Spectral Diffusion due to Tunneling Processes at very low Temperatures

Hans Maier, Karl-Peter Muller, Siegbert Jahn, and Dietrich Haarer 

Pioneered by the work of Zeller and Pohl [1] it was discovered in the early 1970s that amorphous solids show low-temperature thermal and acoustic properties which are very different from those observed in crystals. For reviews see for example Refs. [2, 3]. The most well-known of these anomalous properties is the specific heat, which is in general considerably larger than would be expected from the Debye model and varies linear with temperature in contrast to the Debye 7 -dependence. Other anomalies are the temperature dependence of the thermal conductivity, the properties of phonon echoes, and ultrasonic absorption. These features seem to be quite universal for all kinds of amorphous solids, irrespective of their chemical composition and structure, i. e. inorganic as well as organic or polymeric. 

The sensitivity of optical experiments on amorphous solids was hindered, in many instances, by the large inhomogeneous broadening which arises from the distribution of local environments of the involved optical transitions. Therefore a very important step was the discovery of persistent spectral hole burning in 1974 [8, 9]. This method of high-resolution laser spectroscopy eliminates the inhomogeneous effects induced by the static disorder in 

68 Macromolecular Systems Microscopic Interactions and Macroscopic Properties Deutsche Forschungsgemeinschaft (DFG) 

Copyright 2000 WILEY-VCH Verlag GmbH, Weinheim. ISBN 978-3-527-27726-1 

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Spectral Diffusion due to Tunneling Processes at very low Temperatures... 

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