Electron or hole liquids in high-Tc cuprates: evidence for d-wave order parameter

Phillips and Jung [J.C. Phillips and J. Jung (2002). Phil. Mag. B, 82, 1163] refute nodal properties of the gap function Delta(k) and propose an isotropic s-wave picture with electron-phonon interaction as the mechanism for high-T-c superconductivity in the cuprates. Here, we compare and contrast predictions of various physical properties with the gap function Delta(k) reflecting three scenarios: (i) isotropic s-wave pairing, (ii) extended s-wave with eight line nodes as championed quite recently by Zhao [G.M. Zhao (2001). Phys. Rev. B, 64, 024503], and (iii) d-wave pairing. By referring to (a) scanning tunnelling microscopy imaging of a Zn impurity, supported by our own subsequent theoretical study, (b) linear decrease of T-c with non-magnetic impurity concentration, and (c) calculations on the extended s-wave scenario applied to tunnelling conductance, strong evidence for nodal properties of Delta(k) is given. The contentions of Phillips and Jung [J.C. Phillips and J. Jung (2002). Phil. Mag. B, 82, 1163] should therefore be treated with considerable caution, isotropic s-wave behaviour being truly exceptional among the high-T-c cuprates.