Anharmonic phonons and the anomalous isotope effect inLa2−xSrxCuO4

A model for the variation of an interionic potential near the orthorhombic to low-temperature-tetragonal structural phase transition in La-Sr-Cu-O can explain variations both above and below 0.5 in the superconducting isotope-effect parameter α. In particular, we posit a quadrupolar potential in which the four outer wells deepen as the phase transition is approached. This model can reproduce the experimentally observed variations in both ${\mathit{T}}_{\mathit{c}}$ and α.