Dislocation-mediated creep of highly separated vortices in a-axis-oriented ${\mathrm{HgBa}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{6+\delta }$ thin films

Using ac susceptibility, we determine the critical current density $J_c$ and the flux creep activation energy U of an a-axis-oriented ${\mathrm{HgBa}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{6+\delta }$ thin film. The critical current density at helium temperatures is found to be $4.6\times {10}^4{\mathrm{A}/\mathrm{c}\mathrm{m}}^2,$ 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 ${U(T,H}_{\mathrm{ac}}{)=U}_0(1\mathrm{}-t^4{)H}_{\mathrm{ac}}^{-1/2}$ with ${t=T/T}_c,$ $T_c=120\mathrm{}\mathrm{K},$ and $U_0=0.77\mathrm{}{\mathrm{eV}\mathrm{}\mathrm{Oe}}^{1/2}$ for temperatures $T>\mathrm{}45\mathrm{}\mathrm{K}$ and in the field range studied. The activation energy is of the same order as that found in c-axis-oriented films. Below $T=45\mathrm{}\mathrm{K}$ the activation energy is observed to decrease as thermally assisted quantum creep becomes increasingly important.