Crossover from Thermal to Quantum Regime in Vortex Motion in Conventional Type II Superconductors

Abstract

Recently much attention has been given to the dynamical nature in magnetic and superconducting systems due to the first-order phase transition in which the fluctuations of system of either thermal or quantum mechanical origin play an essential role and in a macroscopic system metastable state undergoes transitions to a lower state. In disorder magnetic systems like SmCo3.5Cu1.5 magnetic domain walls move between pinning centers provided by the disorder. At high temperature this is known to occur via thermal activation: at low temperature we have shown previously that there is a crossover to quantum tunnelling [1,2]. It is expected that such behavior of the decay of metastable state is also observed in type II superconductors. In a previous paper [3,4], we have shown that the concept of mean activation energy leads to coherent results on the mixed state nature in several high-Tc superconductors (HTSCs). We have found that the time decay of the zero field cooled diamagnetic magnetization of YBa2Cu3O7-8 and Bi(Pb)2Sr2Ca2Cu3Oy can be explained in terms of a mean activation energy E which is found to be proportional to the reciprocal magnelic field. The temperature variation of the penetration and exclusion rate of vortices obeys an Arrhenius type equation with an effective temperature T*=(θ/2)coth(θ/2T) . Here, θ is a characteristic temperature closely related to the crossover from thermal process to quantum tunneling of vortices. It is natural to ask whether the same behavior of vortex state also exists in other superconductors not only in HTSCs but also in conventional type-II superconductors.