FIGURE 3. The many-electron ground states and their explanation by spin-orbit interaction.

a, G = dI/dV_sd, measured as a function of gate voltage, V_g, and magnetic field, , showing Coulomb blockade peaks (carrier addition spectra) for the first four electrons and the first two holes (data are offset in V_g for clarity). b, Incorrect interpretation of the addition spectrum shown in a using a model with exchange interactions between electrons. Dashed/solid lines represent addition of down/up spin moments. The two-electron ground state at low fields, indicated at the left, is a spin triplet. c, Comparison of the measured two-electron addition energy from a with the one-electron excitation spectrum from Fig. 2e. d, Schematic explanation of the data in a using electronic states with spin–orbit coupling: The two-electron ground state at low fields, indicated on the left, is neither a spin-singlet nor a spin-triplet state.