Long-Time Heat Release in the Quasi-1D Conductor (TMTSF)2PF6 at Low Temperatures

We have measured the heat capacity and the heat release of the quasi-1D conductor (TMTSF)₂PF₆ in its spin-density wave ground state. Below 1K the presence of low-energy excitations is at the origin of slow heat release over time scales from about 1s up to 10⁵s. In a first attempt, we analyzed this behaviour with a modified version of the standard tunneling model used in the case of amorphous solids, introducing a short-time cut-off in the distribution of the relaxation times of tunneling states. A remarkable feature of the heat release in (TMTSF)₂PF₆ is its high absolute value, which is 3–4 orders of magnitude larger than in structural glasses. The heat capacity contains a Schottky and a quasi-linear term.