Evidence for spin singlet-triplet transitions of two electrons in a quantum dot observed via single-electron tunneling

We report magnetotunncling in a double-barrier resonant tunneling nanometer device in the single-electron charging regime, in particular for the case of two electrons in the well. Sharp rising steps appear in current-voltage characteristics, the Nth step appearing at a voltage V_N when the Coulomb blockade for tunneling of Nth electron into the well is overcome. The first current step width is ΔV₁ = V₂ - V₁ (the second step width is ΔV₂ = V₃ - V₂), corresponding to the difference of energies required for having two (three) electrons and having one (two) electron(s) in the well. As a function of magnetic field, we observe ΔV₁ to exhibit upward cusps, while ΔV₂ exhibits downward cusps at the same magnetic field. Such behavior is expected to manifest the spin singlet-triplet transitions of the two-electron state confined in a quantum dot.