Superconductivity and Ferromagnetism from Effective Mass Reduction
  Within a simple model Hamiltonian, both superconductivity and metallic ferromagnetism can be understood as arising from lowering of kinetic energy as the ordered state develops, due to a reduction in the carriers effective mass, or equivalently, a bandwidth expansion. Experimental manifestation of this physics has been detected in both high Tc superconductors and large magnetoresistance ferromagnets, as an anomalous transfer of spectral weight in optical absorption from high to low frequencies as the ordered state develops. It is proposed that this general principle is common to the essential physics of superconductivity and ferromagnetism in nature, and hence that these effects in optical properties, although often smaller in magnitude, should exist in all superconductors and metallic ferromagnets. 
J.E. Hirsch