Paramagnetic-like destructive mechanism against superconductivity in Sr₂RuO₄: A triplet scenario versus a singlet one

The most popular description of superconductivity phenomenon in Sr₂RuO₄ is based on a so-called single-band (usually γ-band) "isotropic p-wave order parameter". In a magnetic field parallel to the conducting planes, such triplet "isotropic p-wave phase" is not destroyed by the Clogston-Chandrasekhar paramagnetic limiting field and can be destroyed only by the Meissner currents (i.e., the orbital effects). We analyze the orbital destructive effects against superconductivity for in-plane magnetic field (when electron orbits are open) and find that H_c2^∥(0) ≃ 0.75 T_c(dH_c2^∥(T)/dT)_Tc (which is a little bigger than the Werthamer-Helfand-Hohenberg value for an isotropic 3D case). We point out that the experimentally determined ratio H_c2^∥(0)/T_c(dH_c2 ^∥(T)/dT)T_c ≃ 0.44 - 0.5 in Sr₂RuO₄ is significantly less than the calculated value 0.75. Since the upper critical field, K_c2^∥(T). is a well experimentally defined quantity in Sr₂RuO₄ (unlike high-T_c superconductors) we conclude that the single-band triplet "isotropic p-wave order parameter" seems to be inappropriate description of superconductivity in this material. Two possibilities are discussed: 1) Three-band nature of triplet superconductivity; 2) Singlet (d-wave) nature of superconducting pairing (in this case, the destructive actions of both the orbital effects and the Clogston-Chandrasekhar paramagnetic effects result in an agreement with the experimentally observed value of H_c2^∥(0)/T_c(dH_c2 ^∥(T)/dT)_Tc).