| The universal low temperature properties of amorphous solids are well described by the two level system (TLS) model. The model assumes that the low energy excitations in glasses consist of atoms or groups of atoms which can tunnel between two low lying energy states. The assumption made in the original model is that the parameters which describe each TLS are widely distributed, and it is this broad distribution which leads to the universality seen at low temperatures. However, there is no a priori reason for assuming a flat distribution, and there is evidence suggesting that the distribution of TLS changes at low energy. This thesis focuses on three sets of experiments which probe the density of states at low temperatures. The first two experiments study the effects of large DC fields on the dielectric constant of amorphous solids, and the third is a measurement of the thermal conductivity down to 10 mK, lower than ever previously achieved. The experiments are presented and discussed, taking into account implications on the density of states of TLS. |
