Low temperature specific heat of blue bronze K _0.30 MoO ₃

Abstract:

We report on specific heat measurements of the quasi-one-dimensional model system blue bronze K_0.3MoO₃, performed over a wide temperature interval 0.1-30 K, in its charge density wave (CDW) ground state. In this T range, the specific heat is successively determined by the presence of low-energy excitations (LEE) due to the disordered nature of the CDW ground-state, the acoustical phonon contribution which obeys an exact T³ law between 0.7 and 3.5 K, and a large excess of C / T ³ centered at 11.5-13 K. The contribution of the LEEs is of particularly low amplitude in comparison to other CDW systems. Also, feeble time-dependent effects could be detected. This implies a less disordered ground state in this compound. The C / T ³ bump, mainly of phononic origin, can be partly ascribed to a phason-excitation contribution, with a low-energy cut-off defined by the pinning energy. This interpretation is supported by the sensitivity of the low-T side of the bump to the purity of samples, and therefore to their pinning frequency. A similar effect was already measured in the specific heat of the CDW compound (TaSe₄)₂I, by comparison between pure and doped samples. Finally, we outline once more that some generic thermodynamic properties of CDW systems are similar to structurally disordered systems.