Stability of spin states in quantum dots

Abstract

We have investigated electronic transport through a Coulomb-blockaded quantum dot in which interactions are strong. Linear changes in conductance peak spacings with in-plane magnetic field are observed and interpreted in terms of Zeeman splitting of single-particle levels. Thereby, the measurements allow tracking changes in the dot's ground-state spin as the dot is gradually opened to the leads and the electron number is changed. Spin states have been identified in the weak- $(kTg\ensuremath{\Gamma}),$ intermediate- $(\ensuremath{\Gamma}\ensuremath{\approx}kT),$ and strong- $(\ensuremath{\Gamma}gkT)$ coupling regime. It is found that ground states with spin $S=0$ or $S=1/2$ are most likely, while larger total spins $Sg~1$ can occasionally occur, despite the large number of 50--100 electrons. A g factor close to the bare bulk GaAs value has been determined experimentally for the majority of the spin states. A perpendicular magnetic field applied to the dot in the same state allows the investigation of spin-pair candidates under conditions where orbital effects dominate the evolution of conductance peaks. Strong correlations in the position and in the amplitude of neighboring peaks allow the final identification of spin pairs. The method of combining parallel and perpendicular magnetic fields for identifying spin states and spin-pairs works well for intermediate and strong coupling of dot states to the leads while the data in the weak-coupling regime is less conclusive. Our results indicate that the spin degree of freedom is remarkably stable and the spin states are well described within a single-particle picture.