MAGNETIC-FIELD DEPENDENCE OF THE LONDON PENETRATION DEPTH IN THE VORTEX STATE OF YBA2CU3O6.95

The magnetic penetration depth in the vortex state of YBa2Cu3O6.95 has been measured as a function of temperature T and magnetic field H app lied along the (c) over cap axis. The internal field distribution in s ingle crystals was fit assuming a triangular vortex lattice with an av erage in-plane penetration depth lambda(ab) = (lambda(a) lambda(b))(1/ 2) = 1155(3) Angstrom, extrapolated to T = O and H = O; lambda(H,T) in creases with both T and H. The large T-independent increase in lambda( ab) with H is attributed to a strong nonlinear response as a result of the unconventional pairing in the superconducting state.