Splitting or not splitting of the Van Hove singularity in the high T c superconductors

2014 The tetragonal-orthorhombic (T-O) structural phase transition observed in the high Tc superconductor La2-xSrxCuo4-y is analysed. It is shown that the tilt of the CUO6 octahedra is the primary order parameter of this phase transition and that the symmetry of the shear distortion induced by the square to rectangle transformation of the oxygen basal plane of the octahedra does not allow a splitting of the Van Hove singularity in the density of states. Consequences on the mechanism of the T-O phase transition and the variations of the superconducting temperature Tc with the band filling are analysed. The T-O transition observed in the other series of high Tc superconductors, YBa2Cu3O6+03B4 is briefly discussed from the same symmetry point of view. Tome 49 N° 3 MARS 1988 LE JOURNAL DE PHYSIQUE J. Phys. France 49 (1988) 375-381 MARS 1988,