The effect of chemical doping and hydrostatic pressure on T-c of Y1-yCayBa2Cu3Ox single crystals

We performed susceptibility measurements on Y1-yCayBa2Cu3Ox single crystals under high He pressure. For each Ca content various samples with different oxygen contents have been prepared to probe the influence of Ca on T-c(x), dT(c)/dp(x) and T-c,T-max. Starting from the parabolic T-c(n(h)) behavior we calculated n(h) values from T-c and T-c,T-max for each sample, It is sho wn that in the overdoped region dT(c)/dp can be described by a pressure ind uced charge transfer with dn(h)/d p approximate to 3.7 10(-3) GPa(-1) and a dT(c,max)/dp value of 0.8 K/GPa, irrespective of the Ca content. In the un derdoped region additional pressure effects lead to a peak in dT(c)/dp at a pproximate to 0.11 holes/CuO2 plane. However, with increasing Ca content th is peak is strongly depressed. This is explained in terms of an increasing disorder in the CuO chain system due to doping. Deviations in dT(c)/dp at v ery low n(h) values can be assigned to the ortho II ordering in the CuO cha in system. (C) 1999 Elsevier Science B.V. All rights reserved.