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

We have investigated the effect of the magnetic field (B) on the very low-temperature equilibrium heat capacity c<Subscript>eq</Subscript> of the quasi-1 D organic compound (TMTTF)<Subscript>2</Subscript>Br, characterized by a commensurate Spin Density Wave (SDW) ground state. Below 1 K, c<Subscript>eq</Subscript> is dominated by a Schottky-like A<Subscript>S</Subscript>T<Superscript>-2</Superscript> 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<Subscript>eq</Subscript> extracted from the time constant, increases enormously. For B ≥ 2–3 T, c<Subscript>eq</Subscript> varies like B<Superscript>2</Superscript>, 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<Subscript>eq</Subscript>, 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. Copyright EDP Sciences/Società Italiana di Fisica/Springer-Verlag 2007