Electromagnetic response of superconductors and optical sum rule
  The interrelation between the condensation energy and the optical sum rules has been investigated. It has been shown that the so called 'partial' sum rule violation is related mainly to a temperature dependence of the relaxation rate rather than to the appearance of superconductivity itself. Moreover, we demonstrate that the experimental data on the temperature dependence of the optical sum rule can be explained rather well by an account of strong electron-phonon interaction. 
A.E. Karakozov (L.F. Vereshchagin Institute for High Pressure Physics,   RAS, Troitsk, Moscow region, Russia), E.G. Maksimov...