Field-dependent specific heat study of the spin-Peierls transition in TTF–AuS4C4(CF3)4

The compound TTF–AuS4C4(CF3)4 is one of the very few existing examples of the spin-Peierls transition, in which a system of 1D antiferromagnetic chains dimerizes spontaneously as a consequence of magnetoelastic coupling with the surrounding lattice. We have studied the spectific heat anomaly accompanying the SP transition for this compound as a function of an applied magnetic field. Our H=0 data differ considerably from those obtained earlier by Wei et al., and in fact remove the discrepancy found by them between their results for TTF-AuS4C4(CF3)4 and the isomorphous TTF–CuS4C4(CF3)4. The differences between the two experiments are probably due to thermal relaxation effects. The combination of the specific heat data now available confirms the conclusion drawn from our previous field-dependent differential susceptibility studies on the Au compound, namely that the SP transition involves a considerable degree of short-range order, starting to develop below T=1.2 Tsp. We have been able to follow the specific heat anomaly for fields up to the multicritical point in the H–T phase diagram.