DMRG study of the impurity-induced transition from the spin-Peierls state to the antiferromagnetic state

We investigate impurity effects in the spin-Peierls systems coupled with an interchain exchange interaction. The density matrix renormalization group method is used. The interchain interaction is treated within the mean field approximation. In the impurity-free case, the ground state makes a first-order transition from the spin-Peierls (SP) state to the antiferromagnetic (AF) state at a critical strength of the interchain interaction. Impurities are doped to the spin-Peierls system which is located close to the SP-AF phase boundary in the phase diagram. A single impurity suppresses the bond alternation and induces a staggered magnetic moment locally around the doped site. If the impurity density is larger than a critical value, the spin-Peierls system makes a transition to the antiferromagnetic state. It is shown that a two-impurity problem is essential to understand the physical mechanism of the phase transition.