Equilibrium low temperature heat capacity of the spin density wave compound (TMTTF)2Br: effect of a magnetic field

We have investigated the effect of the magnetic field (B) on the very low-temperature equilibrium heat capacity c_eq of the quasi-1 D organic compound (TMTTF)₂Br, characterized by a commensurate Spin Density Wave (SDW) ground state. Below 1 K, c_eq is dominated by a Schottky-like A_ST^-2 contribution, very sensitive to the experimental time scale, a property that we have previously measured in numerous DW compounds. Under applied field (in the range 0.2–7 T), the equilibrium dynamics, and hence c_eq extracted from the time constant, increases enormously. For B ≥ 2–3 T, c_eq varies like B², in agreement with a magnetic Zeeman coupling. Another specific property, common to other Charge/Spin density wave (DW) compounds, is the occurrence of metastable branches in c_eq, induced at very low temperature by the field exceeding a critical value. These effects are discussed within a generalization to SDWs in a magnetic field of the available Larkin-Ovchinnikov local model of strong pinning. A limitation of the model when compared to experiments is pointed out.