JAHN-TELLER-EFFECT MEDIATED HOLE-PAIR TUNNELING AND SUPERCONDUCTIVITY IN HIGH Tc COPPER OXIDES

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Abstract
A detailed description is given of a new mechanism of high temperature superconductivity recently proposed by us. In this model a CuO 2 square planar layer is formed by the cooperative Jahn-Teller distortion. As a result the bonding character between Cu and O atoms in a layer becomes partly covalent because of short interatomic distance. In doped (La1−xMx)2CuO4 the extra d holes in | z 2> orbitals in Cu 3+ ions form a hole pair by the superexchange interaction through intervening oxygen p (and/or s) orbitals. Then the coherent tunneling of a hole–pair occurs by a non-adiabatic process and the hole—pairs make a Bose condensation at the zero wavevector state. This causes superconductivity. Based on this mechanism, important key temperatures to give a measure of T c, the Bose condensation temperature and the pair-breaking temperature, are discussed. It is pointed out that T c is determined by a pair-breaking temperature of a hole–pair and thus it does not depend on the electron-lattice interactions. It is also suggested that various means to increase covalency such as the bending of square planar layers and the change of Cu 3+ concentration will lead to the discovery of higher T c materials.
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