Universality of anomalous excitations and diffusion effects in amorphous solids and glasses

professor M. Schulz (Ulm University)


Thermodynamic and electric properties of glasses were intensively investigated during the last decades. An important result of this research was the discovery of several universal properties that are only weakly dependent on the special structure of the amorphous solid. These `anomalous' properties are normally unknown for crystals. Such properties can be observed in the low temperature behavior of the specific heat, the thermal conductivity, the propagation of ultrasound, dielectric loss of glasses as well as in phenomena related to the transport and diffusion of ions close to and below the glass transition point. Firstly, we will give a general proof for the universal existence of anomalous, non-Debye excitations in amorphous is proposed. Starting from an ideal amorphous solid with fixed mean positions of all atoms and harmonic interactions, it was demonstrated that any increase of disorder leads stringently to an increase of the spectral density for low (and also very high) frequencies. That means, the density of states shows additional contributions to the low frequency part of the standard Debye density, even the wave length of corresponding phonon like excitations is of an order of magnitude on which the amorphous solid is still homogeneous. Secondly, a mesoscopic model will be presented which allows the description of anomalous transport effects in glasses at and below the glass transition point. Such effects are known as mixed alkali or mixed mobile ion effect and they dominate the AC and DC properties of several glasses. It will be shown that the interplay of an effectively short ranged two particle interaction between the typical charge carriers and a local, component-sensitive interaction between the charged particles and their disordered environment leads above a critical interaction strength to a universal anomalous dependence of the conductivity on the composition ratio of the mobile charge carriers. The consideration of possible structural relaxations of the glassy environment supports these effects essentially, so that especially the difference between the conductivity of glasses obtained from an partial ion exchange above and below the glass transition point can be explained.