Strong electron-electron interactions in Si/SiGe quantum dots

TL;DR

This study investigates how strong electron-electron interactions influence the properties of silicon-based quantum dots, revealing that weak confinement enhances interactions and can suppress singlet-triplet splitting, with implications for quantum computing design.

Contribution

It combines tight-binding and full-configuration-interaction methods to analyze two-electron wavefunctions in Si/SiGe quantum dots, accounting for atomic-scale disorder and revealing interaction effects on qubit properties.

Findings

Strong interactions can suppress singlet-triplet excitation energy.

Weak valley-orbit interactions allow near noninteracting singlet-triplet splitting.

Disorder and confinement strength critically influence quantum dot behavior.

Abstract

Interactions between electrons can strongly affect the shape and functionality of multi-electron quantum dots. The resulting charge distributions can be localized, as in the case of Wigner molecules, with consequences for the energy spectrum and tunneling to states outside the dot. The situation is even more complicated for silicon dots, due to the interplay between valley, orbital, and interaction energy scales. Here, we study two-electron wavefunctions in electrostatically confined quantum dots formed in a SiGe/Si/SiGe quantum well at zero magnetic field, using a combination of tight-binding and full-configuration-interaction (FCI) methods, and taking into account atomic-scale disorder at the quantum well interface. We model dots based on recent qubit experiments, which straddle the boundary between strongly interacting and weakly interacting systems, and display a rich and diverse range of behaviors. Our calculations show that strong electron-electron interactions, induced by weak confinement, can significantly suppress the low-lying, singlet-triplet (ST) excitation energy. However, when the valley-orbit interactions caused by interfacial disorder are weak, the ST splitting can approach its noninteracting value, even when the electron-electron interactions are strong and Wigner-molecule behavior is observed. These results have important implications for the rational design and fabrication of quantum dot qubits with predictable properties.