Anomalous Effect of Quantum Interference in Organic Spin Filters

Abstract

The molecular topology in the single-molecular nano-junctions through which the de-Broglie wave propagates plays a crucial role in controlling the molecular conductance. The enhancement and reduction of the conductance in para and meta connected molecules due to constructive and destructive Quantum Interference (QI) respectively are quite well established. Herein, we investigated the effect of localized spin centers on spin transportation using organic radicals as molecular junctions. The role of the localized spins on the QI as well as on spin filtering capability is investigated employing density functional theory in combination with non-equilibrium Green s function (NEGF-DFT) techniques. Various organic radicals including nitroxy (NO˙), phenoxy (PO˙) and methyl (CH 2˙) attached to the central benzene ring of pentacene with different terminal connections (para and meta) to gold electrodes are examined. Due to more obvious QI effects, para connected pentacene is found to be more conductive than meta one. Surprisingly, on incorporating a radical center, along with spin filtering, a significant reversal of QI effects is observed which manifests itself in such a way that the conductance of meta coupled radicals is found to be more than para by two orders of magnitude. The anomaly in QI patterns induced by radical center is analyzed and discussed in terms of orbital and structural perspectives.

Quantum Interference Controlled Spin-Polarized Electron Transmission in Graphene Nanoribbons

“…Apart from the electron transport, a few of the recent reports have also witnessed the spintronic applications of QI effects. An anomalous effect of QI in organic-radical based spin filters with the reversal of para-meta conductance was realized in our previous work which is recently certified by an experimental observation . Pal et al demonstrated the spin-polarized currents in a silver–vanadocene–silver single molecule junction stemming from the spin-dependent QI features .…”

“…PDOS analysis reveals that these transmission dips are associated with strongly localized DOS peaks near Fermi level arising from the edge carbon atoms (Figure S10). This is in contrast to the multipath DQI antiresonances, wherein no DOS peaks are observed in PDOS spectra corresponding to sharp dips in transmission spectra …”

Understanding Charge Transport in Triarylmethyl-Based Spintronic Nanodevices

“…In previous work on similar radical systems, − features of the local current have been analyzed in terms of quantum interferences. The arrows of the current give the orientation of the electronic flux density at a given point in its quasi-stationary state.…”

“…We aim at understanding their transport mechanism and relating them to the structural and electronic properties of these systems. Previous efforts have been made by others to understand the local and/or global transport behavior in similar systems, also in the context of spin filtering. , In these previous studies, spin-dependent transport was studied for devices in which an organic side group bore the radical. In contrast, the radical centers in the system studied here are found on the current pathway.…”

Quantum Circuit Rules for Molecular Electronic Systems: Where Are We Headed Based on the Current Understanding of Quantum Interference, Thermoelectric, and Molecular Spintronics Phenomena?

“…For example, Saraiva-Souza et al exploited the destructive QI property of the polyacetylene by adding electron donor/acceptor groups to control the spin transport of the zigzag graphene nanoribbon (ZGNR) system. Recently, Bajaj et al found that meta-connected pentacene ( m -pentacene) molecule shows a conductance that is 2 orders of higher magnitude than its para-connected counterpart when a spin filtering organic radical is incorporated, which is completely counterintuitive according to the QI rules. They attribute this anomalous behavior to the splitting of the molecular energy states due to the exchange energy, which transforms the destructive QI feature of m -pentacene to constructive QI for the studied spin orientation.…”