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Peter Schwab
Impurity Spin Dynamics and Quantum Coherence in Mesoscopic Rings
Supervisor: Prof. U. Eckern [Theoretical physics II]
Date of oral examination: 03/08/1996
80 pages, english
In the presence of a magnetic field, a normal-metal ring carries an equilibrium current, usually called persistent current. In rings where the electron motion is diffusive, several mechanisms which produce a persistent current have been found: A persistent current exists, if the electrons can diffuse around the ring without loosing their phase coherence. However, none of the mechanisms known can explain the amplitude of the currents measured in the experiments. We study the effect of paramagnetic impurities on the persistent current. Magnetic impurities tend to destroy quantum coherence. However, by freezing out the spin dynamics in a magnetic field, the amplitude of the typical current, here given by the current fluctuations, is of the same order as without magnetic impurities. If the Thouless energy $E_c$ and the temperature are below the Kondo temperature, the impurity spins are effectively screened, the magnetic impurities scatter like nonmagnetic impurities. Although we cannot explain the large currents observed experimentally, our results for the current as a function of parameters like the impurity concentration, magnetic field and so on, may serve as a test for the applicability of the theoretical concepts (in comparison with future, systematic experiments). We also consider quantum corrections to the free energy of the magnetic impurities; due to these corrections we identify a new contribution to the persistent current. The current as a function of temperature is of the order $I \sim (E_c^2/\phi_0 T) \cdot \exp( -T/3E_c )$, which is larger than the peristent current without magnetic impurities. The current depends crucially on spin-orbit scattering: Without spin-orbit scattering, we find diamagnetic currents, and for strong spin-orbit scattering, we find paramagnetic currents.