Spin-Peierls Transition in CuGeO_3: Critical, Tricritical, or Mean Field?

The spin-Peierls phase transition in CuGeO_3 has been extensively studied utilizing a variety of experimental techniques. Interpretations of the phase transition behavior vary from tricritical to mean field to Ising critical to XY critical. We show that the behavior in the vicinity of the phase transition of each of the order parameter, the magnetic energy gap and the heat capacity can be quantitatively fitted with few adjustable parameters with a mean field model incorporating a tricritical to mean field critical crossover in the transition region.