Unusual vortex dynamics in the quantum-liquid phase of superconductors

量子涨落如何影响II型超导体中的涡旋动力学？在量子涡旋液体（QVL）相中，驱动涡旋是如何流动的？这些都是非常有趣的问题，因为它们可能与^4He的超流性（量子液体）和涡旋的宏观量子隧穿密切相关。然而，这些问题尚未得到实验或理论上的回答。在这项工作中，我们研究的变化与静态涡旋状态的变化，从热量子液体通过测量涡流产生的电压噪声。我们制备了具有中等强度钉扎的非晶（a）-Mo_xSi_<1-x>2和a-Mg_xB_2<1-x>薄膜，以研究材料依赖性。我们还研究了<1-x>钉扎较弱的a-MoxGe_2薄膜，以阐明钉扎对平衡涡旋态和涡旋动力学的可能影响，发现这些非晶薄膜在低温T下的涡旋液态是相似的，都涉及到QVL相。我们通常检测到不寻常的噪音涡流在QVL阶段，这些电影。这些结果表明，QVL相的存在和异常的涡旋动力学在QVL是普遍的，无关于钉扎强度以及非晶薄膜的材料。基于噪声谱，我们提出噪声流的起源是由于QVL稳态流中由量子隧穿控制的随机脱钉和钉扎的涡旋束，并利用锁模共振研究了a-Mo_xGe_2薄膜中的涡旋动力学<1-x>.在测量的最低T（=0.1 K）下观察到驱动涡旋物质的动态有序性。我们发现，在低T下，动态熔化场比静态熔化场B_c受到显著抑制，表明在B_c以下发生的运动晶体的量子涨落诱导熔化或涡旋晶格的本征（无钉扎）量子驱动熔化。

How do quantum fluctuations affect the vortex dynamics in type-II superconductors? How do driven vortices flow in the quantum-vortex-liquid (QVL) phase? These are very interesting questions, because they may be closely related to superfluidity (quantum liquid) of^4He and macroscopic quantum tunneling of vortices. However, these questions have not yet been answered experimentally or theoretically. In this work, we study the change in vortex dynamics associated with the change in static vortex states from the thermal to quantum liquid by measuring voltage noise generated by vortex flow. We have prepared amorphous (a)-Mo_xSi_<1-x> and a-Mg_xB_<1-x> films with moderately strong pinning to examine a material dependence. We have also studied a-MoxGe_<1-x> films with weaker pinning to clarify possible effects of pinning on the equilibrium vortex states and vortex dynamics.We find that the vortex liquid states for these amorphous films are similar to each other, involving the QVL phase at low temperature T. We commonly detect the unusual noisy vortex flow in the QVL phase for these films. These results indicate that the presence of the QVL phase and unusual vortex dynamics in QVL are universal, independent of the pinning strength as well as the material of amorphous films. We propose based on noise spectra that the origin of noisy flow is due to vortex bundles that are depinned and pinned randomly, dominated by quantum tunneling, in the stationary flow of QVL.We also study the vortex dynamics in the solid phase of a-Mo_xGe_<1-x> films by using a mode-locking resonance. Dynamic ordering of driven vortex matter is observed down to the lowest T (=0.1 K) measured. We find that at low T, the dynamic melting field is significantly suppressed compared to the static melting field B_c, indicative of quantum-fluctuation induced melting of moving crystals or intrinsic (pin-free) quantum-driven melting of vortex lattice, which occurs below B_c.