Observation of Quantum Interference in Molecular Charge Transport (original) (raw)
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Journal of Physics: Condensed Matter
We study the transport through a molecular junction exhibiting interference effects. We show that these effects can still be observed in the presence of molecular vibrations if Coulomb repulsion is taken into account. In the Kondo regime, the conductance of the junction can be changed by several orders of magnitude by tuning the levels of the molecule, or displacing a contact between two atoms, from nearly perfect destructive interference to values of the order of 2e 2 /h expected in Kondo systems. We also show that this large conductance change is robust for reasonable temperatures and voltages for symmetric and asymmetric tunnel couplings between the source-drain electrodes and the molecular orbitals. This is relevant for the development of quantum interference effect transistors based on molecular junctions.
Angewandte Chemie International Edition, 2019
Together with the more intuitive and commonly recognized conductance mechanisms of charge-hopping and tunneling,q uantum-interference (QI) phenomena have been identified as important factors affecting charge transport through molecules.C onsequently,e stablishing simple and flexible molecular-design strategies to understand, control, and exploit QI in molecular junctions poses an exciting challenge.H ere we demonstrate that destructive quantum interference (DQI) in meta-substituted phenylene ethylenetype oligomers (m-OPE) can be tuned by changing the position and conformation of methoxy (OMe) substituents at the central phenylene ring. These substituents play the role of molecularscale taps,w hich can be switched on or off to control the current flowt hrough am olecule.O ur experimental results conclusively verify recently postulated magic-ratio and orbitalproduct rules,a nd highlight an ovel chemical design strategy for tuning and gating DQI features to create single-molecule devices with desirable electronic functions.
The Journal of Physical Chemistry C
We studied the interplay between quantum interference (QI) and molecular asymmetry in charge transport through a single molecule. Eight compounds with five-membered core rings were synthesized and their single-molecule conductances were characterized using the mechanically controllable break junction (MCBJ) technique. It is found that the symmetric molecules are more conductive than their asymmetric isomers and there is no statistically-significant dependence on the aromaticity of the core. In contrast, we find experimental evidence of destructive QI in fivemembered rings, which can be tuned by implanting different heteroatoms into the core ring. Our findings are rationalized by the presence of anti-resonance features in the transmission curves calculated using non-equilibrium Green"s functions. This novel mechanism for modulating QI effects in charge transport via tuning of molecular asymmetry will lead to promising applications in the design of single-molecule devices.
Kondo blockade due to quantum interference in single-molecule junctions
Nature Communications, 2017
Molecular electronics offers unique scientific and technological possibilities, resulting from both the nanometre scale of the devices and their reproducible chemical complexity. Two fundamental yet different effects, with no classical analogue, have been demonstrated experimentally in single-molecule junctions: quantum interference due to competing electron transport pathways, and the Kondo effect due to entanglement from strong electronic interactions. Here we unify these phenomena, showing that transport through a spin-degenerate molecule can be either enhanced or blocked by Kondo correlations, depending on molecular structure, contacting geometry and applied gate voltages. An exact framework is developed, in terms of which the quantum interference properties of interacting molecular junctions can be systematically studied and understood. We prove that an exact Kondo-mediated conductance node results from destructive interference in exchange-cotunneling. Nonstandard temperature dependences and gate-tunable conductance peaks/ nodes are demonstrated for prototypical molecular junctions, illustrating the intricate interplay of quantum effects beyond the single-orbital paradigm.
