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Disordered Matter


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Disordered matter is ubiquitous in our daily live. Especially its electrical applications are numerous, e.g. the use of doped crystalline and amorphous semiconductors as standard materials for electronics or solar cells, glasses and polymers as insulator materials, or ionic conductors as solid electrolytes for accumulators and fuel cells. In our group, disordered matter is mainly investigated using broadband dielectric measurements, covering a very broad frequency range of more than 18 decades. In addition, we perform nonlinear dielectric spectroscopy, where extremely high voltages up to 2000 V are applied to the samples.  

Among the materials investigated are amorphous materials as glass forming liquids and glasses, including also ionically conducting glass formers, plastic crystals, where the molecules are disordered with respect to their orientational degrees of freedom only, and doped semiconductors having substitutional disorder.

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Typical loss spectrum of glassforming liquids

 

More details on the investigation of disordered matter in our group can be found at the following links:

strukturFrom a theoretical point of view, the dielectric response and ac conductivity of disordered matter is of interest for a number of reasons. For example, dielectric spectroscopy is the ideal tool to investigate the zoo of different dynamic processes prevailing in supercooled liquids, which to a large extend are not understood very well until now and often are considered as the key to achieve a better understanding of the glass transition. In addition, charge transport in disordered matter very often is dominated by hopping transport of localized charge carriers which shows a characteristic frequency-dependence of the complex conductivity, giving hints towards the nature of charge carriers and transport mechanisms. Considerable interest also focuses on doped semiconductors that are close to the metal-to-insulator transition, where e.g. the role of Anderson localization needs to be clarified. Interestingly, in the past years a number of universalities in the response of disordered matter to electromagnetic fields were found, indicating common microscopic processes in such different materials as, e.g., supercooled liquids and doped semiconductors, a finding that certainly deserves further investigation.

hopping 

Also from a technological point of view, it is important to learn more about the response of disordered matter to electromagnetic fields. For example, currently an increasing number of devices (for example in wireless communication systems or computer techniques) operate at progressively higher frequencies extending well into the microwave regime. Thus the knowledge of the response of materials usedFliege2 for electronic applications as, e.g., insulators or capacitive elements, in as broad a frequency range as possible is becoming increasingly important. In addition, having in mind that the frequency spectrum used by modern mobile communications systems will more and more extend far into the GHz regime, the knowledge of the interaction of this ultra-high frequency radiation with biological systems (mostly being composed of disordered matter) will be of utmost importance to fix liminal values for the exposure of man to non-ionizing radiation.