Charge ordering and spin-Peierls transition in α′-NaV2O5

Recent X-ray analysis and NMR experiments have shown that alpha'-NaV2O5 is not a conventional spin-Peierls (SP) compound because above the transition at 33 K it is in a homogeneous mixed valent (MV) state and atomic spin chains do not exist. Furthermore, thermal expansion experiments have identified the existence of a double phase transition. We present a theoretical model which qualitatively explains these observations, We derive an effective Hamiltonian for the insulating state which describes the transitions as primary charge order leading to linear spin chains in the b-direction and a secondary SP transition. We show that this scenario explains in a natural way the anomalous BCS ratio observed in alpha'-NaV2O5.