Equilibrium low temperature heat capacity of the spin density wave compound (TMTTF)_2Br: effect of a magnetic field

摘要

We have investigated the effect of the magnetic field (B) on the very low-temperature equilibrium heat capacity ceq of the quasi-1 D organic compound (TMTTF)2Br, characterized by a commensurate Spin Density Wave (SDW) ground state. Below 1 K, ceq is dominated by a Schottky-like AST-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 ceq extracted from the time constant, increases enormously. For B 2–3 T, ceq 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 ceq, 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.