Spin-fluctuation high-T-c superconductivity mechanism and the order parameter symmetry

The notion that electrons in HTSC cuprates pair via antiferromagnetic spin fluctuations is discussed and the symmetry of the superconducting order par ameter analyzed. Three approaches to the problem, one phenomenological (wit h an experimental dynamic magnetic susceptibility) and two microscopic (inv olving, respectively, the Hubbard model and the tJ model) are considered an d it is shown that in each case strong-coupling theory leads to a d-symmetr ic order parameter with zeros at the Fermi surface. The paper then proceeds to consider experimental techniques in which the d-symmetry of the order p arameter may manifest itself. These include low-temperature thermodynamic m easurements; measurements of the penetration depth and the upper critical f ield; Josephson junction experiments to obtain the phase of the superconduc ting order parameter; and various spectroscopic methods. The body of data f avor the view that the order parameter in cuprates has d(x2-y2) symmetry. F or a superconductor with a d-symmetric order parameter, Ginzburg-Landau the ory is outlined and both an isolated vortex and a vortex lattice are invest igated. Finally, some theoretical aspects of the effects of nonmagnetic imp urities on a d-type superconductor are considered.