Re-entrant behavior of low-field flux creep in c-axis-oriented ${\mathrm{HgBa}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{6+\delta }$ thin films

The temperature, ac and dc field, and current dependent activation energy $U(T,H)\mathrm{}\left[{(J}_{c0\mathrm{}}{/J}_c{)}^{\mu }-1\right]/\mu$ governing low-field flux creep in epitaxial c-axis-oriented ${\mathrm{HgBa}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{6+\delta }$ thin films has been determined from measurements of the frequency-dependent in-phase ac susceptibility. Above 35 K three different thermally activated flux creep regimes can be identified: (i) dislocation-mediated plastic flux creep, described by ${U(T,H)=U}_0(1\mathrm{}-t^4{)H}^{-1/2}$ and $\mu =0,$ (ii) elastic collective flux creep which decreases with temperature and has a weaker field dependence of $H^{0.22}$ above a field-dependent temperature $T_{\mathrm{cm}}\mathrm{}\left(H\right)\mathrm{}$ where μ acquires finite values, and (iii) reappearance of dislocation-mediated plastic flux creep which rapidly increases as $T_c$ is approached. It is argued that the re-entrant plastic-elastic-plastic vortex creep behavior is driven by the underlying temperature and field dependence of the shear modulus $c_{66}.$ $T_{\mathrm{cm}}\mathrm{}\left(H\right)\mathrm{}$ marks a line in the $H-T$ plane where the increasing $c_{66}$ promotes long-range correlations in the dilute vortex phase and creep becomes collective. At high H and T, $c_{66}$ again decreases and plastic creep reappears as the ordered phase starts to melt. Evidence for thermally assisted quantum creep is observed up to temperatures as high as $T_0=35\mathrm{}\mathrm{K}.$