Quantum liquid of vortices at T=0 in thick a-Mo xSi 1− x films

Abstract

Effects of disorder (or quantum fluctuations) on the vortex phase diagram at low temperature T have been studied on the basis of measurements of dc and ac complex resistivities for thick amorphous Mo x Si 1− x films with different disorder (normal-state resistivity). Irrespective of disorder, the vortex–glass transition (VGT) persists down to T∼0.05 T c0 up to B∼0.9 B c2(0), where T c0 and B c2(0) are the mean-field transition temperature and upper critical field at T=0, respectively. In the limit T=0, the VGT line B g( T) is independent of T and extrapolates to a field below B c2(0), indicative of the presence of a quantum–vortex–liquid (QVL) phase at T=0 in the regime B g(0)< B< B c2(0). The width of the T=0 QVL phase, [ B c2(0)− B g(0)]/ B c2(0), is wider for a more disordered film. This result is consistent with a view that the QVL phase is driven by quantum fluctuations, which are enhanced with increasing disorder.