Physical Review B, 2010
We report a first-principles study of quantum transport in a prototype two-terminal device consisting of a molecular nanowire acting as an interconnect between two gold electrodes. The wire is composed of a series of bicyclo͓1.1.1͔pentane ͑BCP͒ cage-units. The length of the wire ͑L͒ is increased by sequentially increasing the number of BCP cage units in the wire from 1 to 3. A two terminal model device is made out of each of the three wires. A parameter free, nonequilibrium Green's function approach, in which the bias effect is explicitly included within a many body framework, is used to calculate the current-voltage characteristics of each of the devices. In the low bias regime that is considered in our study, the molecular devices are found to exhibit Ohmic behavior with resistances of 0.12, 1.4, and 6.5 ⍀ for the wires containing one, two, and three cages respectively. Thus the conductance value, G c , which is the reciprocal of resistance, decreases as e −L with a decay constant ͑͒ of 0.59 Å −1. This observed variation of conductance with the length of the wire is in excellent agreement with the earlier reported exponential decay feature of the electron transfer rate predicted from the electron transfer coupling matrix values obtained using the two-state Marcus-Hush model and the Koopman's theorem approximation. The downright suppression of the computed electrical current for a bias up to 0.4 V in the longest wire can be exploited in designing a three terminal molecular transistor; this molecular wire could potentially be used as a throttle to avoid leakage gate current.
Probing electron-phonon excitations in molecular junctions by quantum interference
Scientific reports, 2016
Electron-phonon coupling is a fundamental inelastic interaction in condensed matter and in molecules. Here we probe phonon excitations using quantum interference in electron transport occurring in short chains of anthraquinone based molecular junctions. By studying the dependence of molecular junction's conductance as a function of bias voltage and temperature, we show that inelastic scattering of electrons by phonons can be detected as features in conductance resulting from quenching of quantum interference. Our results are in agreement with density functional theory calculations and are well described by a generic two-site model in the framework of non-equilibrium Green's functions formalism. The importance of the observed inelastic contribution to the current opens up new ways for exploring coherent electron transport through molecular devices.
Electron-transfer properties of quantum molecular wires
Chemical Physics, 1995
It is shown that interelectrode tunnel current through a quantum molecular wire (QMW) depends essentially on the relation between the dynamic properties of QMW and the relaxation processes within electrodes and donor (acceptor) units. Within the framework of the adiabatic approximation, it is found that image forces from the electrodes change the tunnel current by several orders of magnitude. It depends on: (1) the position of the donor-bridge chain-acceptor (DBA) system with respect to the electrode surfaces; (2) the ratios between the static and high-frequency permittivities of the interelectrode medium; (3) the effective radii of donor (acceptor) and QMW units. The orientation effects connected both with the orientation of DBA units and with mutual spin orientations of neighbouring magnetic QMW units are studied. It is shown that even small changes of the orientation of porphyrin molecules may cause changes of several orders of magnitude of the QMW-mediated tunnel current. In the case of an antiferromagnetically ordered QMW, the interelectrode tunnel current can be regulated by a magnetic field in a wide region up to 7-8 orders of magnitude because of the ability of the magnetic field to influence changes in the spin orientations.
Molecular Wires: Charge Transport, Mechanisms, and Control
Annals of the New York Academy of Sciences, 1998
By molecular wires, one generally means molecular structures that transmit a signal between two termini. We discuss some theoretical models and analysis for electronically conductive molecular wires in which a single molecule conducts charge between two electrodes. This situation resembles both intramolecular non-adiabatic electron transfer, in which electronic tunneling between donor and acceptor is seen, and mesoscopic quantum transport.
The Relation between Structure and Quantum Interference in Single Molecule Junctions
Nano Letters, 2010
Quantum interference (QI) of electron pathways has recently attracted increased interest as an enabling tool for singlemolecule electronic devices. Although various molecular systems have been shown to exhibit QI effects and a number of methods have been proposed for its analysis, simple guidelines linking the molecular structure to QI effects in the phase-coherent transport regime have until now been lacking. In the present work we demonstrate that QI in aromatic molecules is intimately related to the topology of the molecule's π system and establish a simple graphical scheme to predict the existence of QI-induced transmission antiresonances. The generality of the scheme, which is exact for a certain class of tight-binding models, is proved by a comparison to first-principles transport calculations for 10 different configurations of anthraquinone as well as a set of cross-conjugated molecular wires.