Temperature dependence of the magnetic penetration depth in YBa2Cu3O6.95

We report precision transverse field μSR measurements of the internal field distribution in the vortex state of crystalline YBa2Cu3O6.95. A novel low background apparatus was used to study a mosaic sample of three high quality single crystals (Tc=93.3K). The observed frequency spectra in magnetic fields of 5kG and 15 kG applied along the c-axis have the characteristic features expected for a regular vortex lattice with some additional broadening. From a preliminary analysis we find that [λ(0)/λ(T)]2 has a linear temperature coefficient for T < 30 K. Such a term is inconsistent with simple s-wave pairing in the superconducting state. These results support recent microwave measurements of λ(T) on similar crystals in zero applied field but differ significantly from previous μSR reports on sintered powders and crystals with lower Tc. (orig.)