Applications of ionic models to the high-temperature superconductor La 2CuO 4

Abstract

The electronic structures of superconducting oxides such as La 2CuO 4 include significant ionic contributions. Application of ionic models to these materials complements band structure studies; ionic models provide useful reference models for studying the high- T c superconductors. In order to make this comparison useful, it is important to use a non-empirical model that does not involve fitting to experiment. We have applied the Potential Induced Breathing (PIB) Model, a non-empirical model which has shown success in predicting properties of insulating oxides, to La 2CuO 4. The PIB overlapping ion change density is compared to a self-consistent LAPW charge density. The out-of-plane oxygen and lanthanum ions are quite ionic, but the in-plane oxygen and copper show significant distortions, primarily displaying movement of charge out of the overlap region. Static minimum energy structures were calculated in tetragonal and orthorhombic symmetries using the PIB model. The ionic model predicts the observed tetragonal to orthorhombic distortion. PIB predicts a lower symmetry ground state. Phonon dispersion curves were calculated for both the tetragonal and orthorhombic structures. Unstable phonon branches are found for both structures using the ionic model. In the tetragonal structure these unstable branches are related to the X-point rotation of the in-plane oxygens, the octahedral tilting associated with the phase transition to the orthorhombic structure, and sliding of the O(Z) and La atoms in the x−y plane throughout the Brillouin zone.