Superconductivity and ferromagnetism from effective mass reduction

Within a simple model Hamiltonian, both superconductivity and metallic ferr omagnetism can be understood as arising fi om lowering of kinetic energy as the ordered state develops, due to a reduction in the carriers effective m ass, or equivalently, a bandwidth expansion. Experimental manifestation of this physics has been detected in both high T-c superconductors and large m agnetoresistance ferromagnets, as an anomalous transfer of spectral weight in optical absorption from high to low frequencies as the ordered state dev elops. It is proposed that this general principle is common to the essentia l 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.