathrm{\text{−}}{}^4\mathrm{He}$ mixture, under extremely high magnetic field/temperature conditions ($B\le 14.8\mathrm{T}$, $T\ge 1.5\mathrm{mK}$). The spin-splitting energy $\mu B$ is substantially greater than the Fermi energy $k_B{T}_F$; as a consequence the polarization tends to unity and $s$-wave quasiparticle scattering is suppressed for $T\ll T_F$. Using a novel composite vibrating-wire viscometer an enhancement of the viscosity is observed by a factor of more than 500 over its low-field value. Good agreement is found between the measured viscosity and theoretical predictions based upon a $t$-matrix formalism.

We have studied nuclear forward scattering of synchrotron radiation for the 67.41 keV resonance of ${}^{61}\mathrm{Ni}$ using a silicon crystal monochromator with low-index reflections and a multielement detector. This approach can be extended to other high-energy Mössbauer transitions and does not pose any restrictions on the sample environment. Under conditions of large sample thickness and short nuclear lifetime, typical for work with high-energy nuclear resonances, the nuclear decay follows a universal dependence where both thickness effects and hyperfine interactions are taken into account by time scaling.

Whether positronium (Ps) can be physisorbed on a material surface is of great fundamental interest, since it can lead to new insight regarding quantum sticking and is a necessary first step to try to obtain a ${\mathrm{Ps}}_2$ molecule on a material host. Experiments in the past have produced evidence for physisorbed Ps on a quartz surface, but firm theoretical support for such a conclusion was lacking. We present a first-principles density-functional calculation of the key parameters determining the interaction potential between Ps and an $\alpha$-quartz surface. We show that there is indeed a bound state with an energy of $0.14\mathrm{eV}$, a value which agrees very well with the experimental estimate of $\sim 0.15\mathrm{eV}$. Further, a brief energy analysis invoking the Langmuir-Hinshelwood mechanism for the reaction of physisorbed atoms shows that the formation and desorption of a ${\mathrm{Ps}}_2$ molecule in that picture is consistent with the above results.

The simultaneous measurement of structural and chemical information at the atomic scale provides fundamental insights into the connection between form and function in materials science and nanotechnology. We demonstrate structural and chemical mapping in ${\mathrm{Bi}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{MnO}}_3$ using an aberration-corrected scanning transmission electron microscope. Two-dimensional mapping is made possible by an adapted method for fast acquisition of electron energy-loss spectra. The experimental data are supported by simulations, which help to explain the less intuitive features.

Critical velocities have been observed in an ultracold superfluid Fermi gas throughout the BEC-BCS crossover. A pronounced peak of the critical velocity at unitarity demonstrates that superfluidity is most robust for resonant atomic interactions. Critical velocities were determined from the abrupt onset of dissipation when the velocity of a moving one-dimensional optical lattice was varied. The dependence of the critical velocity on lattice depth and on the inhomogeneous density profile was studied.

We examine the structure of the distribution of single particle displacements (van Hove function) in a broad class of materials close to glass and jamming transitions. In a wide time window comprising structural relaxation, van Hove functions reflect the coexistence of slow and fast particles (dynamic heterogeneity). The tails of the distributions exhibit exponential, rather than Gaussian, decay. We argue that this behavior is universal in glassy materials and should be considered the analog, in space, of the stretched exponential decay of time correlation functions. We introduce a dynamical model that describes quantitatively numerical and experimental data in supercooled liquids, colloidal hard spheres, and granular materials. The tails of the distributions directly explain the decoupling between translational diffusion and structural relaxation observed in glassy materials.

We report the coexistence of ferromagnetic order and superconductivity in UCoGe at ambient pressure. Magnetization measurements show that UCoGe is a weak ferromagnet with a Curie temperature $T_C=3\mathrm{K}$ and a small ordered moment $m_0=0.03{\mu }_B$. Superconductivity is observed with a resistive transition temperature $T_s=0.8\mathrm{K}$ for the best sample. Thermal-expansion and specific-heat measurements provide solid evidence for bulk magnetism and superconductivity. The proximity to a ferromagnetic instability, the defect sensitivity of $T_s$, and the absence of Pauli limiting, suggest triplet superconductivity mediated by critical ferromagnetic fluctuations.

We report laboratory experiments on rippled granular surfaces formed under rolling wheels. Ripples appear above a critical speed and drift slowly in the driving direction. Ripples coarsen as they saturate and exhibit ripple creation and destruction events. All of these effects are captured qualitatively by 2D soft-particle simulations in which a disk rolls over smaller disks in a periodic box. The simulations show that compaction and segregation are inessential to the ripple phenomenon. We describe a simplified scaling model which gives some insight into the mechanism of the instability.