Specific heat of at its spin-Peierls transition

We have measured the specific heats of small (≈1 mg) crystalline flakes of (Formula presented) A large sharp peak is observed at its (33 K) spin-Peierls transition. The size of the peak (Formula presented) is more than an order of magnitude greater than the value estimated from mean-field theory, consistent with large fluctuations expected from its one-dimensional magnetic structure. However, the peak is somewhat sample dependent and does not have the symmetry about (Formula presented) expected for either Gaussian or critical fluctuations, suggesting either that the samples are near a multicritical point or that there is some heterogeneous broadening, in which case we are underestimating the true size of the anomaly. The low-temperature behavior is consistent with a Debye temperature between 345 and 414 K, depending on the size of magnetic contributions to the specific heat.