Flux-creep activation energy in high-Tcsuperconductors

We investigated the relationship between the effective flux-creep activation energy U and the current density j (∼M), for both the ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{\mathit{x}}$ and ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_{\mathit{x}}$ systems. We found that the U-versus-M curves can be best fitted by a power law. More specifically, for a single crystal of ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_{\mathit{x}}$ at a given temperature, a logarithmic relation between U and M is found in the low-temperature region, while a power-law behavior takes place near the irreversibility line. The validity of the apparent activation energy ${\mathit{U}}_0$ is also discussed.