Exciton Fine Structure in Lead Chalcogenide Quantum Dots: Valley Mixing and Crucial Role of Intervalley Electron–Hole Exchange

Abstract

We study the exciton fine structure in quantum dots of multivalley lead chalcogenides. We demonstrate that intervalley electron−hole exchange interaction, ignored in previous studies, dramatically modifies the exciton fine structure and leads to appearance of the ultrabright valley-symmetric spin-triplet exciton state dominating interband optical absorption. Valley mixing leads to brightening of other symmetry-allowed spin-triplet states which dominate low-temperature photoluminescence.

Optical transitions, exciton radiative decay, and valley coherence in lead chalcogenide quantum dots

“…The first term in the right-hand side of Eq. ( 60) gives the resonant frequency renormalization due to the electronhole long-range exchange interaction [15]. For the imaginary part of Eq.…”

“…Recently it has been shown [15] that the inter-valley electron-hole exchange interaction leads to a formation of the valley-symmetric ultrabright spin-triplet state of the direct exciton (i.e. exciton with the electron and the hole sharing a valley) and renders all other exciton states optically inactive.…”

“…The splitting between the ultrabright triplet and all the other states is N = 4 times as large as the spin splitting between the bright and dark states obtained in the model of independent valleys. Mixing of the exciton states from different valleys at the surface of the QD distorts the ultrabright state [15]. Nevertheless, physics underlying its formation needs special attention.…”

“…In Ref. 15 emergence of the ultrabright state was described for a Coulomb exciton. Now the question arises, how formation of the ultrabright state can be described in terms of the mechanical exciton interacting with the longitudinal electric field.…”

Interplay of Bright Triplet and Dark Excitons Revealed by Magneto‐Photoluminescence of Individual PbS/CdS Quantum Dots

“…than those of II-VI and III-V QDs, which are only composed of eight states. [18] Theoretical models predict the roles of the electron-hole exchange interaction [18][19][20][21] and the valley mixing [19][20][21][22][23][24] in splitting this highly degenerate state into several groups of states that are distributed closely in energy but are widely varying in optical transition dipole moment from completely forbidden to fully allowed. It is remarkable that, as in the case of II-VI and III-V QDs, some theories predict the lowest energy state of this manifold to be the optically forbidden, or "dark," exciton state located tens of meV below the first optically active "bright" exciton.…”