Kunio Ishida

Possible mechanism of superconductivity in electron superconducting cuprates

Abstract Based on the recent result of cluster calculations for electron doped superconductor that electrons are accommodated in the 4s level of Cu atoms, we propose a unified treatment of electron- and hole-doped superconductivities, i.e. the spin-polaron pairing mechanism developed by Kamimura et al. . For this purpose we perform a computer simulation of a 2D quantum spin system with two impurity spins and show that the long-ranged attractive interaction exists between two spin-polarons. Thus a spin-polaron system forms Cooper pairs and condensates to a usual BCS type wave function. Our mechanism can explain the observed doping concentration and pressure dependences of Tc.

Computational Study on the Ground State of a Dopant Hole in a Two-Dimensional Quantum Spin System

In order to examine the Kamimura–Suwa model for the electronic structure of cuprates, computational study is made for a 4 ×4-site 2D square system. In this system the ground state of a dopant hole which is interacting with a two-dimensional quantum spin system is investigated numerically by the exact diagonalization method. Present numerical calculations show that antiferromagnetic ordering of the underlying host spins continues to exist even in the presence of a dopant hole. Allowing the dopant hole to adopt two kinds of orbital states through alternate Cu sites, the hole becomes itinerant, following the Kamimura–Suwa model. It is also shown in the case that the dopant hole occupies only a single orbital state b 1g , it tends to form a localized spin-polaron.

Theoretical Study of Transient X-ray Speckles in Photoinduced Nucleation

Transient x-ray speckle patterns at 0 K in the early stage of photoinduced nucleation processes are theoretically studied. By employing a model of localized electrons coupled with quantized phonons, we found that various types of speckles appear in diffraction patterns, and that the nature of the incubation period and/or coalescence of photoinduced domains is discussed by their time-dependent profiles or their two-time correlation function g (2) . Thus, measurement of transient x-ray speckles by ultrashort coherent x-ray pulses will be a direct way to reveal the mechanism of photoinduced structural change.

Anomalous below-gap absorption by bulk highz.sbnd;Tc superconductors

Abstract Optical absorption spectra for various high z . sbnd ; T c superconductors have been measured by using an optically pumped far-infrared laser at the temperature region between 12 and 160 K, and the nature of electronic states is discussed.

Electronic states and origin of superconductivity in La2-xSrxCuO4

The ground state of CuO6 in La-compounds is 3B1g in the superconducting phase, supporting Kamimuras two-storey house model. Based on this model the Sr concentration dependence of Tc and the normal state Hall effect and the temperature dependence of the normal state resistivity are calculated, and all the results are in good agreement with experiments.