Figure 4. Illustration of the many-body states and the related energy level diagrams.

(a) In the Kondo-screened ground state, the spin (white arrow) in scattering channel k=1 is screened, illustrated by grey arrows and shading, and the tunnelling electron can enter with its spin (blue arrow) parallel to the spin in k=2, increasing the excited state's total spin to S*=1. The excitation scheme including the anisotropy-split excited state is shown by the energy-level diagram with indicated anisotropy parameters D and E. (b) In the free-spin ground state, the spin in k=1 (red arrow) is only partially screened. The tunnelling electron has to enter this state in an anti-parallel alignment, obeying the Pauli exclusion principle and reducing the spin in the excited state to S*=1/2. Red lines in the excitation scheme symbolize the thermal occupation of the anisotropy-split ground state as determined by the Boltzmann distribution function (black line).