Optical conductivity and the sum rule in the DDW state
  The density-wave with d-wave order parameter (DDW) is possibly realized in the underdoped regime of high-T_c cuprates. The DDW state is characterized by two branches of low-lying electronic excitations, and the quantum mechanical current has in particular an inter-branch contribution. The latter component causes a finite-frequency response in the optical conductivity and a reduction of the Drude contribution. We show that this redistribution of the spectral weight leaves the optical sum mostly intact, so that the restricted optical sum rule is only weakly violated. 
D. N. Aristov and R. Zeyher