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We report on a comparative study of the transport properties of lightly La-doped layered CaO (CaMnO 3 ) m (mϭ1, 2, and ϱ͒ and similar Mn 3ϩ /Mn 4ϩ ratios. In contrast to the large and rather nonsystematic variations of the resistivity (T) for samples with the same doping and various m, the absolute thermopower ͑S͒ exhibits remarkable systematics. In the paramagnetic ͑PM͒ state, S varies linearly with temperature and its extrapolation to Tϭ0 has a finite intercept. The intercept and the slope of these lines vary systematically with m. In particular, within the experimental error, the inverse slopes of the straight lines increase linearly with 1/m, which in its turn varies linearly with the average number of Mn-Mn near neighbors. The implications of this simple systematics are discussed using a model developed by Cutler and Mott ͓Phys. Rev. 181, 1336 ͑1968͔͒ for transport by activated hopping of carriers obeying nondegenerate statistics. This model ͑or a possible extension into the semidegenerate regime͒ leads also to a simple, qualitative interpretation of S(T) of these materials on crossing the Néel temperature (T N ) from the PM state and below T N .

Using the transverse field muon spin relaxation technique we measure the temperature dependence of the magnetic field penetration depth λ, in the NaxCoO2 · yH2O system. We find that λ, which is determined by superfluid density ns and the effective mass m * , is very small and on the edge of the TF-µSR sensitivity. Nevertheless, the results indicate that the order parameter in this system has nodes and that it obeys the Uemura relation. By comparing λ with the normal state electron density we conclude that m * of the superconductivity carrier is 70 times larger than the mass of bare electrons. [4] found no coherence peak, questioning the previous result. Therefore, an additional and different experimental approach is required. A possible approach is to measure the temperature dependence of the magnetic field penetration depth λ. At low temperatures, λ is sensitive to low-lying excitations, and, in the case of a complete gap λ(T ) − λ(0) should vary exponentially as a function of T . On the other hand, nodes in the gap lead to a power-law dependence of this penetration depth difference. A study of λ can help addressing the other two questions as well. For the third question, one of the most universal correlations among the unconventional superconductors is the relation between the transition temperature T c and the width of the transverse field muon spin rotation (TF-µSR) line at low temperatures, σ(0) ∝ λ −2 . Uemura et. al.[5] were able to show that the same relation holds for the underdoped cuprates, the bismuthates, Chevrel-phase and the organic superconductors. This relation has no explanation in the frame-work of the BCS theory, and it is usually explained in terms of phase coherence establishment in a theory of local fluctuations of the order parameter [6]. It is interesting to know if Na x CoO 2 · yH 2 O also obeys this relation. The second question would be addressed by the absolute value of σ(0).The aim of this work is to measure the temperature dependence of λ with TF-µSR in Na x CoO 2 · yH 2 O. TF-µSR is a very useful way to study superconductors in the mixed state. In this method 100% spin polarized muons are implanted in the sample, which is cooled in a field perpendicular to initial muon spin. Above T c , where the external field penetrates the sample uniformly, the second moment of the field distribution at the muon stopping site ∆B 2 is relatively small and determined only by fields produced by nuclear moments. Consequently the muon spins rotate in a coherent way and weak depolarization of the muon ensemble is observed. When the sample is cooled below T c a flux lattice (FLL) is formed in the sample resulting in an inhomogeneous field and a therefore a larger second moment at the muon site. This increase in ∆B 2 leads, in turn, to a high muon spin depolarization rate in the sample. The penetration depth is related to the field distribution width bywhere λ ⊥ is the in-plane penetration depth, Φ 0 is the flux quanta, and F ∼ 0.44 for anisotropic compounds [7].Polycrystalline samples of Na 0.7 CoO 2 ...

BaCoS 2 is a Mott insulator with a Neél temperature (T N ) close to room temperature. In the ordered antiferromagnetic phase, the magnetic moments are confined in the Co 2 S 2 planes, and arranged in alternatingly directed ferromagnetic chains. We report on resistivity and thermopower measurements and on roomtemperature cell constants of samples of BaCoS z with nominal sulfur content 1.9рzр2.1. We discuss the effect of sulfur content on cell constants and band filling. The main observation is a crossover from activated electronic transport at high temperatures, to high resistivity almost independent of temperature, and accompanied by a vanishingly small thermopower, at low temperatures ͑far below T N ͒. We interpret the transport mechanism in the low-temperature regime as tunneling in the Co 2 S 2 planes, to nearest neighbors along the ferromagnetic chains and to next-nearest neighbors in the perpendicular direction, or alternatively to nearest neighbors along the ferromagnetic chains with conducting inclusions acting as shorts.

The conditions (temperature, pressure and time) for attaining equilibrium with oxygen for different YBCO materials are reported. The equilibrium oxygen content was used in order to determine more exact values for the enthalpies and entropies of interaction of YBa2Cu3Oy with oxygen. Internal equilibrium in YBCO is proposed and is able to explain the effects of ageing and low temperature annealing on Tc. Homogeneity and transitions to superconductivity of ceramics having equilibrium and non-equilibrium oxygen contents are discussed.