| speaker: | Hans Kroha Universität Bonn, Germany |
| time: | Fr., 24.08, 11:10-12:00 |
We propose a physical realization of the two-channel Kondo (2CK) effect, where a dynamical defect in a metal has a partially broken SU(3) symmetry with a unique ground state and twofold degenerate excited states. The defect can be comprised of an interstitial atom moving in a modulated Mexican hat potential, which is formed by the lattice. A perturbative renormalization group analysis shows that the coupling to the conduction electrons renormalizes the excited defect levels below the non-interacting ground state, thus stabilizing the 2CK fixed point. For a wide range of parameters the level crossing occurs in the weak coupling region. The model contains two correlation-induced, exponentially different low-temperature scales, corresponding to transitions between the degenerate defect levels and to transitions between the ground and the excited impurity states, respectively. Using perturbative renormalization group calculations out of equilibrium, we show that the model describes in a natural way spikes in the differential conductance of a point contact with 2CK impurities, whose width is much sharper than the 2CK Kondo temperature. These spikes are due to Kondo-enhanced transitions between the ground and the excited defect levels. Thus, the model explains, on the same footing, the two main features observed previously in ultrasmall metallic point contacts [D. C. Ralph and R. A. Buhrmann, Phys. Rev. Lett. 69, 2118 (1992)], i.e. the zero-bias anomaly and the sharp conductance spikes at elevated bias.