Impurity-Induced States in Spin-Peierls Systems

Impurity effects in the spin-Peierls system are studied. The Heisenberg model with the spin-lattice coupling is treated within the adiabatic approximation. The spin system is transformed into the fermion system with the help of the Jordan-Wigner representation. The fermion system is treated within the mean-field approximation. We consider two kinds of impurities: site-type and bond-type. The site-type impurity generates a lattice soliton that has no spin. If more than one impurity is doped to a single chain and if odd number of magnetic ions are left between the neighboring impurities, a spin soliton with s = 1/2 is generated between the impurities. The bond-type impurity does not generate lattice solitons, while it generates spin solitons.