Optical conductivity and the sum rule in the DDW state

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The density-wave with d-wave order parameter (DDW) is possibly realized in the underdoped regime of high-Tc 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.