Vortex creep and critical current densities in superconducting (Ba,K)Fe_2As_2 single crystals
Résumé
The surprisingly rapid relaxation of the sustainable current density in the critical state of single crystalline (Ba,K)Fe_2As_2 is investigated for magnetic fields oriented parallel to the c-axis and to the ab-plane respectively. Due to the inadequacy of standard analysis procedures developed for flux creep in the high temperature superconducting cuprates, we develop a simple, straightforward data treatment technique that reveals the creep mechanism and the creep exponent µ. At low magnetic fields, below the second magnetization peak, µ varies only slightly as function of temperature and magnetic flux density B. From the data, we determine the temperature- and field dependence of the effective activation barrier for creep. At low temperatures, the measured current density approaches the zero--temperature critical current density (in the absence of creep) to within a factor 2, thus lending credence to earlier conclusions drawn with respect to the pinning mechanism. The comparable values of the experimental screening current density and the zero-temperature critical current density reveals the limited usefulness of the widely used ''interpolation formula''.
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