Possibility of high-temperature superconductivity in an ordered tetragonal crystal structure in RBa2Cu3O7 (R= rare earth) superconductors.

Abstract

The R${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ compounds are superconducting in the orthorhombic phase but are insulating in the tetragonal case. It is now well understood that while crystal symmetry per se does not play any role in the occurrence of superconductivity in these compounds, the orthorhombic phase is superconducting due to the formation of ordered chains that act as dopants for the two-dimensional ${\mathrm{CuO}}_{2}$ planes. In the tetragonal phase these chains are destroyed, thus inhibiting the charge transfer from the ${\mathrm{CuO}}_{2}$ planes. We propose a fully ordered tetragonal crystal structure for the R${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ (R= rare earth) superconductors with space group P4/mmm, which is different from the orthorhombic phase. This structure is formed from the orthorhombic crystal structure of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6.5}$ but now the chains run along the a axis as well. In this structure the Cu(1) atoms in the plane of the chains have twofold, square-planar fourfold, and sixfold coordinations, in contrast to the purely square-planar coordinations in orthorhombic ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$. Such a structure has previously been invoked by Fu et al. for the interpretation of their crystal-structure data on LaBa(Ca,Sr)${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ compounds. Results of our electronic-structure calculations show that the hole density and the Fermi level in this tetragonal crystal structure are practically identical to those found in the orthorhombic phase. This suggests that this phase, if it could be stabilized by appropriate chemical substitutions and heat treatment, would have a ${\mathit{T}}_{\mathit{c}}$ identical to that in the orthorhombic structure, and could be interesting for many technological applications.