engleski

Phase excitations in charge density wave systems versus boson peak and TLS in glasses

The dynamics of charge density waves (CDW) is described by the temporal changes of the amplitude and the phase of the complex CDW order parameter. Phasons (phase excitations) are acoustic-like low energy degrees of freedom and therefore govern low-frequency CDW response. Dynamics of the CDW dielectric glass transition strongly resembles scenario of freezing in supercooled liquids [1]. The well known CDW pinning is characterized by the resonant response at GHz frequencies. Screening by free carriers is tightly bound with the deformation of CDW and its relaxational dynamics giving the criterium for freezing at Tg [1]. CDW glassy phenomenology is completed with low-energy excitations, the peak in heat capacity (Cp/T3), long-time energy relaxation and aging found at low-T [2]. Relevant degrees of freedom in this new class of glass concern CDW superstructure on characteristic scales given by the size of the phase coherence length (lϕ~μm). As the lattice distortion associated with the CDW can be thought of as being a “frozen” phonon, we discuss its possible manifestations in relation with the phenomenology of Boson peak in glasses as freezing of incomplete softening of superstructural mode near glass transition temperature. The Boson peak frequency in CDW glass phenomenology can be seen as Ioffe-Regel limit for phason propagation, consistent with the pinning resonance (gap in the phason dispersion). Very well understood aspects of the microscopic picture in CDW systems dealing with pinning, screening and phasons as relevant excitations of defective superstructure (on mesoscopic, μm scales) and the parallels we draw with glass forming systems as defective soft structures (on nm scales) [3] might be of mutual interest for both fields.

glasses; low-energy excitations

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