Symmetry aspects of spin filtering in molecular junctions: Hybridization and quantum interference effects

Abstract

Control and manipulation of electric current and, especially, its degree of spin polarization (spin filtering) across single molecules are currently of great interest in the field of molecular spintronics. We explore one possible strategy based on the modification of nanojunction symmetry which can be realized, for example, by a mechanical strain. Such modification can activate new molecular orbitals which were inactive before due to their orbital mismatch with the electrodes conduction states. This can result in several important consequences such as (i) quantum interference effects appearing as Fano-like features in electron transmission and (ii) the change in molecular level hybridization with the electrodes states. We argue that the symmetry change can affect very differently two majority-and minority-spin conductances and thus alter significantly the resulting spin-filtering ratio as the junction symmetry is modified.We illustrate the idea for two basic molecular junctions: Ni/benzene/Ni (perpendicular vs tilted orientations) and Ni/Si chain/Ni (zigzag vs linear chains). In both cases, one highest occupied molecular orbital (HOMO) and one lowest unoccupied molecular orbital (LUMO) (out of HOMO and LUMO doublets) are important. In particular, their destructive interference with other orbitals leads to dramatic suppression of majority-spin conductance in low-symmetry configurations. For a minorityspin channel, on the contrary, the conductance is strongly enhanced when the symmetry is lowered due to an increase in hybridization strength. We believe that our results may offer a potential route for creating molecular devices with a large on-off ratio of spin polarization via quantum interference effects.PACS numbers:

Giant anisotropic magnetoresistance through a tilted molecular π -orbital

“…Therefore, molecular orbitals are expected to preserve their symmetry and localized nature. In the collinear magnetic case without SOC, it has been shown that a nonmagnetic organic molecule can act as a "halfmetallic" conductor due to either orbital symmetry arguments [27,28] or quantum interference effects [29,30], leading to nearly fully spin-polarized conduction. In addition, a nearly perfect spin filtering was reported when a vanadium-benzene wire is placed between two magnetic electrodes [31].…”

The Anomalous Effect of Quantum Interference in Organic Spin Filters

“…[25][26][27] Recently, Li et al proposed a mechanism for tuning the spin filtering ratio based on QI effects in a nickel based junction. 28 Ratner and coworkers have demonstrated that how DQI features can be controlled by addition of a donor or acceptor group to the polyacetylene chain bridging graphene nanoribbons (ZGNR). 29 But the role of QI in tuning the overall conductance and the spin filtering efficiency of organic radical molecular junction, which are expected to be the most efficient spin filters has not been explored yet.…”

Robust conductance zeros in graphene quantum dots and other bipartite systems

“…In the following considerations we are interested in the general multi-terminal case of a quantum dot (QD) connected to a number of N l one-channel transport leads, arXiv:2007.04700v1 [cond-mat.mes-hall] 9 Jul 2020 indexed by α or β = 1 • • • N l . The leads are described by 1D tight-binding or discrete chain [19][20][21] and the contact points between them and QD are individual sites denoted with i α and i β .…”