  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.
