Vortex states at low temperatures and quantum fluctuations in disordered superconductors

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We have studied the vortex states at low temperatures (T) and high fields (B) for thick (100 nm) amorphous Mo_xSi_<1-x> films with uniform and strong disorder. We have found that the vortex-liquid phase, so-called the quantum-vortex-liquid(QVL) phase, persists down to T=0. Upon cooling in fixed fields which correspond to the QVL phase at T→O, curvature in the Arrehenius plots of the resistivity changes from downward to upward at certain temperature T_Q, indicative of crossover from temperature dominated to quantum driven fluctuations in the liquid phase. Moreover, the relative width of the QVL phase has been shown to increase along the B and T axes nearly proportional to the normal-state resistivity ρ_n. This is in accord with the view that the QVL phase is caused by strong quantum fluctuations, which are enhanced with increasing ρ_n.We have also studied a change in the vortex dynamics associated with a change in the vortex state from the thermal to quantum liquid by measuring the fluctuating component of the flux-flow voltage δ V(t) about the average voltage. We have found the unusual vortex dynamics in the low-temperature liquid phase. For the thick film, δ V(t) originating from the vortex motion is clearly visible in the QVL phase, where the distribution of δ V(t) is anomalously asymmetric, having a tail which extends to the direction of flux motion. This result implies large velocity and/or number fluctuations of driven vortices in the QVL phase. We have observed large broadband noise there. For the thin (6 nm) film the similar unusual vortex motion is observed in nearly the same reduced-temperature regime. We suggest based on these results that the vortex dynamics in the low-temperature liquid phase of thick and thin films is dominated by common physical mechanisms related to quantum-fluctuation effects.