Out of Equilibrium Quantum Systems

In practically all disordered solds - in glasses but also in crystals with certain point defects structural rearrangements take place via quantum tunneling of atoms or groups of atoms.  These degrees of freedom drastically change the macroscopic behaviour of these materials at low temperautures. Within several projects we are investigating the properties of atomic tunneling centers in various model systems. The spectrum of research activities spans from the investigation of the microscopic nature and the dynamics of individual tunneling systems to the complex collective behaviour of coupled systems.

Phase Coherence and Energy Relaxation of Atomic Tunneling Systems

Phase coherence and energy relaxation of atomic tunneling systems in dielectric amorphous solids are investiged by means of polarisation echo experiments. The results of such measurements allow to draw conclusions of the distribution function of tunneling systems and their coupling with thermal phonons. At ultralow temperatures the interaction between tunneling systems and resulting collective tunneling processes are studied.

Bulk Metallic Glasses at Ultralow Temperatures

Atomic tunneling systems in bulk metallic glasses  interact with phonons and conduction electrons. The latter interaction can be switched on and off in superconductive metallic glasses using sufficiently high magnetic fields. In this way renormalization effects of the density of states of tunneling systems by conduction electrons and electron induced relaxational processes can be studied.

Influnence of Nuclear Spin on Atomic Tunneling Systems

Glasses show a surprizing magnetic field dependence of their dielectric properties at ultralow temperatures. These effects are caused by nuclear moments and can be used to obtain detailed information about the largely unknown microscopic nature of atomic tunneling systems in amorphous solids. Measurements of the magnetic fingerprint of model glasses are performed at ultralow temperatures und compared to numerical calculations.

Noise as Signal

The thermal noise limits the precision of measurements but can also be used as an interesting and unique signal. Measurements of the dielectric noise at the glass transition are used to draw conclusions about the dynamics of structural units of glasses even under non-equilibrium conditions. In addition, the thermally driven current noise of metallic conductors is utilised the develop novel thermometers for ultralow temperatures.

 
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