The order parameter symmetry of doped YBa2Cu3O7

A two-band model that reproduces the Fermi surface seen in angle-resolved photoemission experiments is used to study the symmetry of the order parameter of doped YBa2Cu3O7. The model incorporates the effects of tunnelling in the c-direction between the planes and chains in YBCO. It is observed that a suitable choice of the phases of the symmetric pair wave-function in the planes and chains leads to both s- and d-wave-like features. We calculate the mean-field phase diagram of this system, observe the shift in the position of the node(s) of the order parameter(s) on the Fermi surface with doping and try to reconcile the seemingly contradictory experimental observations as regards the presence of features that resemble s-wave and/or d-wave symmetry. The effect of the leading-order, two-dimensional fluctuations on the mean-field transition temperature is shown to be small.