Mixed-valence magnetic molecular cell for quantum cellular automata: Prospects of designing multifunctional devices through exploration of double exchange

In this article, we propose to use multielectron square-planar mixed-valence (MV) molecules as molecular cells for quantum cellular automata (QCA) devices. As distinguished from previous proposals in this area, in multielectron cells, the electronic pair encoding binary information is shared among localized spins (“spin cores”). Hopefully, this will allow exploring not only charge degrees of freedom encoding binary information in the antipodal electronic distributions but also spin degrees of freedom through the magnetic interactions such as double exchange and Heisenberg−Dirac−Van Vleck (HDVV) exchange. To develop the proposed route, the square-planar tetrameric cells for QCA have been theoretically modeled. The considered cells comprise a pair of excess electrons shared among four spin cores and hence they exhibit double exchange and HDVV exchange. Such cells can be based either on the square-planar mixed-valence molecular clusters or on the similar square arrays of multielectron quantum dots. The detailed case study of the cell representing the transition-metal tetramer of the type of d²−d²−d¹−d¹ shows that depending on the relative strength of the second-order double exchange and HDVV exchange, the isolated cell has either ground localized spin-triplet or one of the two delocalized spin-singlets, exhibiting different extents of electron delocalization. Due to different sensitivities of these states to the quadrupole electrostatic field induced by the polarized neighboring driver cell, the latter is shown to be able to produce switching between different ground states (including spin-switching between spin-singlet and spin-triplet), which leads to the nonmonotonic behavior of the cell−cell response function. This opens new perspectives for the designing of multifunctional devices combining the QCA functionality with spin-switching function.