ANISOTROPIC UNIAXIAL PRESSURE EFFECTS IN YBA2CU3O7-DELTA

We present measurements of the uniaxial pressure dependence of T(c) of untwinned YBa2Cu3O7-delta crystals with various oxygen stoichiometrie s. For all samples investigated, T(c) decreases for pressure along a, increases for pressure along b, and, in oxygen deficient samples, incr eases strongly for pressure along c. These results are compared to the behavior found in the La2-xSrCuO4 and YBa2Cu4O8 systems. Neither the model of pressure-induced charge transfer nor coupling to orthorhombic distortions can explain all the data. However, the presence of singul arities in the electronic density of states close to the Fermi energy is a possible origin of the observed behavior. Our preliminary data on the pressure dependence of the c-axis and in-plane resistivities in t winned crystals are consistent with this view.