Two- and three- energy level mixing effects in vertically coupled quantum dots

We investigate high bias single electron resonant tunneling through sub-micron gated AlGaAs/InGaAs/AlGaAs/InGaAs/AlGaAs triple barrier structures for which the tunnel coupling energy between the two quantum dots is very weak (less than 0.1meV). The two quantum dot ``disks'' in the vertical diatomic artificial molecule located in the circular device mesa can be almost circular or elliptically deformed. In a device where the constituent dots are elliptically deformed, the single particle states of each dot evolve almost ideally with magnetic field, except at several of the two- and three- energy level crossings. At these crossing points, we see pronounced two level anti- crossing behavior, with levels split by hundreds of micro-eV, and intriguing level crossing phenomena, like mixing of three resonances leading to resonance suppression. We analyze the observed quantum level mixing effects using a simple three level mixing model.