engleski

Contribution to the investigation of low energy excitations in quasi one-dimensional systems

The basic properties of the charge density wave (CDW) ground state have been thoroughly studied experimentally and explained theoretically, but the additional features due to low energy excitations (LEEs) typical for glassy systems have been considered systematically only in rare instances. This work represents such an effort to characterise in detail the LEEs in the CDW state of the quasi one-dimensional material o-TaS_3. We focused mostly on the dielectric properties, which we measured in a wide temperature (5 K-300 K) and frequency (10 mHz-100 MHz) range using dielectric spectroscopy and thermally stimulated discharge techniques. In addition, we measured the thermal capacity in the same temperature range. Our results show the existence of a new low temperature state of the CDW. The transition to the new state is represented by a dynamical crossover, typical for glasses, indicating the continuous freezing of certain degrees of freedom at a finite temperature. In the light of the existing theories of CDW dynamics, in the high temperature state the CDW acts as an elastic medium allowing for the continuous large scale deformations that govern the dynamics. The freezing of elastic deformations is due to Coulomb interactions that becomes important as the screening by free carriers decreases with its density. The freezing temperature corresponds to the temperature at which the density of free carriers decreases below one carrier per domain of CDW coherence. In the low temperature state the elastic degrees of freedom are frozen and the dynamics is governed by the local plastic deformations of CDW, i.e. topological defects such as solitons or dislocation loops. The observation of the crossover at the freezing temperature in the scaling analysis of the thermal capacity data indicates a strong interaction between the CDW and the host lattice in the absence of screening.

charge-density-wave systems; collective effects; dielectric properties of solids and liquids; domain structure; hysteresis; glass transitions; thermal properties of amorphous solids and glasses: heat capacity; thermal expansion; etc.

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