Abstract

Hg-based high-temperature superconductors (HgBCCO) encompass a series of materials described by HgBa₂Ca_n-1Cu_nO_2n+2+δ, where n = 1, 2, 3,…, and many of the chemical elements in the formula could be partially cross-substituted with others. The first report of superconductivity was in HgBa₂CuO_4+δ (Hg-1201) with a superconducting transition temperature (T _c) of 94 K [1]. Subsequent reports on HgBa₂CaCu₂O_6+δ (Hg-l212) [2] and HgBa₂Ca₂Cu₃O_8+δ (Hg-1223) [3] revealed higher T _c values of 125 K and 135 K, respectively. These discoveries placed the HgBCCO at a unique position among the high-temperature superconductors (HTS), since most members of the HgBCCO family have T _c's above 100 K. In particular, the T _c of 138 K discovered in (Hg_0.8,Tl_0.2)-1223 remains as the highest value so far achieved in superconductors without applying pressure [4], making HgBCCO one of the most interesting systems in the investigation of the fundamental physics underlying the unresolved mechanism of high-temperature superconductivity. For practical applications, a higher T _c implies higher device operating temperatures and lower cost. A particular niche for HgBCCO-related applications is at temperatures exceeding 77 K as illustrated in various prototype devices including superconducting quantum interference devices, photodetectors, microwave passive devices, wires and tapes, fault current limiters, etc.