Vortex melting in polycrystallineYBa2Cu3O7fromO17NMR

Line-shape analysis of ${}_{}{}^{17}{}_{}{}^{}\mathrm{O}$ NMR spectra is used to probe vortex melting and dynamics in aligned powders of ${\mathrm{YBa}}_2$ ${\mathrm{O}}_3$ ${\mathrm{O}}_7$. Vortex transitions are identified by comparing their dynamics with the NMR time scale. Line- shape changes indicate a well-defined melting transition at a temperature, ${\mathrm{T}}_{\mathrm{m}}$. Below ${\mathrm{T}}_{\mathrm{m}}$ there is a coexistence regime of solid and liquid vortices with a lower bound, ${\mathrm{T}}_{\mathrm{g}}$, which marks complete vortex freezing.