Low field vortex dynamics over seven time decades in a Bi_2Sr_2CaCu_2O_{8+\delta} single crystal for temperatures 13 K < T < 83 K

TL;DR

This study investigates vortex dynamics in a Bi_2Sr_2CaCu_2O_{8+} single crystal across seven decades of time and a temperature range of 13 K to 83 K, revealing different regimes of vortex pinning and flux creep behavior.

Contribution

It provides detailed measurements of flux creep activation barriers, current-voltage characteristics, and critical current density over a wide temperature and time range, elucidating vortex pinning regimes.

Findings

At T < 20 K, vortices are pinned individually with logarithmic U(j).

At T > 40 K, U(j) follows a power-law with exponent 0.6.

An intermediate regime shows no unique U(j) dependence, with a predicted second magnetization peak.

Abstract

Using a custom made dc-SQUID magnetometer, we have measured the time relaxation of the remanent magnetization M_rem of a Bi_2Sr_2CaCu_2O_{8+\delta} single crystal from the fully critical state for temperatures 13 K < T < 83 K. The measurements cover a time window of seven decades 10^{-2} s < t < 10^5 s, so that the current density j can be studied from values very close to j_c down to values considerably smaller than j_c. From the data we have obtained: (i) the flux creep activation barriers U as a function of current density j, (ii) the current-voltage characteristics E(j) in a typical range of 10^{-7} V/cm to 10^{-15} V/cm, and (iii) the critical current density j_c(0) at T = 0. Three different regimes of vortex dynamics are observed: For temperatures T < 20 K the activation barrier U(j) is logarithmic, no unique functional dependence U(j) could be found for the intermediate temperature interval 20 K < T < 40 K, and finally for T > 40 K the activation barrier U(j) follows a power-law behavior with an exponent mu = 0.6. From the analysis of the data within the weak collective pinning theory for strongly layered superconductors, it is argued that for temperatures T < 20 K pancake-vortices are pinned individually, while for temperatures T > 40 K pinning involves large collectively pinned vortex bundles. A description of the vortex dynamics in the intermediate temperature interval 20 K < T < 40 K is given on the basis of a qualitative low field phase diagram of the vortex state in Bi_2Sr_2CaCu_2O_{8+\delta}. Within this description a second peak in the magnetization loop should occur for temperatures between 20 K and 40 K, as it has been observed in several magnetization measurements in the literature.