On the two-parametric theory of superconductivity

The concept of spin fluctuations in the Hubbard model is used for deriving a closed system of equations for the superconducting order parameter $\Delta$ and spin susceptibility. The limiting cases of low temperatures and temperatures close to the superconducting transition temperature are considered. The temperature dependences of the parameter $\Delta$ and the Knight shift are obtained. The conditions under which the second-order phase transition turns to the first-order phase transition are established.