Two-Stage Kondo Effect in a Quantum Dot at a High Magnetic Field

We report a strong Kondo effect (Kondo temperature $\sim 4\mathrm{K}$) at high magnetic field in a selective area growth semiconductor quantum dot. The Kondo effect is ascribed to a singlet-triplet transition in the ground state of the dot. At the transition, the low-temperature conductance approaches the unitary limit. Away from the transition, for low bias voltages and temperatures, the conductance is sharply reduced. The observed behavior is compared to predictions for a two-stage Kondo effect in quantum dots coupled to single-channel leads.