P-Wave Pairing in Superconducting Sr₂RuO₄

Abstract

Although there is strong evidence that Sr₂RuO₄ is a triplet superconductor [2, 3], the full symmetry of the equilibrium state below T_c remains open to debate [2-14]. There exist strong indications for broken time-reversal symmetry in the superconducting state [15, 10] and equally convincing measurements showing that the order parameter d(k) has a line of nodes on the Fermi surface [16-19]. The reason why this state of affairs represents a puzzle is that for all odd parity spin triplet pairing states in tetragonal crystals, group theory does not require the simultaneous presence of both broken time-reversal symmetry and line nodes [20]. To explain this inconsistency different three dimensional models of pairing have been proposed [11-13]. In fact the experimental results on heat transport [19] seem to favour the horizontal, with respect to (ab) crystal plane, line nodes. Usually in a multi-band BCS like model, with different coupling constants for each band, one generically finds multiple phase transitions as the different sheets of the Fermi surface are gaped on lowering the temperature. Since experimentally there is only one jump in the specific heat, at T _c = 1.5 K, in constructing a sensible model one must eliminate such multiple transitions. Zhitomirsky and Rice [12], in their simplified two band model, considered an interaction which couple the order parameters of the different symmetry. The presence of this inter-band interaction leads to a single superconducting transition. Microscopically such coupling of different bands comes from effective three site interactions. They assumed that one band in Sr₂Ru0₄ is the most important for superconducting pairing, while two other are gaped via an inter-band proximity effect.