The role of point defects in melting of solid He

We have recently observed that the resistance to shear of solid ⁴He decreases dramatically near the first-order BCC–HCP transition. In our view, the solid shears via a diffusive counter-flow of atoms and point defects. The mechanism of self diffusion couples point defects with one specific phonon which softens as the transition is approached. Since such a scenario can possibly lead to melting, it is important to understand (a) which type of point defect is associated with the reduction of shear resistance, and (b) can the presence of point defects lead to the softening of phonons. We report here the results of numerical simulations and analytic modeling. Our results indicate that split interstitials are much more effective than vacancies in lowering the resistance to shear. We suggest that these excitations can be generated as a result of a “local mode” excited in the crystal.     

Observation of a new excitation in bcc 4He by inelastic neutron scattering

We report neutron-scattering measurements of the phonons in bcc solid 4He. In general, only three acoustic phonon branches should exist in a monatomic cubic crystal. In addition to these phonon branches, we found a new "opticlike" mode along the [110] direction. One possible interpretation of this new mode is in terms of localized excitations unique to a quantum solid.     

Observation of spontaneous flux generation in a multi-josephson-junction loop

We describe observations of spontaneous flux generation inside a YBa(2)Cu(3)O(7-delta) loop made of 214 Josephson junctions in series. The flux is generated spontaneously during cooldown into the superconducting state. The experiment is motivated by the Kibble-Zurek scenario of formation of topological defects in condensed matter systems. The transition from decoupled superconducting segments into a coherent loop is determined by the strength of thermal fluctuations in the junctions. Values of the flux measured at the end of each cooldown follow a normal distribution, and are consistent with the instantaneous phase differences across the junctions adding up as the loop becomes coherent.