Vortex states at low temperatures and quantum fluctuations in disordered superconductors

本文研究了具有均匀和强无序的非晶Mo_xSi_2厚膜（100 nm）在低温（T）和高场（B）下的涡旋态<1-x>。我们已经发现，涡旋-液相，即所谓的量子涡旋-液相（QVL）相，持续到T=0。当在对应于T→ 0处的QVL相的固定场中冷却时，电阻率的Arrehenius图中的曲率在特定温度T_Q处从向下变为向上，指示从温度主导到液相中的量子驱动波动的交叉。QVL相的相对宽度沿B和T轴沿着增加，与正常态电阻率ρ n几乎成正比。这与QVL相是由强的量子涨落引起的观点是雅阁的。我们还通过测量磁通流电压δ V（t）关于平均电压的涨落分量，研究了涡旋态从热态到量子态的变化所引起的涡旋动力学的变化。我们发现了低温液相中不寻常的涡旋动力学。对于厚膜，在QVL相中清晰可见源于涡旋运动的δ V（t），其中δ V（t）的分布是不对称的，具有延伸到通量运动方向的尾部。这一结果意味着大的速度和/或数量波动的驱动涡在QVL阶段。我们在那里观察到了很大的宽带噪声。对于薄（6 nm）的薄膜，观察到类似的不寻常的旋涡运动在几乎相同的降低温度制度。我们建议基于这些结果，厚膜和薄膜的低温液相中的涡旋动力学是由量子涨落效应相关的共同的物理机制。

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.