Excitation spectroscopy of few-electron states in artificial diatomic molecules

TL;DR

This paper investigates the excitation spectra of few-electron states in artificial diatomic molecules formed by double quantum dots, revealing how electron number influences excited states and identifying complex states like the quadruplet.

Contribution

It provides a detailed classification of excited states in double quantum dots using spectroscopy and Hubbard model calculations, including the identification of normally forbidden quadruplet states.

Findings

Odd and even electron numbers lead to distinct excited states.

Higher excited states involve additional single-particle levels.

Identification of the quadruplet state despite spin blockade.

Abstract

We study the excitation spectroscopy of few-electron, parallel coupled double quantum dots (QDs). By applying a finite source drain voltage to a double QD (DQD), the first excited states observed in nonequilibrium charging diagrams can be classified into two kinds in terms of the total effective electron number in the DQD, assuming a core filling. When there are an odd (even) number of electrons, one (two)-electron antibonding (triplet) state is observed as the first excited state. On the other hand, at a larger source drain voltage we observe higher excited states, where additional single-particle excited levels are involved. Eventually, we identify the excited states with a calculation using the Hubbard model and, in particular, we elucidate the quadruplet state, which is normally forbidden by the spin blockade caused by the selection rule.