Superconductivity at 94 K in HgBa2Cu04+δ

FOLLOWING the discovery 1 of high-transition-temperature (high- T c ) superconductivity in doped La 2 CuO 4 , several families of related compounds have been discovered which have layers of CuO 2 as the essential requirement for superconductivity: the highest transition temperatures so far have been found for thallium-bearing compounds 2 . Recently the mercury-bearing compound HgBa 2 Rcu 2 O 6+δ (Hg-1212) was synthesized 3 (where R is a rare-earth element), with a structure similar to the thallium-bearing superconductor TlBa 2 CaCu 2 O 7 (Tl-1212), which has one T1O layer and two CuO 2 layers per unit cell, and a T c of 85 K (ref. 2). But in spite of its resemblance to Tl-1212, Hg-1212 was found not to be superconducting. Here we report the synthesis of the related compound HgBa 2 CuO 4+δ (Hg-1201), with only one CuO 2 layer per unit cell, and show that it is superconducting below 94 K. Its structure is similar to that of Tl-1201 (which has a T c of < 10 K) 4 , but its transition temperature is considerably higher. The availability of a material with high T c but only a single metal oxide (HgO) layer may be important for technological applications, as it seems that a smaller spacing between CuO 2 planes leads to better superconducting properties in a magnetic field 5 .