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 have studied the doping dependence of the in-plane and out-of-plane superfluid density, ρ s (0), of two monolayer high-Tc superconductors, HgBa2CuO 4+δ and La2−xSrxCuO4, using the low frequency ac-susceptibility and the muon spin relaxation techniques. For both superconductors, ρ s (0) increases rapidly with doping in the under-and optimally doped regime and becomes nearly doping independent above a critical doping, pc ∼ 0.20.

Recent research suggests that the charge carriers in the paramagnetic state of the magnetoresistive manganites are small polarons. Here we report studies of the oxygen-isotope effects on the intrinsic resistivity and thermoelectric power in several ferromagnetic manganites. The precise measurements of these isotope effects allow us to make a quantitative data analysis. Our results do not support a simple small-polaron model, but rather provide compelling evidence for the presence of small immobile bipolarons, i.e., pairs of small polarons. Since the bipolarons in the manganites are immobile, the present result alone appears not to give a positive support to the bipolaronic superconductivity theory for the copper-based perovskites.

To gain insight into the out-of-plane electrodynamics of high-T c superconductors we have measured the absolute values and temperature dependence of the c-axis magnetic penetration depth c (T) for two typical single-layer high-T c cuprates, La 2Ϫx Sr x CuO 4 and HgBa 2 CuO 4ϩx , as a function of doping. A distinct change in the behavior of c is observed near 0.20 holes/Cu, which is related to the opening of the normal-state pseudogap. The variation of c with doping is qualitatively similar to that of the in-plane component, ab , which suggests that the c-axis superfluid response is mainly governed by the superconducting condensation energy. The strong doping dependence of c (0) for pϽ0.20 provides an explanation for the discrepancies in the literature. RAPID COMMUNICATIONSPRB 61 R3809 c-AXIS SUPERFLUID RESPONSE AND PSEUDOGAP IN . . . RAPID COMMUNICATIONS R3810PRB 61 PANAGOPOULOS, COOPER, XIANG, WANG, AND CHU

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