Structural phase transitions and superconductivity in La-M-Cu-O compounds

Abstract

The structural phase transitions D¹⁷ _4h→D¹⁸ _2h→D¹⁶4h in La_2-x M _x CuO₄ compounds and their influence on electronic spectrum for oxygen doped holes in CuO₂ plane are considered on the basis of a micro-scopical anharmonic model. The free energy and equation for the two-component order parameter are calculated in the self-consistent phonon approximation. Taking into account the electron-phonon interaction with the soft tilting mode and other phonon branches the dependence of the superconducting temperature T_c on hole concentration n is obtained. A dip in T_c(n) dependence is found at the transition to the low temperature tetragonal phase (LTT) owing to splitting of two oxygen bands at half-filling.

Key Words:

• Structural phase transitions
• soft tilting mode
• local normal coordinates
• high temperature superconductors