GINZBURG-LANDAU PARAMETER IN YBA2CU3O6.95 BELOW THE IRREVERSIBILITY TEMPERATURE AS MEASURED BY MU(-FIELDS()SR IN HIGH MAGNETIC)

The vortex state of a type-II superconductor produces a distinctive mu (+)SR line shape with features determined by the average internal held B-0, the magnetic penetration depth lambda, the superconducting coher ence length xi, and the degree of disorder in the vortex lattice. Only in the high field regime (lambda much greater than L>xi, where L is t he intervortex spacing) do the vortex cores (of radius approximate to xi) occupy a large enough area that they are observable in the line sh ape as a high field cutoff. Our mu(+)SR measurements of the held distr ibutions in a mosaic of single crystals of YBa2Cu3O6.95 in fields of 1 .9, 4.1, 4.7, and 6.5 T(B-0 parallel to ($) over cap c) show all the f eatures of the line shape. We find lambda = 0.15+/-0.01 mu m at 10 K a nd the Ginzburg-Landau parameter kappa=lambda/xi=69.6+/-1.4 constant b etween 30 and 75 K; this is the only measurement to date of kappa in Y Ba2Cu3O6.95 below the irreversibility temperature. Due to disorder in the vortex lattice, either-from pinning or from vortex fluctuations th at are quasistatic on the time scale of mu(+)SR, the observed line sha pe is ''smeared'' relative to that predicted for a perfect lattice. Fr om the degree of smearing, we estimate an upper limit of 5.5% for the rms deviation of individual vortices from their ideal positions.