Sum Rules and Asymptotic Behaviors for Optical Conductivity of   Nonequilibrium Many-Electron Systems
  For many-electron systems, we consider a nonequilibrium state (NES) that is driven by a pump field(s), which is either an optical field or a longitudinal electric field. For the differential optical conductivity describing the differential response of the NES to a probe optical field, we derive exact sum rules and asymptotic behaviors, which open wide possibilities for experiments. In deriving these results, we have also derived universal properties of general differential response functions of time-dependent NESs of general systems. 
Akira Shimizu and Tatsuro Yuge
cond-mat.str-el cond-mat.stat-mech cond-mat.supr-con