CHARGE-TRANSFER AND SUPERCONDUCTIVITY IN ND(BA1-XNDX)2CU3O7+X PREPARED IN HIGH-PRESSURE OXYGEN

By annealing in oxygen at high pressure around 600 approximately 1000 atm, we have been able to make a series of the single-phase solid solution system Nd(Ba1-xNdx)2Cu3O7+x i.e. delta = x in Nd(Ba1-xNdx)2Cu3O7+delta, x-dependences of both hole concentration and superconductivity have been investigated in this system. In this series of samples, the hole concentration deduced from the Hall effect measurement changes monotonically with x while the average charge of a [Cu-O] unit, i.e. the total hole concentration, deduced from a chemical analysis is not changed for all samples. This clearly demonstrates that the mobile hole concentration in this system is determined by a charge transfer between CuO2 sheet-layer and CuO1+x chain-layer. This charge transfer is understood by consideration of the electrostatic energy based on a simple model. x-dependence of the superconductivity transition temperature T(c) is interpreted as the dependence of T(c) on the mobile hole concentration of the CuO2 plane, which is expressed by an almost linear relation up to the highest T(c) of 95 K.