Spin Stark effect in nanostructures

We would like to present studies of the Stark effect on electronic structure in molecules and coherently coupled quantum dots. Our interest is focused on influence of an electric field on the spin system and on switching processes between different spin configurations. The studies are performed within the Hubbard model taking into account strong electron correlations. Using a canonical transformation we explicitly derive exchange couplings in an effective spin Hamiltonian. We show that the electric field can induced a singlet-triplet transition in triple quantum dots and in triangular molecules. The singlet-triplet switching results from competition between direct and super-exchange processes. Moreover we consider the case with three electrons (spins) in the system for which dark spin states can occur. The dark state can be formed either from a singlet state or two triplet states by adding third electron to unoccupied dot. The added spin is then uncoupled from the others. We present also a linear and nonlinear spin Stark effect, i.e. influence of the electric field and its orientation on spin-spin correlations. The model is also applied for studying electronic transport. Since electron transfer rates are anisotropic, the current characteristics are anisotropic as well, differing for small and large electric fields. We show how the singlet-triplet transition and the dark spin state can be observed in measurement of electric current.

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