Vortex states at low temperatures and quantum fluctuations in disordered superconductors

本文研究了具有均匀性和强无序性的厚（100 nm） Mo_xSi_<1-x>非晶薄膜在低温(T)和高场(B)下的涡流态。我们发现涡旋-液相，即所谓的量子涡旋-液相（QVL）相，一直持续到T=0。在T→O处对应QVL相的固定场冷却后，电阻率的Arrehenius图曲率在一定温度T_Q下由下向上变化，表明液相从温度主导到量子驱动的波动发生了交叉。此外，QVL相的相对宽度沿B轴和T轴的增加几乎与正常状态电阻率ρ_n成正比。这与QVL相是由强量子涨落引起的观点一致，而强量子涨落随着ρ_n的增大而增强。我们还通过测量平均电压δ V(t)的波动分量，研究了从热流体到量子流体的涡旋状态变化所引起的涡旋动力学变化。我们在低温液相中发现了不寻常的涡旋动力学。对于厚膜，δ V(t)在QVL相中明显可见，δ V(t)的分布异常不对称，有一条向通量运动方向延伸的尾巴。这一结果表明在QVL阶段驱动涡的速度和/或数量波动较大。我们观察到那里有很大的宽带噪声。对于薄（6纳米）薄膜，在几乎相同的降低温度状态下观察到类似的不寻常的涡旋运动。基于这些结果，我们认为厚膜和薄膜低温液相中的涡旋动力学是由与量子涨落效应相关的常见物理机制主导的。

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.