Cavitation in 3He-4He liquid mixtures at low temperatures

Bubble formation in solutions of 3He and 4He is studied within a density-functional approach. In particular, the temperature dependence of the cavitation pressure for different 3He concentrations is calculated at low temperatures and compared to that of pure 4He. The presence of Andreev states lowers the surface tension and, consequently, nucleation barriers are drastically reduced. This fact means that even at low 3He concentrations the cavitation process takes place at higher pressures than the spinodal pressure, which is not the case for pure 4He.

Grain boundaries in vortex matter

We explore the statistical properties of grain boundaries in the vortex polycrystalline phase of type-II superconductors. Treating grain boundaries as arrays of dislocations interacting through linear elasticity, we show that self-interaction of a deformed grain boundary is equivalent to a nonlocal long-range surface tension. This affects the pinning properties of grain boundaries, which are found to be less rough than isolated dislocations. The presence of grain boundaries has an important effect on the transport properties of type-II superconductors as we show by numerical simulations: our results indicate that the critical current is higher for a vortex polycrystal than for a regular vortex lattice. Finally, we discuss the possible role of grain boundaries in vortex lattice melting. Through a phenomenological theory we show that melting can be preceded by an intermediate polycrystalline phase.