Vortex dynamics in single-crystalYBa2Cu3O7probed byCu63nuclear spin echo measurements in the presence of transport current pulses

We report ${}_{}{}^{63}{}_{}{}^{}\mathrm{Cu}$ NMR spin-echo experiments on a high-quality single crystal of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ (${\mathit{T}}_{\mathit{c}}$=93 K), to which we have bonded leads for both transport current pulse applications and four-point resistance measurements. For a 9 T field with ${\mathit{H}}_0$ parallel to c, the resistive transition onsets at 90 K, and R fully reaches zero at ${\mathit{T}}_{\mathit{R}=0\mathrm{}}$=76 K. The superconducting state NMR linewidth, however, does not exceed the normal-state value until T<${\mathit{T}}_{\mathit{R}=0\mathrm{}}$, where it is in agreement with predictions based on measured penetration depths. We discuss the possibility that a vortex liquid is present within the resistive transition, with vortex diffusion occurring at a rate fast enough to induce motional narrowing of the vortex lattice contribution to the NMR linewidth. We use the Einstein relation to show that this rapid vortex diffusion implies an upper bound for the correlation length for vortex motion. Inclusion of transport current pulses in the spin-echo pulse sequence is found to have no effect on the magnitude of the spin-echo signal for T both greater and less than ${\mathit{T}}_{\mathit{R}=0\mathrm{}}$.