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

Local magnetic measurements in a highly anisotropic Nd-Ce-Cu-O crystal reveal a sharp onset of an anomalous magnetization peak at a temperature-dependent field B on . The same field marks a change in the field profiles across the sample, from profiles dominated by geometrical barriers below B on to Bean-like profiles above it. The temperature dependence of B on and the flux distribution above and below B on imply a disorder-induced transition at B on from a relatively ordered vortex lattice to a highly disordered, entangled vortex solid. Local magnetic relaxation measurements above B on show evidence for plastic vortex creep associated with the motion of dislocations in the entangled vortex structure.[S0031-9007 (97)04113-6]

We report inelastic x-ray scattering studies of charge excitations in insulating Nd 2 CuO 4 as a function of incident photon energy. An excitation of ϳ6 eV is observed when the incident photon energy is tuned through the Cu K edge. This is interpreted as resonantly enhanced inelastic x-ray scattering. Numerical calculations identify the 6 eV feature as a charge-transfer excitation to the antibonding state and suggest that nonlocal effects play a role in determining the shape of the resonance. [S0031-9007(98)06181-X] PACS numbers: 74.25.Jb, 71.27. + a, Understanding the normal state electronic properties of the high-T C copper oxides is an important prerequisite for a theory of high temperature superconductivity. In addition, the properties themselves are highly unusual, exhibiting non-Fermi-like behavior [1] and they have therefore attracted much interest from the more general perspective of understanding electronic behavior in strongly correlated transition metal oxides.A useful theoretical approach has been to treat the copper oxide planes within the framework of a microscopic electronic Hamiltonian, such as the extended Hubbard model [2]. The strong electron correlations preclude the possibility of successful band structure calculations, and a variety of numerical techniques have therefore been applied, utilizing small clusters of ions for which on-site interactions can be treated explicitly [3], for example, impurity and cluster interaction models [4,5]. These models are typically local, that is, the translational symmetry of the lattice is neglected. However, as first emphasized by Veenendaal, Eskes, and Sawatzky [6], solid state (nonlocal) effects can be important, and recently calculations have been performed for a number of clusters connected in a planar geometry for which some degree of translational symmetry is restored [7].A crucial test of such treatments is to compare their predictions with measurements of the electronic excitation spectrum. To this end a variety of spectroscopies have been applied to the cuprates. However, each has its limitations. For example, photoemission experiments only probe to the photoelectron escape depth and it can be difficult to ensure that bulklike properties are measured. Also finalstate effects (i.e., those due to the presence of a core hole in the final state) are large, and electrostatic charging may be a problem for insulators. X-ray absorption spectroscopy (XAS) offers bulklike penetration, but still suffers from a final-state core hole, and optical spectroscopies are essentially limited to q 0 properties and energy transfers of a few eV. There remains a clear need for new spectroscopies to elucidate the electronic excitation spectrum of the high-T C cuprates.Inelastic x-ray scattering in the hard x-ray regime exhibits none of the above disadvantages. The scattering process is charge neutral so that final-state effects are eliminated and the excitation spectrum is measured directly. Further, bulklike properties are measured. It would thus seem an ideal tool in thi...

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.

No abstract

No abstract

No abstract