Unconventional superconductors: Contrasting cuprates and ruthenates

Abstract

Sr 2RuO 4 and the high-T c cuprates are highly 2-dimensional metals with similar crystal structures which transform into unconventional superconductors. In Sr 2RuO 4 p-wave superconductivity arises at a low temperature (T c ∼ 1.5K) out of a well formed Landau-Fermi liquid. This behavior contrasts strongly with the high T c and anomalous normal state of the cuprates and sharpens the question, what is so special about the cuprates? A key distinguishing feature is the existence of Umklapp electron-electron scattering processes near the saddle points of the single band cuprate Fermi surface-processes which are not allowed on the multi-band ruthenate surface. These U-processes are responsible for the Mott charge gap at stoichiometry. In the case of the 2-leg ladder they reinforce pairing and lead to the RVB insulating spin liquid at half-filling. A key question then is whether similar effects can occur in 2-dimensions leading to a partial truncation of the Fermi surface near the saddle points without broken translational symmetry. Scattering in the Cooper channel between the open and truncated parts of the Fermi surface would then be responsible for the high-T c superconductivity. The results of 1-loop RG investigations of a 2-patch model, restricted to the neighborhoods of the saddle points, and of a multi-patch model of the full Fermi surface are promising.