HIGH-FIELD VORTEX DYNAMICS IN YB2CU3O7 FROM O-17 NUCLEAR-MAGNETIC-RESONANCE

The magnetic field-temperature phase diagram for vortex dynamics in YB a2Cu3O7 has been extended up to 24 T based on O-17 NMR experiments on aligned powders. Both spectrum analysis and spin-spin relaxation exper iments indicate that vortices are pinned below a temperature T-p. whic h for low fields is consistent with T-m, the first order melting trans ition for clean, untwinned crystals. At high fields 10-24 T, T-p is 80 K and field independent. Pinned vortices have a Lorentzian spectral d ensity of field fluctuations with a sharp onset. [S0031-9007(98)05393- 9].