Synthesis and superconductivity of highly underdoped HgBa2CuO4+δ

Abstract The highest transition temperature superconductors are found within the complex homologous series HgBa 2 Ca n −1 Cu n O 2 n +2+ δ ( n =1–7), with the third member, HgBa 2 Ca 2 Cu 3 O 8+ δ possessing the record-high transition temperature ( T c ) of 135 K at room pressure. The first member of this family, HgBa 2 CuO 4+ δ having a T c of up to 97 K, displays the highest transition temperature for any analogous compounds with a single copper-layer. The chemical reaction for the formation of this material is intrinsically complex due to the natural high volatility of mercury-bearing compounds; chemical synthesis has been postulated to proceed via a solid–vapour reaction. With this in mind, we have developed a mixed solid/vapour phase synthesis for HgBa 2 CuO 4+ δ using what one might term a `remote' source of mercury, in this case elemental Hg itself. Interestingly, because of the zero oxidation state of elemental mercury in the reagent mixture, the synthesis reaction proceeds under reducing conditions. By this route, a highly underdoped state ( T c ≤35 K) of the superconducting phase HgBa 2 CuO 4+ δ is readily obtained. This level of underdoping is extremely difficult to achieve by more conventional synthetic routes. We comment on the unusually high oxygen affinity of the resulting underdoped compound, in relation to other cuprate superconductors, and the implied mobility of oxygen defects within the crystal structure.