DECREASE OF T(C) WITH LOW-TEMPERATURE OXYGEN ORDERING IN 90-K SUPERCONDUCTORS YBA2CU3O6- EVIDENCE OF THE OVER-HOLE-DOPED STATE(X )

The influence of the low-temperature oxygen rearrangement on the norma l-state and superconducting properties was studied on YBa2Cu3O6+x cera mics with various oxygen contents in the ''90 K plateau'' range. The s uperconducting transition temperature attained a maximum value at the optimum oxygen content x0 almost-equal-to 0.90, and gradually decrease d with further increase of x. The quenching or fast-cooling of the sam ples was followed by a relaxation process of the oxygen ordering. It w as found that aging at 250 K induces an increase of T(c) and a narrowi ng of the transition in samples with oxygen content below the optimum x0, and on the other hand, a decrease of T(c) and a broadening of the transition at x>x0. The variation of the superconducting properties wa s explained by the enhancement of the hole density in the CuO2-planes induced by the oxygen ordering. The excessive hole doping of the super conducting CuO2-planes is suggested to be responsible for the decrease of T(c) at high oxygen concentrations.