Non-Equilibrium Disordered Quantum Systems

Structural disorder typically affects the dynamics of non-equilibrium quantum systems and leads to a variety of unique phenomena that are investigated at extremely low temperatures with new methods. The focus is on the physical realization of such systems in the form of amorphous solids, disordered crystals and spin glasses. Fundamental questions on the interplay of disorder and many-particle interaction, on the microscopic nature of low-lying states, on relaxation and decoherence channels, on dissipative dynamics and on the occurrence of complex collective phenomena are investigated. Current projects focus on the surprising influence of nuclear degrees of freedom on the dynamics of atomic tunnel systems in structurally disordered systems such as multicomponent glasses and polymers. In particular, the nonlinear response and phase coherence of such systems are investigated.

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.