Charge gap and ferroelastic lattice distortion in high-temperature superconductors

It is shown that a ferroelastic lattice distortion is associated with the superconducting transition both in low- and high-temperature superconductors. A low-temperature ferroelastic transition in crystalline solids produces also a maximum in the temperature dependence of their thermal conductivity. In layered tetragonal superconductors with a sufficiently high transition temperature, a ferroelastic distortion is improper and is caused by an out-of-plane charge ordering. The relation between the out-of-plane charge gap and the maximum superconducting transition temperature for these superconductors is obtained.