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

The real chi' and imaginary chi'' components of the AC susceptibility for a ceramic sample of superconducting HgBa2CuO4+delta have been meas ured at H(AC) = 0.1 Oe over the temperature range 80-300 K under purel y hydrostatic pressure conditions. Upon cooling, a slight but sharp de crease in slope dchi'/dT occurs at the ''upper'' transition temperatur e, T(c)u congruent-to 98 K, which we attribute to the onset of superco nductivity 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. Th e imaginary or loss component chi'' (T) shows an onset at 95.5 K follo wed by a maximum at 93.3 K. Under an applied He-gas pressure to 0.9 GP a, 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 optimal ly doped HgBa2CuO4+delta at ambient pressure probably lies near 98 K.