GIANT PRESSURE EFFECT IN OXYGEN-DEFICIENT YBA2CU3OX

The pressure effect on T-c of polycrystalline and single crystalline Y Ba2Cu3Ox has been investigated as a function of oxygen content x by ac -susceptibility measurements under helium pressure. In the overdoped r egion x > 6.93 the single crystals show a negative dT(c)/dp, as expect ed from the charge transfer model. For optimally doped samples with x = 6.93 we find dT(c)/dp = 0.4 K/GPa which points to pressure effects o n T-c aside from charge transfer. In the underdoped region x < 6.93 th e dT(c)/dp values obtained from the experiment depend strongly on the storage temperature of the sample during the experiment. When the samp les are stored at temperatures well below 240 K throughout the entire experiment including pressure application and pressure release, dT(c)/ dp increases to approx. 7 K/GPa at x = 6.7 but with a further decrease of the oxygen content the dT(c)/dp drops to approx. 2 K/GPa at x = 6. 4. These effects are intrinsic to the YBa2Cu3Ox structure and can be e xplained by considering the anisotropic structure of YBa2Cu3Ox. The de crease of the c-axis lattice parameter results in a charge transfer to the CuO2-planes mainly [1], whereas the compression of the a- and b-a xis lattice parameter is known to produce different pressure effects w hich are responsible for the peak in dT(c)/dp at x = 6.7 [2]. When pre ssure is changed at room temperature oxygen ordering effects occur whi ch cause a relaxation of T-c to the equilibrium value T-c(p) at this p ressure with a time constant depending on the oxygen content x. A decr ease of x results in a peak effect in dT(c)/dp at x = 6.7 again, which is enhanced to approx. 12 K/GPa. If the oxygen content is decreased f urther, dT(c)/dp first drops to 5 K/GPa at x = 6.6, but then increases to values of more than 20 K/GPa for x < 6.42. These giant pressure ef fects at low oxygen contents are mainly caused by a reversible T-c inc rease (dT(c)/dp)(0) due to pressure induced oxygen ordering via oxygen motion between unit cells.