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%'e present a fast algorithm using the F-V' transformation for calculating the conductance of large two-dimensional random resistor lattices. The procedure can also be used to calculate other quantities for which Y-V' transformations can be defined. As an example of the method's usefulness, we present extensive data on square random resistor lattices as large as 1000 bonds on a side.Using this data, eve estimate that the conductivity exponent t =1.303+p' &&4 in two dimensions.

We report a comprehensive study of the in-plane transport properties of Nd2 "Ce Cu04 & epitaxial thin films and crystals by both increasing and decreasing 6 with Ce content fixed at x = 0.15. We find a remarkable correlation between the appearance of superconductivity and (1) a positive magnetoresistance in the normal state, (2) a positive contribution to the otherwise negative Hall coefficient, and (3) an anomalously large Nernst effect. These results strongly suggest that both holes and electrons participate in the charge transport for the superconducting phase of Nd2, Ce"Cu04~. PACS numbers: 74.76.Bz, 72.15.Eb, 72.15.Gd, 74.25.Fy In most high-T, cuprates, such as La2, Sr Cu04 and YBa2Cu307, the charge carriers are doped holes. On the other hand, in Nd2 "Ce,Cu04 s (NCCO), where superconductivity is induced by substituting Nd3+ with Ce4+, the Cu02 planes are believed to be doped with electrons [1]. The "electron-doped" character of NCCO gives a strong constraint on the possible mechanisms for hightemperature superconductivity in copper oxides [2,3]. Recently this system has attracted much more interest because of its possible simple BCS s-wave pairing in the superconducting state [4], in contrast to d-wave behavior proposed for hole doped high-T, cuprates [5]. However, questions still remain concerning the nature of the charge carriers in the superconducting phase of NCCO. For instance, both

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We report an observation of a sign change in the Hall resistivity p"~i n the superconducting state of the n-type superconductor Ndl 8&Ceo»Cu04~. This anomaly in other superconductors has widely been attributed to extrinsic effects, such as pinning or thermoelectric effects, or else to complicated band structures. However, the behavior of the Hall effect in the n-type cuprate Ndl 8,Ceo»Cu04 y and the systematics of the anomaly in other superconducting materials together provide strong evidence against such models. The data instead indicate that p"~r eveals an intrinsic property of vortex motion.

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