Prof. Dr. Stefan Kehrein
Arnold Sommerfeld Center
for Theoretical Physics
4th floor, Room 418
My main research interest is the non-equilibrium behavior of quantum many-body systems. In the past decades, condensed matter theory has mainly focused on understanding the equilibrium or linear response properties of condensed matter systems. Recent experimental advances in ultracold gases and nanostructures have opened up the exciting new field of non-equilibrium quantum many-body physics, where many fundamental questions like return to equilibrium, physics beyond the linear response regime, etc., are largely unanswered. This has led to a lot of activity and we are pursuing some of these questions using mainly analytical methods, especially the flow equation approach (method of infinitesimal unitary transformations). [More]
D. Taubert, C. Tomaras, G. J. Schinner, H. P. Tranitz, W. Wegscheider, S. Kehrein, and S. Ludwig
Relaxation of hot electrons in a degenerate two-dimensional electron system: transition to one-dimensional scattering
arXiv:1104.1645, Phys. Rev. B 83, 235404 (2011)
The energy relaxation channels of hot electrons far from thermal equilibrium in a degenerate two-dimensional electron system are investigated in transport experiments in a mesoscopic three terminal device. We observe a transition from two dimensions at zero magnetic field to quasi-one-dimensional scattering of the hot electrons in a strong magnetic field. Theoretical calculations of electron-electron scattering and the emission of optical phonons underline our interpretation in terms of a transition to one-dimensional dynamics.
Atomic force micrograph of the Hall bar sample.
C. Tomaras and S. Kehrein
Scaling approach for the time-dependent Kondo model
arXiv:1011.1281, Eur. Phys. Lett. 93, 47011 (2011)
We develop a generalization of the flow equation method to time-dependent Hamiltonians. We apply these ideas to a Kondo model with a ferromagnetic exchange coupling switched on over a time scale τ. The asymptotic expectation value of the impurity spin interpolates continuously between its quenched and adiabatic value.
Non-adiabacity of the interaction quench as a function of the time scale τ: Weak instantaneous quenches have μ=2, the adiabatic limit corresponds to μ=1.
M. Heyl and S. Kehrein
The X-ray edge singularity in quantum dots
We investigate the X-ray edge singularity problem realized in noninteracting quantum dots. We analytically calculate the exponent of the singularity in the absorption spectrum near the threshold and extend known analytical results to the whole parameter regime. In particular, we verify the validity of the Hopfield rule of thumb in this exactly solvable nonequilibrium problem.
Possible experimental realization of x-ray edge physics in a noninteracting quantum dot.
M. Heyl and S. Kehrein
The Crooks relation in optical spectra - universality in work distributions for weak local quenches
We show that work distributions and non-equilibrium work fluctuation theorems can be measured in optical spectra for a big class of quantum systems. For the particular case of a weak local perturbation, the Crooks relation establishes a universal relation in absorption as well as in emission spectra. Due to a direct relation between the spectra and work distribution functions this is equivalent to universal relations in work distributions for weak local quenches. As two explicit examples we treat the X-ray edge problem and the Kondo exciton.
Crooks relation in the absorption spectrum of a Kondo exciton.
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