Phase transition from a dx2-y2 to a dx2-y2+dxy superconductor

We study the phase transition from a dx2-y2 to a dx2-y2+dxy superconductor using the tight-binding model of two-dimensional cuprates. As the temperature is lowered past the critical temperature Tc, first a dx2-y2 superconducting phase is created. With further reduction of temperature, the dx2-y2+dxy phase is created at temperature T=Tc1. We study the temperature dependencies of the order parameter, specific heat, and spin susceptibility in these mixed-angular-momentum states on a square lattice and on a lattice with orthorhombic distortion. The above-mentioned phase transitions are identified by two jumps in specific heat at Tc and Tc1. ©1999 The American Physical Society.