Superconductivity at 94 K in HgBa₂Cu0_4+δ

Abstract

FOLLOWING the discovery¹ of high-transition-temperature (high-T_c) superconductivity in doped La₂CuO₄, several families of related compounds have been discovered which have layers of CuO₂ as the essential requirement for superconductivity: the highest transition temperatures so far have been found for thallium-bearing compounds². Recently the mercury-bearing compound HgBa₂Rcu₂O_6+δ (Hg-1212) was synthesized³ (where R is a rare-earth element), with a structure similar to the thallium-bearing superconductor TlBa₂CaCu₂O₇ (Tl-1212), which has one T1O layer and two CuO₂ 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 HgBa₂CuO_4+δ (Hg-1201), with only one CuO₂ 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 CuO₂ planes leads to better superconducting properties in a magnetic field⁵.