Magnetic Field Influence on the Low Temperature Heat Capacity and Heat Release of (TMTSF)2PF6

We have measured under applied magnetic field the heat capacity cp and heat release \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document} $${\dot Q}$$ \end{document} of the quasi-1D conductor (TMTSF)2PF6 in its spin density wave (SDW) ground state. The low-temperature heat capacity (T < 0.5 K) is dominated by a Schottky anomaly contribution. A corresponding term was also found for the heat release. In this T-range both properties are strongly sensitive to moderate magnetic fields (i.e., below 0.5 T): their amplitudes have a sharp maximum for Hc=0.2 T. We show that the corresponding density of two-level states NS undergoes a sharp maximum at Hc, which rules out an extrinsic origin for the two-level states. Instead we suppose an intrinsic origin, such as defects of commensurability of the SDW, as the (slow) excitations related to the domain walls. This effect is found to be independent on the field orientations H ∥ a and H ⊥ a.