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

The magnetoresistance of epitaxial Nd0.7Sr0.3MnOδ thin films has been studied. A giant magnetoresistance, with more than 4 orders of magnitude change in resistance (−ΔR/RH≳106%), was obtained at ∼60 K and a magnetic field of 8 T. This giant magnetoresistance (GMR) ratio is about one order of magnitude larger than the highest value reported previously which was observed in La–Ca–Mn–O film. We have also obtained a large GMR ratio with −ΔR/RH≳3000% for H=5 T in an in situ Nd0.7Sr0.3MnOδ thin film, a much larger effect than the previous results in doped manganese oxide films in which a large GMR ratio was obtained only in postannealed samples. Our results also show that the GMR effect in these films can be strongly influenced by the thin-film preparation conditions.

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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We consistently observe a magnetic-field-dependent conductance peak at zero-bias voltage in a wide range of superconductor/noble-metal junctions fabricated from oxide superconductors ͑YBa 2 Cu 3 O 7Ϫ␦ and Tl 2 Ba 2 CaCu 2 O͒ that have been reported to exhibit d-wave pairing behavior; however, no measurable peak appears in similar junctions made from an s-wave oxide superconductor (Nd 1.85 Ce 0.15 CuO 4 ). Explanations of this correlation are considered in terms of the Appelbaum-Anderson model for magnetic interface scattering and the midgap-state model for d-wave interface states.