Bond and charge-density waves in (DI-DCNQI)_2Ag (DI-DCNQI=2,5-diiodo- N,N′-dicyanoquinediimine) studied by single-crystal infrared and Raman spectroscopy

摘要

The charge distribution and molecular arrangement of a quarter-filled quasi-one-dimensional system (DI-DCNQI)_2Ag (DI-DCNQI=2,5-diiodo-dicyanoquinediimine) has been studied by IR and Raman spectroscopy. The charge localization of this material was believed to be a one-dimensional generalization of a Wigner crystal driven by intersite Coulomb repulsion. While charge disproportionation is confirmed via the splitting of b_u modes in the infrared spectrum, the appearance of intense IR vibronic bands of a_g modes strongly suggests the presence of the dimerization that is not expected by the proposed 1010 charge ordering model. In addition, the selection rules for the IR and Raman signals cannot be explained without a further symmetry reduction of the unit cell. To explain the vibrational behavior observed for a single crystal, we show that a more appropriate model for the charge ordering is 0110 (2k_F charge-density wave and 4k_F bond-order wave).