Cation size control of structure, structural fluctuations, and superconductivity in L1.85M0.15CuO4

Abstract

The structural effects of changing the cation size variance σ2 in L1.85M0.15CuO4 superconductors (A site cations: L3+=La, Nd; M2+=Ca, Sr, Ba) have been determined through neutron powder-diffraction studies of two series of samples, each with a fixed average A site radius 〈rA〉. The variance couples strongly to the orthorhombic strain leading to a strong linear increase of the I4/mmm to Abma symmetry and Abma to Pccn symmetry structural phase-transition temperatures with σ2. The latter transition is shown to occur at a critical orthorhombic strain. Both the mean value and the root-mean-square fluctuations of the Cu-O-Cu angle increase linearly with σ2 and the suppression of the superconducting Tc by the fluctuations is greater than the suppression by the mean angle by a factor of ∼2. The results have been used to construct phase diagrams, projected on the chemical window, for the L1.85M0.15CuO4 system that show a large region in which superconductivity is suppressed by formation of the low-temperature tetragonal P42/ncm superstructure.

Source-reported materials — not catalogue approval

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