  We report measurements of the magnetic penetration depth in YBCO films at
various oxygen concentrations. At optimal doping, critical fluctuation effects
are absent, and the penetration depth from 4 K to 0.99 Tc is well described by
d-wave, BCS, strong-coupling theory with a gap, Delta0/ kT ~ 3.3. This implies
that the T-dependence of the penetration depth comes largely from
single-particle excitations. As in crystals, underdoping reduces the zero
temperature superfluid density without affecting the low-T slope or curvature
of the penetration depth. We show that these results, as well as heat capacity
measurements, are well described by an ad hoc model in which superfluid is lost
from regions of the Fermi surface occupied by the pseudogap while the low lying
excitations near the nodes remain unaffected.

Superconductivity