Unusual vortex dynamics in the quantum-liquid phase of superconductors

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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.