VORTEX LATTICE AND VORTEX DYNAMICS IN YBA2CU4O8 FROM Y-89 NMR

Y-89 NMR spectra in oxygen-deficient, pure and calcium-doped YBa2Cu4O8 powdered samples below T(c) are presented. The square root of the derived second moment of the field distribution inside the vortex lattice at low temperatures appears to be linearly related to T(c). The values derived for <DELTA-B2>, however, are remarkably different from the ones obtained from mu +SR. At higher temperatures a narrowing of the lines is observed and a crossover between two different temperature dependences of the vortex dynamics is evidenced.