Spin of a multielectron quantum dot and its interaction with a neighboring electron

TL;DR

This study explores the spin states of multielectron quantum dots and their interaction with neighboring electrons, revealing occupancy-dependent spin configurations and exchange behaviors using spectroscopy and coherent measurements.

Contribution

It provides the first detailed experimental characterization of spin states in multielectron quantum dots across multiple occupancies, supported by a Hubbard model interpretation.

Findings

Even occupancies mostly show spin-0 ground states with no exchange interaction.

Odd occupancies exhibit spin-1/2 ground states with sign-changing exchange interactions.

A Hubbard model with Hund's rule and orbital effects qualitatively explains the observed exchange profiles.

Abstract

We investigate the spin of a multielectron GaAs quantum dot in a sequence of nine charge occupancies, by exchange coupling the multielectron dot to a neighboring two-electron double quantum dot. For all nine occupancies, we make use of a leakage spectroscopy technique to reconstruct the spectrum of spin states in the vicinity of the interdot charge transition between a single- and a multielectron quantum dot. In the same regime we also perform time-resolved measurements of coherent exchange oscillations between the single- and multielectron quantum dot. With these measurements, we identify distinct characteristics of the multielectron spin state, depending on whether the dot's occupancy is even or odd. For three out of four even occupancies we do not observe any exchange interaction with the single quantum dot, indicating a spin-0 ground state. For the one remaining even occupancy, we observe an exchange interaction that we associate with a spin-1 multielectron quantum dot ground state. For all five of the odd occupancies, we observe an exchange interaction associated with a spin-1/2 ground state. For three of these odd occupancies, we clearly demonstrate that the exchange interaction changes sign in the vicinity of the charge transition. For one of these, the exchange interaction is negative (i.e. triplet-preferring) beyond the interdot charge transition, consistent with the observed spin-1 for the next (even) occupancy. Our experimental results are interpreted through the use of a Hubbard model involving two orbitals of the multielectron quantum dot. Allowing for the spin correlation energy (i.e. including a term favoring Hund's rules) and different tunnel coupling to different orbitals, we qualitatively reproduce the measured exchange profiles for all occupancies.