Glass-like features in the heat capacity of charge density wave systems

Charge density wave (CDW) is a spatial modulation of conducting electrons accompanied by the lattice modulation which appears in some quasi one-dimensional metallic crystals at low temperatures. Due to the strong polarizability at QCDW=2kF, kF being the Fermi wave vector, quasi one-dimensional electron gas screens and softens the phonons of corresponding wave length until a new periodicity sets in and the gap opens at the Fermi level. Near QCDW, electron and phonon degrees of freedom combine to give new spectrum of the excitations of the complex CDW order parameter: acoustic-like phase excitations (phasons) and optic-like amplitude excitations (amplitudons) [1]. We demonstrate that the heat capacity Cp in these systems is nevertheless very similar to Cp of ordinary glass formers such as a-SiO2. The heat capacity of CDW systems o-TaS3, K0.3MoO3, (TaSe4)2I, NbSe3 and K2Pt(CN)4Br0.3 x 3.2 H2O (KCP) is characterized by the power law contribution at lowest temperatures [2, 3] and the maximum in Cp/T3 in excess to the regular Debye heat capacity [3]. These features correspond very well to the power law contribution from low energy excitations and the Boson peak in glasses. We discuss the results in the light of the glass transition observed in the low frequency dielectric response of CDW systems [4]. The phase excitations which govern the low energy properties of CDW are strongly affected by the interaction with random impurities. Impurity pinning breaks CDW in the domains of correlated phase, opens the gap in the acoustic-like phason spectrum and leads to the resonant response in the 10-100 GHz frequency range [1]. Low frequency CDW dynamics freezes at temperatures where the number of free carriers excited across the CDW gap is not sufficient to screen the long range CDW deformations across the domain boundaries [4]. Below the glass transition the coherent phase excitations are confined to the domains of characteristic micron sizes which determines a sort of as Ioffe-Regel limit for phason propagation. The maximum in Cp/T3 in CDW systems can be well described by the acoustic dispersion with the low frequency cut-off corresponding to the pinning resonance frequency and consistent with the phase domain size [5]. [1] G. Grüner, Rev. Mod. Phys. 60 (1988) 1129 and references therein [2] K. Biljaković, Phys. Rev. Lett. 96 (2006) 039603 [3] K. Biljaković, et al., Physica B 404 (2009) 456 and references therein [4] D. Starešinić et al., Phys. Rev. B 65 (2002) 165109 ; ibid. 69 (2004) 113102 [5] K. Biljaković et al., Phys. Rev. Lett. 57 (1986) 1907 ; J. Odin et al., Eur. Phys. J. B 24 (2001) 315