Anomalous Transport Properties in SuperconductingNd1.85Ce0.15Cu

Abstract

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

Influence of doping level on the Hall coefficient and on the thermoelectric power inNd2−xCexCuO4+δ

“…2,3,4,5,6,7 Similar feature has been observed with changing the oxygen content. 4,8 In another ntype cuprate P r 2−x Ce x CuO 4+δ (PCCO), the change of R H sign with temperature 9,10,11 and doping 12 were also observed. Those strange behaviors strongly indicate that at moderate doping, NCCO and PCCO have two bands with different types of charge carriers, one is hole carrier and the other is electron carrier.…”

“…1 However, R H and thermoelectric power gradually increase and eventually change sign to positive with further doping Ce. 2,3,4,5,6,7 Similar feature has been observed with changing the oxygen content. 4,8 In another ntype cuprate P r 2−x Ce x CuO 4+δ (PCCO), the change of R H sign with temperature 9,10,11 and doping 12 were also observed.…”

“…Especially for x=0.025, such low temperature insulating behavior becomes obvious around 240 K, similar to the report by Onose et al 1 However, the metallic behavior can be observed in the high temperature range. For the overdoped samples, metallic behavior exists in the whole temperature range and superconductivity was observed and slightly overdoped superconducting (x∼0.17) compositions, the change of Hall coefficient sign with temperature has been reported 4,6,8,10 . The conventional single-carrier transport can not explain such behavior adequately, and two types of the carriers have to be used to explain these behaviors.…”

“…It is believed that for the electron-doped cuprates two different types of bands take effect in the transport behavior of those samples as emphasized by many groups 3,4,6 . Here, let us look at it from a different point of view.…”

Magnetic-field dependence of the low-temperature specific heat of the electron-doped superconductorPr1.85Ce0.15CuO4

“…However, point contact tunneling 10 , penetration depth 11,12 and specific heat 13 revealed some s-wave features in the optimally-doped and overdoped samples, and led to the suggestion of a d-wave to s-wave symmetry transition upon doping or lowering the temperature below 4K. Transport and ARPES measurements have shown that the Fermi surface of the n-doped cuprates evolves from electron-like to hole-like upon doping, with a two-carrier behavior around the optimal doping 14,15 . Recently, it was proposed that Fermi surface topology may influence the pairing symmetry which lead to a symmetry mixing and/or a symmetry transition upon doping 9,16 .…”