Phase diagram of a Heisenberg spin-Peierls model with quantum phonons

Using a new version of the density-matrix renormalization group we determin e the phase diagram of a model of an antiferromagnetic Heisenberg spin chai n where the spins interact with quantum phonons. A quantum phase transition from a gapless spin-fluid state to a gapped dimerized phase occurs at a no nzero value of the spin-phonon coupling. The transition is in the same univ ersality class as that of a frustrated spin chain, to which the model maps in the diabatic limit. We argue that realistic modeling of known spin-Peier ls materials should include the effects of quantum phonons.