Fig. 4.

Tuning interdot coupling t_d. a–d Stability diagrams for different voltages on the tuning gate g_d while compensating on plunger gates (T = 30 mK). The plot in d is analyzed in Fig. 3 and the voltage on g_d is decreased in steps of −20 mV from d to a (compensating in steps of 10 and −15 mV on g_N and g_S, respectively). e–h, Stability diagrams generated by the zero-bandwidth model for different t_d (in meV), qualitatively reproducing the experimental behavior in a–d. Orange regions marking ground state transition have singlet-doublet splitting less than 0.015 meV. i, j, m, n Bias spectroscopy of sub-gap states vs. individual plunger gates swept along red, and green arrows in c and a. All plots show clear sub-gap resonances consistent with the ground states (doublet $\mathcal{D}$ or singlet $\mathcal{S}$) for different gate voltages indicated below. k, l, o, p Zero-bandwidth model calculation of sub-gap excitations for t_d = 0.2 meV and t_d = 0.125 meV corresponding to experimental plots i, j, m, n. The triplet excitation has very low spectral weight and therefore does not show up in the measured bias spectroscopy. Parameters are fixed to the same values as in Fig. 3