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The doping dependence of the superfluid density, ρ s ≡ λ ab -2 ∝ n s /m*, of high-T c superconductors is usually considered in the context of the Uemura relation, namely T c proportional to ρ s , which is generally assumed to apply in the underdoped regime. We show that a modified plot of T c /∆ 0 versus ρ s , where ∆ 0 is the maximum d-wave gap at T=0, exhibits universal features that point to an alternative interpretation of the underlying physics. In the underdoped region this plot exhibits the canonical negative curvature expected when a ground-state correlation competes with superconductivity (SC) by opening up a gap in the normal-state DOS. In particular ρ s is suppressed much faster than T c /∆ 0 or indeed T c . The pseudogap is found to strongly modify the SC ground state. PACS numbers 74.25.Ha, 74.25.Bt, 76.75.+i

The absolute values and temperature, T, dependence of the in-plane magnetic penetration depth, λ ab , of La2−xSrxCuO4 and HgBa2CuO 4+δ have been measured as a function of carrier concentration. We find that the superfluid density, ρs, changes substantially and systematically with doping. The values of ρs(0) are closely linked to the available low energy spectral weight as determined by the electronic entropy just above Tc and the initial slope of ρs(T )/ρs(0) increases rapidly with carrier concentration. The results are discussed in the context of a possible relationship between ρs and the normal-state (or pseudo) energy gap.

We review some of the systematic patterns in the normal state properties of high temperature oxide superconductors which may help in the search for the pairing mechanism. A recent analysis of thermodynamic properties, namely the conduction electron entropy determined from high resolution specific heat data and the static magnetic susceptibility, both measured for a wide range of closely spaced compositions, yields much information about the effects of hole doping and zinc substitution on the low energy electronic excitations in these compounds. We attempt to correlate this information with the systematic changes observed in the electrical resistivity, Hall coefficient and most notably the thermoelectric power, for which a new scaling property is reported