Spin transport from doublet state to triplet state in vertical quantum dots

We measured electron transport state spectra of an In_0.05Ga _0.95As vertical double quantum dot. Both the ground and excited states of transport spectra from two electrons to three electrons are measured using a large source-drain voltage. In the obtained transition spectrum, the ground state transition from the 1S² singlet state to the 1S2P triplet state is observed at 5T. Zeeman splitting with a g-factor of 0.36 is clearly observed at magnetic fields higher than 5 T. The observation of two Zeeman sublevels instead of three for the triplet state is explained by the spin selection rules for the S_Z components between the two-electron and three-electron spin states. Transition with the total spin difference between the initial and final states larger than 1/2 is forbidden. Because the transition between doublet states is much longer than electron tunneling, both spin up and spin down can be the initial states from spin transitions. There are four transitions contributing to tunneling processes, but only two energy differences lead to the two Zeeman sublevels in the excitation spectra.