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

The most popular description of superconductivity phenomenon in Sr2RuO4r is based on a so-called single-band (usually gamma -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-Chaldrasekhar 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) similar or equal to 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)(Tc) similar or equal to 0.44 - 0.5 in Sr2RUO4 is significantly less than the calculated value 0.75. Since the upper critical field, H-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 tin 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)/Tc(dH(c2)(//)(T)/dT)(Tc)).