Effect of Orthorhombicity on the Electronic Structure and Superconducting Properties of High-Tc Cuprate Family

Electronic structure in the normal state and properties in the superconducting state of the orthorhombic phase in HTSC cuprates are studied within the multielectron generalized tight-binding (GTB) approach. The joint effect of variation of average Cu-O distance and orthorhombic distortion on the Fermi contour, band structure, and concentration dependence of T-c is studied. Quasiparticle excitations were constructed within the framework of the five-band p-d model for the layer of the CuO6 octahedra. The electronic structure of quasiparticle excitations in the effective Hubbard model is calculated using the equation of motion for Green's functions in the generalized mean-field approximation. Orthorhombic distortion leads to asymmetric with respect to the nodal direction of the Brillouin zone dispersion surface of quasiparticle excitations and to splitting of each van Hove singularity into two peaks. Transformation of the Fermi contour from hole pockets to the large hole and electron pockets occurs as a result of two quantum-phase transitions at dopings x(c1) and x(c2). Simultaneous average Cu-O distance elongation and orthorhombic distortion decreasing result in T-c decrease. T-c dependence on average Cu-O distance in the orthorhombic phase is in agreement with the behavior of experimental T-c (max) values when the DOS effect on T-c prevails over the effect of the exchange parameter. There are two T-c maxima in the orthorhombic system with the suppressed exchange parameter, and these maxima appear at concentrations x(c1) and x(c2).