  A novel high magnetic field (8 T) spectrometer for muon spin rotation has
been used to measure the temperature dependence of the in-plane magnetic
penetration depth in YBa2Cu3O6.95. At low H and low T, the penetration depth
exhibits the characteristic linear T dependence associated with the energy gap
of a d_x^2-y^2-wave superconductor. However, at higher fields the penetration
depth is essentially temperature independent at low T. We discuss possible
interpretations of this surprising new feature in the low-energy excitation
spectrum.
