Multiple-welled tunnelling systems in glasses at low temperatures.

The thesis examines anomalous effects in the physical properties of some multicomponent glasses at low temperatures in the absence and in the presence of a magnetic field. These anomalous effects can be explained well in terms of a new (anomalous) tunnelling systems (ATS) model. ATS in glasses are provided by the process of partial devitrification of the glassy network due to the presence of network-modifying ions in real glasses. A new probability distribution, inversely proportional to the energy asymmetry, takes into account partial devitrification. The effect of the magnetic field is explained by introducing the Aharonov-Bohm coupling into the Hamiltonian of a single ATS. The multi-welled ATS together with the standard two-level tunneling systems explain qualitatively, as well as quantitatively and with reasonable parameters, the relative change of the dielectric permittivity and loss at zero and weak magnetic fields for multicomponent glasses such as AlBaSiO (or BAS) and BK7. They also explain the relative change of the dielectric permittivity and heat capacity with temperature T and with alkali concentration x in the mixed (SiO2)1-x(K2O)x glass. The polarization echo’s theory in glasses has been improved and extended to the case of the independent ATS model describing glasses in a magnetic field. The agreement between theory and experiment is highly satisfactory, given the simplifications used in the theory. The isotope substitution effect on the dipole-echo amplitude also can be explained in a simple way in our model. The interpretation of the extracted material parameters brings us to confirm the existence of coherent tunneling clusters of N true particles, with a value of N ranging from about 25 at the lowest temperatures, to about 600 at the higher temperatures.