  We consider a two-orbital impurity system with intra and inter-level Coulomb
repulsion that is coupled to a single conduction channel. This situation can
generically occur in multilevel quantum dots or in systems of coupled quantum
dots. For finite energy-spacing between spin-degenerate orbitals, an in-plane
magnetic field drives the system from a local singlet ground state to a
"mixed-level" Kondo regime, where the Zeeman-split levels are degenerate for
opposite spin states. We use the numerical renormalization group approach to
fully characterize this mixed level Kondo state and discuss its properties in
terms of the applied Zeeman field, temperature and system parameters. Under
suitable conditions, the total spectral function is shown to develop a Fermi
level resonance, so that the linear conductance of the system peaks at a finite
Zeeman field while it decreases as function of temperature. These features, as
well as the local moment and entropy contribution of the impurity system are
commensurate with Kondo physics, which can be studied in suitably tuned quantum
dot systems.
