Spin exchange energy for a pair of valence band holes in artificial molecules

Abstract

We study the exchange energy for two interacting holes in vertically stacked InGaAs quantum dots using the configuration interaction method and an axially symmetric 4-band Kohn-Luttinger Hamiltonian that introduces heavy-hole and light-hole subbands mixing due to the spin–orbit interaction. We demonstrate that, as a consequence of the band mixing, a singlet–triplet degeneracy appears for a single specific value of the interdot barrier, for which the bonding and antibonding heavy hole have identical energy. We explain this degeneracy using a simple model matrix Hamiltonian as due to the interaction effects for a degenerate ground state of the hole. We demonstrate that, in conditions of the singlet–triplet degeneracy due to hybridization of the bonding and antibonding energy levels, the spin exchange energy becomes insensitive to external electric fields.