Field Induced Reduction of the Low-Temperature Superfluid Density inYBa2Cu3O6.95

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 ${\lambda }_{\mathrm{ab}}$ in ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.95}$. At low $H$ and low $T$, ${\lambda }_{\mathrm{ab}}$ exhibits the characteristic linear $T$ dependence associated with the energy gap of a $d_{x^2{-y}^2}$-wave superconductor. However, at higher fields ${\lambda }_{\mathrm{ab}}$ is essentially temperature independent at low $T$. We discuss possible interpretations of this surprising new feature in the low-energy excitation spectrum.