LATTICE INSTABILITIES, ISOTOPE EFFECT, AND HIGH-TC SUPERCONDUCTIVITY IN LA2-XBAXCUO4

Unlike La(2-x)Sr(x)CUO4, La(2-x)Ba(x)CUO4, x approximately x(cr) = 0.12, undergoes a structural change to a low-temperature tetragonal (LTT) phase below 60 K, and this transformation is known to suppress T(c) drastically. We present self-consistent local-density-functional calculations for this system that indicate the CUO6 octahedrons are unstable to an X-point tilt in any direction, thereby leading to strongly anharmonic X-point vibrations. The LTT phase is most favored energetically, consistent with experiment. The effect of the LTT distortion on the electronic structure accounts for the suppression of T(c) in the LTT phase, and for the enhanced isotope effect at x = x(cr).