Tunable Few-electron Charge and Spin States in Parallel-Coupled Quantum Dots in InAs Nanowires

High-resolution spin spectroscopy is performed on parallel-coupled double quantum dots in the few-electron regime. The system consist of a InAs nanowire of zinc blende crystal structure, having a single quantum dot (QD) epitaxially defined by two thin segments of wurtzite, acting as tunnel barriers (offset ~100 meV) [M. Nilsson et al, PRB 93, (2016)]. The small axial extension of the QD (<10 nm) leads to a strong quantum confinement and enables the QD to be fully depleted of electrons. By using pairs of local side gates and a global back gate the system can be tuned from one QD into parallel double QDs, for which we can control the populations down to the last electrons. The combination of hard-wall barriers to source and drain, shallow inter-dot tunnel barriers, and very high single-QD excitation energies (up to 27 meV), allow an order of magnitude tuning of the strength for the first intramolecular bond. In addition, the consistently large |g*|-factors (~9) facilitate detailed studies of the B-field dependence of the 1- and 2-electron states. Specifically, we find that it is possible to tune the magnitude of the B-field induced singlet-triplet anti-crossing by changing the inter-dot tunnel coupling. We model the experimental data using a simple few-electron Hamiltonian.