Dislocation-mediated creep of highly separated vortices ina-axis-orientedHgBa2CaCu2O6+δthin films

Abstract

Using ac susceptibility, we determine the critical current density ${J}_{c}$ and the flux creep activation energy U of an a-axis-oriented ${\mathrm{HgBa}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{6+\ensuremath{\delta}}$ thin film. The critical current density at helium temperatures is found to be $4.6\ifmmode\times\else\texttimes\fi{}{10}^{4}{\mathrm{A}/\mathrm{c}\mathrm{m}}^{2},$ i.e., about two orders of magnitude smaller than for corresponding films with c-axis orientation. The temperature and ac field dependent activation energy is consistent with dislocation-mediated flux creep and well described by ${U(T,H}_{\mathrm{ac}}{)=U}_{0}(1\ensuremath{-}{t}^{4}{)H}_{\mathrm{ac}}^{\ensuremath{-}1/2}$ with ${t=T/T}_{c},$ ${T}_{c}=120\mathrm{K},$ and ${U}_{0}=0.77{\mathrm{eV}\mathrm{}\mathrm{Oe}}^{1/2}$ for temperatures $T>45\mathrm{K}$ and in the field range studied. The activation energy is of the same order as that found in c-axis-oriented films. Below $T=45\mathrm{K}$ the activation energy is observed to decrease as thermally assisted quantum creep becomes increasingly important.