Vortex melting in polycrystallineYBa2Cu3O7fromO17NMR

Line-shape analysis of {sup 17}O NMR spectra is used to probe vortex melting and dynamics in aligned powders of YBa{sub 2}O{sub 3}O{sub 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, T{sub m}. Below T{sub m} there is a coexistence regime of solid and liquid vortices with a lower bound, T{sub g}, which marks complete vortex freezing. {copyright} {ital 1997} {ital The American Physical Society}