Flux-creep crossover and relaxation over surface barriers in ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$ crystals

Flux creep in ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_8$ crystals exhibits two different regimes as a function of time, as well as of temperature and magnetic field. The short-time, low-temperature regime has a peak in current density versus field, which is enhanced by irradiation defects. The long-time, high-temperature regime has a monotonic and sharp falloff (step) in current density versus magnetic field, which is suppressed by irradiation defects. The former is identified with bulk pinning, and the latter with a surface barrier.