Why is Sr₂RuO₄ not a high T_c superconductor? Electron correlation, hund's coupling and p-wave instability

Abstract

Sr₂RuO₄ is a layered superconductor with a T_c ≈ 0.95 K. The electrical resistivity is strongly anisotropic and has an unusual T dependence: ρ_ab ≈ T² for T < K and ρ_ab ≈ T for T > 100 K; the ratio ρ_c/ρ_ab is about 850 around 4 K and 220 around 290 K. dHvA and SdH experiments show the presence of three anisotropic 'Fermi surfaces'. Experiments on Sr₂RuO₄ and related oxides indicate that this conductor is close to a spin 1 Mott insulating state. We present a model for the insulating and conducting states of these compounds and a mechanism for the metal-insulator transition. We argue that there are two bands of non-Fermi liquids which are close to half filling and an almost full band. The residual Hund's coupling in this (primarily 4d) band is argued to cause triplet correlations among the low-energy spinon excitations of the two bands of non-Fermi liquids below about 100 K. Based on this, we suggest that the experimentally observed superconducting state is likely to be a triplet p-wave type. Some other predictions are also made.