On the Possibility of s+d Wave Superconductivity Within a Two-Band Scenario for High Tc Cuprates

A variety of different experimental results show substantial evidence that the order parameter in high-temperature superconducting copper oxides is not of pure d-wave symmetry, but that an s-wave component exists, which especially shows up in experiments that test the c-axis properties. These findings are modeled theoretically within a two-band model with interband interactions, where the superconducting order parameters in the two bands are allowed to differ in symmetry. It is found that the coupling of order parameters with different symmetries (s+d) leads to substantial enhancements of the superconducting transition temperature T_c as compared to order parameters with only s-wave symmetry. An additional enhancement factor of T_c is obtained from the coupling of the bands to the lattice where moderate couplings favor superconductivity while too strong couplings lead to electron (hole) localization and consequently suppress superconductivity.