• Y Nishihara

      Articles written in Pramana – Journal of Physics

    • Magnetic phase diagram of anisotropic layered superconductors via magnetization measurements forHc in Bi2212

      A K Grover F Iga Y Yamaguchi N Aoki K Ino N Goyal Y Nishihara

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      The Bi2Sr2CaCu2O8 system is viewed as an archetypal of superconductors modelled as Josephson coupled CuO2 bilayers. The isothermal and temperature dependent DC and AC magnetization measurements forHc in a single crystal of Bi2212 have been performed. Qualitative changes are observed to occur over a narrow range of temperature values before reaching the superconducting-normal transition. The observed behaviour can be ascribed to the rapid variation in the strength of the coupling between the superconducting CuO2 planes (i.e., bilayers in the case of Bi2212). Strongly coupled planes behave like a 3D superconductor, whereas weakly coupled planes have a two component response attributable to 2D planes and interplanar couplings. We believe that this paper is a plethora of new findings. Our observations imply that resistivity across the planes becomes zero earlier than that within the planes. A new line (designated asH2D(T)) above which the change in the electromagnetic response is dominated by quasi 2D-planes has been determined for the first time. This paper also contains the first observation of Differential Diamagnetic Effect (DDE) in the In-phase AC susceptibility data which signals the onset (atT2D(H)) of dominance of response from 2D-planes. In addition to a host of interesting thermomagnetic history effects which are a consequence of interplay between the diamagnetic responses from the two components, a comparison of irreversibility lines (of the 3D state) determined by different methods on the same specimen of a HTSC is also being presented for the first time. We have come across Paramagnetic Meissner Effect (PME), first recognized in ceramic samples of Bi2212, in the temperature region of dimensional crossover in our single crystal sample, whichinter-alia confirms our labelling of the two component behaviour. A schematic phase diagram summarizing the various transformations that can occur nearTc in the electromagnetic response of an anisotropic layered system has been drawn.

    • Local-moment formation and metal-nonmetal transition in Ca1−xYxVO3 and Ca1−xYxTiO3

      Y Nishihara H Kawanaka H Bando

      More Details Abstract Fulltext PDF

      Electron-doped metallic states of Ca1−xYxVO3 and Ca1−xYxTiO3 change into non-metallic states around x∼0.4 and 0.6, respectively. The residual resistivity in the metallic states increases with increasing effective magnetic moment or coefficient of T2 term of resistivity. The effective moment reaches ∼ 0.5 µB/molecule in Ca1−xYxVO3 and also in Ca1−xYxTiO3 near the metal-nonmetal phase boundary. In these metallic states. ∼ 10% of 3d atoms seem to have large localized magnetic moments. In electron-doped metallic sample of Ca1−xYxVO3, the temperature dependence of resistance shows no resistance-minimum. However, weak negative magneto-resistance is observed for the sample with x=0.2 up to 50 Tesla at 4.2 K.

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