Statistical Physics and Condensed Matter

Low Temperature Properties of Glasses and Mixed Crystals

Heinz Horner, Reimer Kühn,
Cristina Picus, Michael Thesen

Former members:
Jiva Dimitrova, Uta Horstmann, Ludwig Mathey
Peter Nalbach, Peter Neu, Orestis Terzidis
Bernhard Thimmel,Jens Urmann, Alois Würger


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.

The research activities of our group deal with different aspects of tunneling systems, in particular their microscopic characterisation, their interactions, and their coupling to heat baths or external fields.

There exists a strong cooperation with the experimental group of  S. Hunklinger and C. Enss , one of the leading experimental teams in this field.

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last modified: 06.07.2003 rk