Auxiliary master equation approach to the Anderson-Holstein impurity problem out of equilibrium

We introduce a method based on the auxiliary master equation for solving the problem of an impurity with local electron-electron and electron-phonon interaction embedded between two conduction leads with a finite-bias voltage. The Anderson-Holstein Hamiltonian is transformed to a corresponding Lindblad equation with a reduced set of sites, providing an optimal approximation of the hybridization function. The problem is solved in the superfermion representation, using a configuration interaction for fermions and bosons. The phonon basis is shifted and rotated with the intention of permitting a low phonon basis cutoff, even in the strong-coupling regime. We benchmark this approach with the numerical renormalization group in equilibrium, finding excellent agreement. We observe, however, that the rotation brings no advantage beyond the bare shift. This is even more apparent out of equilibrium, where issues in convergence with respect to the size of the phononic Hilbert space occur only in the rotated basis. As an application of the method, we explore the evolution of the phononic peak in the differential conductance spectra with changing phonon frequency.