VORTEX DYNAMICS IN SINGLE-CRYSTAL YBA2CU3O7 PROBED BY CU-63 NUCLEAR-SPIN ECHO MEASUREMENTS IN THE PRESENCE OF TRANSPORT CURRENT PULSES

We report (CU)-C-63 NMR spin-echo experiments on a high-quality single crystal of YBa2Cu3O7 (T-c=93 K), to which we have bonded leads for bo th transport current pulse applications and four-point resistance meas urements. For a 9 T held with H-0 parallel to c, the resistive transit ion onsets at 90 K, and R fully reaches zero at T-R=0=76 K. The superc onducting state NMR linewidth, however, does not exceed the normal-sta te value until T < T-R=0, where it is in agreement with predictions ba sed 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 E instein relation to show that this rapid vortex diffusion implies an u pper bound for the correlation length for vortex motion. Inclusion of transport current pulses in the spin-echo pulse sequence is found to h ave no effect on the magnitude of the spin-echo signal for T both grea ter and less than T-R=0.