The theorist’s ßingle molecule transitor” (resonant level coupled to leads) is one of the simplest examples of a quantal non-equilibrium system. A model in which the resonant level is coupled to a local phonon is formulated as a path integral along the ‘Keldysh’ contour. The strong coupling limit is studied by a nonequilibrium instanton approximation, using methods developed by Ng for the nonequilibrium X-Ray Edge problem. Nonequilibrium effects are explicitly shown to introduce a decoherence, causing the instanton expansion to converge and preventing the formation of a coherence peak. The methods generalize straightforwardly to the nonequilibrium Kondo problem.

Columbia University