Supercoupling between heavy-hole and light-hole states in self-assembled quantum dots

TL;DR

This paper reveals a new mechanism called supercoupling that significantly enhances heavy-hole and light-hole mixing in quantum dots, independent of strain, impacting spintronics and quantum computing applications.

Contribution

It introduces the concept of supercoupling via intermediate states, challenging current models that attribute HH-LH mixing primarily to strain effects.

Findings

Supercoupling amplifies HH-LH mixing in quantum dots.

Intermediate states enable strong coupling regardless of strain.

Unstrained GaAs/AlAs dots show unexpectedly large HH-LH coupling.

Abstract

Spintronics, quantum computing and quantum communication science utilizing cubic semiconductors rely largely on the properties of the hole states, composed of light and heavy hole wavefunction components. The admixture of light-hole (LH) into ground hole state predominately by the heavy hole (HH) would induce unique features of LH in optical transitions, spin relaxation, and spin polarization. We point to an unexpected source of HH-LH mixing in quantum dots, arguing that in contrast with current models the mixing does not reflect the strain between the dot and its matrix and does not scale inversely with the energy splitting between the bulk HH and LH states. Instead, we show via atomistic pseudopotential calculations on a range of strained and unstrained dots of different symmetries that the HH-LH mixing is enabled by the presence in the QD of a dense ladder of intermediate states between the HH and LH states which amplifies and propagates this interaction and leads to "supercoupling" (analogous to super-exchange in magnetism). This explains a number of outstanding puzzles regarding the surprising large coupling seen in unstrained QD (GaAs/AlAs) of ideal shapes and the surprising fact that in strained QD (InAs/GaAs) the coupling is very strong despite the fact that the 12-fold increase in bulk HH-LH splitting overrides the ~4 fold enhancement of the coupling matrix element by strain in comparison with unstrained GaAs QDs.