Coexistence of charge density wave and spin-Peierls orders in quarter-filled quasi-one dimensional correlated electron systems

摘要

Charge and spin-Peierls instabilities in quarter-filled (n=1/2) compoundsconsisting of coupled ladders and/or zig-zag chains are investigated. Hubbardand t-J models including local Holstein and/or Peierls couplings to the latticeare studied by numerical techniques. Next nearest neighbor hopping and magneticexchange, and short-range Coulomb interactions are also considered. We showthat, generically, these systems undergo instabilities towards the formation ofCharge Density Waves, Bond Order Waves and (generalized) spin-Peierls modulatedstructures. Moderate electron-electron and electron-lattice couplings can leadto a coexistence of these three types of orders. In the ladder, a zig-zagpattern is stabilized by the Holstein coupling and the nearest-neighbor Coulombrepulsion. In the case of an isolated chain, bond-centered and site-centered2k_F and 4k_F modulations are induced by the local Holstein coupling. Inaddition, we show that, in contrast to the ladders, a small charge ordering inthe chains, strongly enhances the spin-Peierls instability. Our results areapplied to the NaV_2O_5 compound (trellis lattice) and various phases withcoexisting charge disproportionation and spin-Peierls order are proposed anddiscussed in the context of recent experiments. The role of the long-rangeCoulomb potential is also outlined.