Effect of the pseudogap on the mean-field magnetic penetration depth of YBCO thin films

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.