The superconducting penetration depth from the semiclassical model

The semiclassical approach of Onsager and Pippard has been very successful in relating the electronic properties of a normal metal to its band structure and fermi surface. This paper extends the method to a superconductor. A generalized London equation relating the supercurrent density to the vector potential is obtained, in terms of band and fermi surface parameters and an energy gap which may be anisotropic. The results allow an interpretation of measured penetration depths directly in terms of the electronic and gap structures of the superconductor. They lead to easy physical visualization of the origin of anisotropies and other features of measured penetration depths.