Low Temperature Properties of Glasses and Mixed Crystals


Glassy and amorphous materials exhibit, at low temperatures, a number of properties which are considered anomalous in comparison to those of their crystalline counterparts. Examples are the roughly linear temperature dependence of the specific heat and the approximately quadratic temperature variation of the thermal conductivity at T < 1 K which are in contrast to the T3 behaviour of these quantities in crystals. Moreover, between approximately 1 and 20 K, one observes a crossover to a T3 behaviour of the specific heat and a plateau in the thermal conductivity. The anomalies below 1 K appear to be universal in the sense that they are shared by a large variety of amorphous systems, whereas between approximately 1 and 20 K a stronger dependence on properties of the specific material appears. At still somewhat higher temperatures, a certain degree of universality is again observed, at least in thermal conductivity data.

The anomalous low temperature properties of glasses are commonly believed to originate from localized quantum-mechanical two-level tunneling systems, which couple to phonons and other elementary excitations, as well as to external fields. Although a broad range of experimental observations can be explained in terms of this idea, the microscopic nature of these two-level systems is generally unknown. Therefore, another class of substances is often considered, namely mixed crystals. Here, the microscopic nature of the tunneling units is clear. A substitutional defect (e.g. a Li defect in a KCl host crystal) is located in one of several off-center positions, and the defect can move from one potential well to the other by quantum tunneling. Mixed crystals thus allow an immediate investigation of the physics of tunneling units.

Our research activities deal with different aspects of tunneling systems, in particular their microscopic characterisation, their interactions, and their coupling to heat baths or external fields. Our main emphasis has been on the study of microscopic models of amorphous materials, in which tunneling systems with a broad range of barrier heights and asymmetries are generated through interactions as a collective effect.

Cooperations exist with the experimental group of  C. Enss, A. Fleischmann and S. Hunklinger at the KIP in Heidelberg, one of the leading experimental teams in this field, and with the theory groups of Annette Zippelius in Göttingen and Alois Würger at Bordeaux.

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last modified: 23.09.2009  rk

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