Mixed-Valence Magnetic Molecular Cell for Quantum Cellular Automata: Prospects of Designing Multifunctional Devices through Exploration of Double Exchange

Abstract

In this article, we propose to use multielectron square-planar mixedvalence (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 d 2 −d 2 −d 1 −d 1 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.

Insight Into The Spin‐Vibronic Problem of a Mixed Valence Magnetic Molecular Cell for Quantum Cellular Automata

“…In the initial exchange Hamiltonian defined in the full space comprising both ground and excited Coulomb configurations the HDVV exchange is described by three different exchange parameters: and . However, when the strong U ‐limit occurs, the effective perturbational Hamiltonian acts in the restricted space of only two diagonal‐type configurations D 1 and D 2 in which the and ions alternate and so in this limit and under adopted assumption that only nearest neighboring spins interact, the exchange coupling is described by the only exchange parameter Here, as in paper, [34] this parameter is considered to be negative (antiferromagnetic HDVV exchange), that seems to be the most typical case in transition metal clusters.…”

“…In particular, this is true for the two examples shown in Figure 1. In our recent studies [33–35] we have proposed to substantially extend the class of cells by including MV systems in which the excess charges are delocalized over paramagnetic sites which, as applied to the MV clusters, are conventionally referred to as “spin cores”. The presence of such spin cores along with the delocalized excess electrons is known to give rise to the double exchange (DE) that is sometimes also called “spin‐dependent delocalization”.…”

“…In the short communication [33] the idea of using the MV dimers exhibiting DE as half‐cells, which can be further combined to create bi‐dimeric square cells for QCA and spin switchers has been proposed. As a further development of this idea we have recently considered a more complicated cell which represents square planar transition metal cluster of ‐type comprising four spin cores ( ‐ions) and two excess electrons and exhibits combination of the DE and Heisenberg‐Dirac‐Van‐Vleck (HDVV) exchange [34,35] …”

“…The model developed in articles [34,35] can be regarded as pure electronic one in the sense that only the electronic interactions are considered, namely, the DE, the HDVV exchange and the Coulomb interactions between the excess electrons, while the vibronic coupling has remained out of the scope of these studies. At the same time as it is well known from the theory of MV complexes the vibronic coupling can produce strong impact on the localization‐delocalization phenomenon in MV systems and their properties [36–43] .…”

Pseudo-Tetrahedral vs Pseudo-Octahedral ErIII Single Molecule Magnets and the 'Disruptive' Role of Coordinated TEMPO Radicals

“…11,[14][15][16][17] The second, even more challenging goal, is achieving a certain degree of control over the nanomagnetism of these assemblies via external stimuli lading to the ON/OFF switching [18][19][20][21][22][23] and in the long-run -the multifunctionality, so that they can perform many different functions depending on their environment and thermodynamic conditions. [24][25][26] Usually, the lanthanide of choice for the design of highperformance SMMs is a Kramers ion Dy due to its large magnetic moment and high magnetic anisotropy. However, Dy III SMMs of the most appropriate geometry are mostly electrically charged ions, which prevent for example their sublimation and a straightforward deposition on neutral surfaces.…”