Written on 06-Sep-2008 by oahost
Antibonding hole ground state in artificial molecules
J.I.Climente^1,^2 *, M. Korkusinski^3 , M.F. Doty^4, M. Scheibner^4, A.S. Bracker^4, G. Goldoni^2, D. Gammon^4, and P. Hawrylak^3
^1Departament de Química Física i Analítica, Universitat Jaume I, Castellon, Spain
^2 National Research Center S3, CNR-INFM, Modena, Italy
^3 Institute of Microstructural Sciences, National Research Council, Ottawa, Canada
^4 Naval Research Laboratory, Washington, USA
* Email: firstname.lastname@example.org
Resonant tunneling of carriers between vertically coupled quantum dots enables the formation of hybridized, molecular-like orbitals which are important in many quantum dot-based devices, including those aiming at optically-controlled quantum information storage. The differences in size and composition of quantum dots is overcome by the application of the vertical electric field, which brings the two quantum dot levels into resonance and induces either electron or hole tunneling. The tunneling of electrons is now well understood[1-4], it leads to the formation of bonding molecular ground states in analogy to natural diatomic molecules. However, tunneling of holes does not have a counterpart in diatomic molecules and is less understood. In fact, previous atomistic calculations suggested a reversal of bonding and antibonding hole molecular ground states as the interdot barrier distance increases.[5-7]
In this work, we present theory and experimental observation of the formation of the antibonding hole molecular ground state. Using a 4-band k·p approximation, the hole states are described as Luttinger spinors, which contain all the relevant symmetries. It is shown that the strong spin-orbit interaction in the valence band breaks the parity in the growth direction, mixing bonding and antibonding heavy- and light-hole components of the spinor. This mixing destabilizes (stabilizes) the otherwise pure bonding (antibonding) states, leading to the state reversal. Molecular ground states are then found to have up to ~95% antibonding character. These conclusions are reproduced by numerical, atomistic multi-million-atom calculations using a sp^3d^5s* tight-binding model applied to the realistic self-assembled InGaAs/GaAs double quantum dot structures, including strain, structural asymmetries and vertical electric fields. The results are in qualitative agreement with the k·p theory and predict a bonding-to-antibonding ground state reversal at interdot distances of d»2 nm. Clear experimental evidence of this peculiar hole behavior is found in magneto-photoluminescence experiments of double dots. The character of the hole molecular orbitals is identified from the electric field dependence of the Zeeman splitting of the neutral exciton when resonant hole tunneling is induced. Comparison of samples with different inter-dot separation shows the bonding-to-antibonding ground state reversal in agreement with theory .
 M. Korkusinski et al., Proceedings of the 27th Int. Conf. Phys. Semicond., 685 (2005).