Midgap states in disordered spin-Peierls systems

We investigate effects of a single impurity on the spin-Peierls system using a simplified one-dimensional XY model coupled with lattice distortion in terms of fermions representing spin operators in the continuum limit. An impurity is assumed to reduce the spin-lattice coupling around it. It is shown that midgap hound states appear around the impurity and consequently the lattice distortion is reduced there. Magnetization can be induced there under adequate boundary conditions. The results are compared with the recent results by a phase Hamiltonian.