Spin tunneling in optically excited quantum dot molecules: Controlling g-factors with electric field

We have recently demonstrated coherent tunneling of electron and hole spin between two quantum dots using optical spectroscopy [1,2]. In the case of a hole spin, a very large and resonant enhancement or reduction of g-factor is controlled with an applied electric field [3]. This effect arises because of the corresponding enhancement or suppression of the hole wavefunction in the tunnel barrier for the bonding (symmetric) and anti-bonding (anti-symmetric) states, respectively. This effect was discovered for single holes, but also occurs for two-particle states (two holes or 1 hole and 1 electron). Using this effect to identify the symmetry of the wavefunction, we have now found that the energetic order of the bonding and anti-bonding molecular states goes through a reversal as a function of tunnel barrier thickness. That is, the bonding state is the low energy state for a 2nm barrier thickness (as expected in the simple particle-in-a-box model, or the one-band effective mass theory). But for thicknesses larger than 3nm, a transition occurs such that the anti-bonding state becomes the low energy state. This dramatic and non-intuitive effect arises from the spin-orbit interaction. \newline \newline [1] ``Optical Signatures of Coupled Quantum Dots,'' E. A. Stinaff \textit{et al}, \textit{Science }\textbf{311}, 636 (2006). \newline [2] ``Spin Exchange in Optically Excited Quantum Dot Molecules,'' M. Scheibner, M. F. Doty, I. V. Ponomarev, \textit{et al}., \textit{PRB} \textbf{75}, 245318 (2007). \newline [3] ``Electrically Tuneable g Factors in Quantum dot Molecular Spin States'' M.F. Doty \textit{et al., Phys. Rev. Lett.} \textbf{97}, 197202 (2006).