Single crystal study of the “1201” superconductor Hg 0.8Bi 0.2Ba 2CuO 4+δ

Abstract

Single crystals of the “1201” mercury based cuprate have been grown for the first time. Their structure determination allows the classical Jahn-Teller distortion of the octahedral copper layer to be confirmed and gives evidence for the presence of oxygen at the level of the Hg/Bi layer leading to the formula Hg 0.8Bi 0.2Ba 2CuO 4.3. The [Hg 0.8Bi 0.2O δ] ∞ layers can then be described as built up from HgO 2 sticks that coexist with BiO 6 octahedra, bismuth being nearly pentavalent. The magnetic study of these crystals shows that they exhibit superconductivity up to 75 K ( T c(midpoint = 62 K)), with 100% superconducting volume fraction at low temperature. Moreover the M(B) loops exhibit fishtail shapes which were not present for measurements on ceramics. The fishtail and irreversibility lines of (Hg, Bi)-1201 are studied for the H| c geometry. These data are compared with those of Tl-single crystals; it appears that (Hg, Bi)-1201 is quite similar to Tl monolayers and differs greatly from Tl bilayers. This underlines the influence of structural anisotropy in both Hg- and Tl-cuprates on their superconducting properties.