Coupling between Planes and Chains in Y${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$: A Possible Solution for the Order Parameter Controversy

We propose to explain the contradictory experiments about the symmetry of the order parameter in Y${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ by taking into account the coupling between planes and chains. This leads to an anticrossing of the plane and chain band. We include an attractive pairing interaction within the planes and a repulsive one between planes and chains, leading to opposite signs for the order parameter on planes and chains, and to nodes of the gap because of the anticrossing. Our model blends $s$-wave and $d$-wave features, providing a natural explanation for all the contradictory experiments.