DEPENDENCE OF THE SUPERCONDUCTING TRANSITION-TEMPERATURE OF HGBA2CUO4+DELTA ON HYDROSTATIC-PRESSURE

The real chi' and imaginary chi'' components of the AC susceptibility for a ceramic sample of superconducting HgBa2CuO4+delta have been measured at H(AC) = 0.1 Oe over the temperature range 80-300 K under purely hydrostatic pressure conditions. Upon cooling, a slight but sharp decrease in slope dchi'/dT occurs at the ''upper'' transition temperature, T(c)u congruent-to 98 K, which we attribute to the onset of superconductivity in a small volume (approximately 3%) of the sample which is nearly optimally doped. Below the ''lower'' transition temperature, T(c)l congruent-to 95 K, chi'(T) drops steeply, approaching for T< 90 K a value which corresponds to approximately 80% of ideal shielding. The imaginary or loss component chi'' (T) shows an onset at 95.5 K followed by a maximum at 93.3 K. Under an applied He-gas pressure to 0.9 GPa, both T(c)u and T(c)l increase reversibly at the moderate rate dT(c)/dP congruent-to + 1.75 +/- 0.1 K/GPa which is independent of whether the pressure is changed at room temperature or 80 K. These experiments allow us to speculate that the superconducting transition for optimally doped HgBa2CuO4+delta at ambient pressure probably lies near 98 K.