author: Chris Hooley
Theoretical interest in the single-impurity Kondo model has recently been revived by measurements of the conductance through a quantum dot in the Kondo regime. At low bias voltages, the physics is well understood: spin exchange between the dot and the leads causes the formation of an Abrikosov-Suhl resonance in the density of states, leading to greatly enhanced conductance. On the other hand, the question of what becomes of the state of the system as the voltage bias is increased is still not fully answered. Most of the work done hitherto has either involved approximations such as the non-crossing approximation (NCA) or the use of renormalisation techniques. But the applicability of many of the approximations used is far from obvious, and it is equally clear that the interpretation of renormalisation out of equilibrium may be a subtle matter. It is therefore worthwhile to search for a simple indication of whether a crossover to a non-trivial low temperature state occurs even at large bias voltages. The simplest indication of this in equilibrium is that perturbation theory breaks down at a certain temperature scale T_K. I present the latest results in the analogous (but rather more subtle) calculation for the out-of-equilibrium case, and outline what information on crossover scales can be extracted. Prospects for future work are briefly discussed.