Abstract
Low-temperature anomalies in the thermal, acoustic and dielectric properties of amorphous solids have found increasing attention in recent years.1 Compared with crystals, these materials exhibit an anomalously high specific heat and low thermal conductivity2. This behaviour can be explained by the existence of low-energy excitations with a nearly constant density of states in an energy range extending at least between 10 mK and 10 K. More detailed information has been obtained by ultrasonic and dielectric measurements3 which demonstrate that these excitations are best described as two-level systems. According to a generally accepted model they originate from small groups of atoms, which are energetically nearly equivalent, tunnelling between two positions in the amorphous matrix.4 About the physical nature of the tunnelling “particles”, however, not much has previously been known.
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