Electron Pairing in High-Tc Superconductors

In high-Tc cuprate superconductors, the pairing state of electrons is still spin-singlet, as in conventional BCS superconductors. But the orbital part of the wavefunction, order parameter or gap function, has a dx2−y2 symmetry, rather than the isotropic s-wave symmetry. This exotic symmetry has been confirmed by various experiments, especially the phase-sensitive ones. But the underlying mechanism of d-wave pairing remains controversial. Investigations on Hubbard model provide some ideas.