SUPERCONDUCTIVITY AT HIGHER TEMPERATURES IN THE HG-BA-CA-CU-O COMPOUND SYSTEM

Our recent systematic examination of the pressure effect on high-temperature superconductors revealed that the highest achievable superconducting transition temperature (T(c)) in the layered cuprates may lie between approximately 150 and approximately 180 K. We propose that the newly discovered Hg-Ba-Ca-Cu-O (HBCCO) compound system may be one of the most promising candidates for such a high-T(c) superconductor, because of the possible large range of modulation doping associated with the linear oxygen coordination of divalent Hg in HBCCO. A record-high magnetically determined transition at 135 K with a zero resistivity at 134 K has been obtained by us in HgBa2Ca2Cu3O8+delta. The T(c) of HgBa2Ca2Cu3O8+delta was found to increase continuously with pressure at an increasing rate up to approximately 17 kbar without any sign of saturation. The thermopower shows an underdoped characteristic. These observations suggest that the T(c) of HBCCO can be further enhanced with proper modulation doping without inducing any structural instabilities. The results together with the synthesis steps of HBCCO are summarized and discussed.