Molecular-field approach to the spin-Peierls transition in CuGeO3

We present a theory for the spin-Peierls transition in CuGeO3. We map the elementary excitations of the dimerized chain (solitons) on an effective Ising model. Interchain coupling (or phonons) then introduces a linear binding potential between a soliton-antisoliton pair leading to a finite transition temperature. We evaluate, as a function of temperature, the order parameter, the singlet-tripler gap, the specific heat, and the susceptibility and compare with experimental data on CuGeO3. We find that CuGeO3 is close to a first-order phase transition. We point out that the famous scaling law similar to delta(2/3) Of the triplet gap is a simple consequence of the linear binding potential between pairs of solitons and antisolitons in dimerized spin chains.