Impurity induced midgap state and magnetism in spin-Peierls systems Original Research Article

Effects of impurities on the spin-Peierls systems are investigated by using fermion representation of spin operators within the continuum limit approximation. We use XY model instead of the isotropic Heisenberg exchange interaction. The Bogoliubov–de Gennes-type equations are solved on the assumption that an impurity changes the spin–lattice coupling at the impurity site. It is shown that midgap bound states are formed. The lattice distortion is reduced around the impurities and its spatial dependence is expressed by a composition of parts of the soliton solution in polyacetylenes. There are a few possible impurity states depending on the total Sz, which is determined by the filling of the midgap states. Uniform magnetic moments will be finite around impurities. These results are compared with the results of previous theoretical studies based on the bosonized phase Hamiltonian. Since the previous works are for the isotropic Heisenberg exchange interaction, the behavior of the magnetic moments around the impurity is different from the present case.