Controlling the glass transition temperature in the charge density wave superstructure

The family of glasses includes nowadays numerous systems which are often very different from the classical glassformers, i.e. supercooled liquids. Even such exotic systems as charge density wave (CDW) superstructures occurring in quasi one-dimensional metals exhibit phenomena typical for glasses [1-3] including freezing of the primary relaxation (α) process at a finite temperature Tg [1], occurrence of the secondary (β) relaxation process below Tg [1], “Boson peak”- like [2] and power-law contributions [3] to the heat capacity. Disorder in CDW glasses is introduced on the level of superstructure through the pinning of CDW to crystalline defects which break CDW in domains determined by the coherence of the complex order parameter phase. Domain interaction is controlled by the screening of residual free carriers excited over CDW gap. Increase of the relaxation time at low temperatures follows the activated increase of resistivity which plays the role of viscosity. The glass transition occurs at a temperature Tg at which there is less then one free carrier per domain and the screening is no longer effective [1]. We have varied domain sizes in CDW system o-TaS3 over two orders of magnitude by proton irradiation which created additional defects. Dielectric spectroscopy on irradiated samples shows decrease of the amplitude and the microscopic relaxation time of α process, while the activation energy increases beyond the activation energy of resistivity, demonstrating increased cooperativity of the domains. Corresponding increase of Tg depends on the irradiation dose, and consequently on the domain size, in the activated manner. It follows closely the temperature dependence of free carrier density, which does not change with the irradiation, thus corroborating the freezing criterion for CDW glass. We demonstrate that Tg is closely related to the domain size in CDW glasses. As the microscopic model of CDW dynamics is well-established, our results might contribute to the understanding of the glass phenomenology in general.