Low-temperature vortex dynamics in a high-temperature superconductor

Magnetic-field gradients in the mixed state of a type-II superconductor are studied using ${}_{}{}^{205}{}_{}{}^{}\mathrm{Tl}$ nuclear magnetic resonance (NMR) on ${\mathrm{Tl}}_2$${\mathrm{Ba}}_2$${\mathrm{Ca}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{10+\mathrm{\delta }}$. An anomalous peak was observed in the temperature dependence of the transverse relaxation rate at T/${\mathit{T}}_{\mathit{c}}$≊0.25. We attribute this behavior to magnetic-field flucutations from vortex dynamics. We interpret this behavior as a crossover of the principal time scale for vortex dynamics with that of the NMR experiment, approximately 100 μs. The temperature dependence of this time scale is discussed.