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 o rder parameter". In a magnetic field parallel to the conducting planes, suc h triplet "isotropic p-wave phase" is not destroyed by the Clogston-Chaldra sekhar paramagnetic limiting field and can be destroyed only by the Meissne r currents (i.e., the orbital effects). We analyze the orbital destructive effects against superconductivity for in-plane magnetic field (when electro n orbits are open) and find that H-c2(//)(0) similar or equal to 0.75 T-c(d H(c2)(//)(T)/dT)(Tc) (which is a little bigger than the Werthamer-Helfand-H ohenberg value for an isotropic 3D case). We point out that the experimenta lly 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 t hat the single-band triplet "isotropic p-wave order parameter" seems to be inappropriate description of superconductivity in this material. Two possib ilities are discussed: 1) Three-band nature of triplet superconductivity; 2 ) Singlet (d-wave) nature of superconducting pairing tin this case, the des tructive actions of both the orbital effects and the Clogston-Chandrasekhar paramagnetic effects result in an agreement with the experimentally observ ed value of H-c2(//)(0)/Tc(dH(c2)(//)(T)/dT)(Tc)).