BCS theory assumes equal abundance of spin-up and spin-down electrons. An interesting AMO paper shows that Fermionic superfluidity is robust against a
substantial imbalance in spin populations.
Lecture 9 The BCS Variational Calculation and the Self-Consistent Gap Equation
Calculation of BCS Surface Resistance and Penetration Depth (SRIMP)
Semiclassical model of superconducting penetration depth in anisotropic superconductors (ignores scattering) by Chandrasekhar and Einzel. The key equations have been re-expressed by Prozorov and Giannetta (beware of two errors in Eq. (15))