The figure analyses the low bias source-drain spectroscopy when the tip is scanned along the orange line 3 indicated in
Fig. 3a. ( a) Differential conductance G versus source-drain bias at a fixed gate voltage V gate=−1 V for different tip positions from 0 to 450 nm. Successive curves are shifted upwards by 0.0075 × 2 e 2/ h. Conductance peaks are visible at zero and finite bias on red and blue curves, respectively. ( b) Colour plot of the same data as in a after subtraction of a smooth background to suppress the main gating effect of the tip. Peak positions are indicated by dots. The successive ZBA splittings give a checkerboard pattern. The asymmetry results from the bias-induced change of the QPC position. ( c) Schematic of the QPC potential with one (top) and two (bottom) localized electrons, corresponding, respectively, to a S=1/2 ground state with a zero-bias Kondo peak and to a S=0 ground state with finite-bias Kondo peaks involving the excited state S=1 with singlet-triplet energy splitting J. The expected conductance G versus bias V is shown on the right for each state. ( d) Schematic of the QPC potential with an increasing number of electrons localized by Coulomb interactions. The antiferromagnetic spin coupling in this small 1D Wigner crystal gives either a S=1/2 ground state (ZBA) or a S=0 ground state (splitting of the ZBA), depending on the parity (respectively odd or even).