High-Field Vortex Dynamics inYBa2Cu3O7fromO17Nuclear Magnetic Resonance

The magnetic field–temperature phase diagram for vortex dynamics in $\mathrm{Y}\mathrm{Ba}{}_2\mathrm{Cu}{}_3{\mathrm{O}}_7$ has been extended up to 24 T based on ${}_{}{}^{17}{}_{}{}^{}O$ NMR experiments on aligned powders. Both spectrum analysis and spin-spin relaxation experiments indicate that vortices are pinned below a temperature $T_p$, which for low fields is consistent with $T_m$, the first order melting transition for clean, untwinned crystals. At high fields 10–24 T, $T_p$ is 80 K and field independent. Pinned vortices have a Lorentzian spectral density of field fluctuations with a sharp onset.