Thermodynamics of the spin-density wave transition in (TMTSF)2PF6

We report on calorimetric measurements of the organic spin-density wave (SDW) conductor (TMTSF${)}_2$ ${\mathrm{PF}}_6$. The experiments were carried out from 8 to 20 K, with emphasis on the neighborhood of the 12.1 K SDW transition. The transition anomaly was less than 1% of the total specific heat at that temperature, and deviates substantially from the shape expected for mean-field theory for temperatures approximately 0.15 K on either side of the transition. A comparison to the thermodynamics of the 2D Heisenberg antiferromagnet is made.