Orbital Kondo-effect from tunneling impurities

The article reviews recent results for the low energy physics of fast tunneling centers in metallic environments. For strong enough couplings to the environment these tunneling centers display an orbital Kondo effect and give rise to a non-Fermi-liquid behavior. This latter property is explained by establishing a mapping of the tunneling center model to the multichannel Kondo model via the renormalization group transformation combined with a $1/N_f$ expansion. The case of M-state systems, the role of the splittings and the present experimental situation are also discussed.