Spin-fluctuation mechanism of high-T_c superconductivity and order-parameter symmetry

The notion that electrons in high-T_c cuprates pair via antiferromagnetic spin fluctuations is discussed and the symmetry of the superconducting order parameter is analyzed. Three approaches to the problem, one phenomenological (with an experimental dynamic magnetic susceptibility) and two microscopic (involving, respectively, the Hubbard model and the tJ-model) are considered and it is shown that in each case strong-coupling theory leads to a d-wave order parameter with zeros at the Fermi surface. The review then proceeds to consider experimental techniques in which the d-symmetry of the order parameter may manifest itself. These include low-temperature thermodynamic measurements, measurements of the penetration depth and the upper critical field, Josephson junction experiments to obtain the phase of the superconducting order parameter, and various spectroscopic methods. The experimental data suggest that the order parameter in cuprates is -wave. Ginzburg–Landau theory for a superconductor with a d-wave order parameter is outlined and both an isolated vortex and a vortex lattice are investigated. Finally, some theoretical aspects of the effects of nonmagnetic impurities on a d-wave superconductor are considered.