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 ∼150 and ∼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 HgBa₂Ca₂Cu₃O_8+δ. The T_c of HgBa₂Ca₂Cu₃O_8+δ was found to increase continuously with pressure at an increasing rate up to ∼17 kbar without any sign of saturation. The thermo-power 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.