Constructive Quantum Interference Research Articles

Conformational Control of σ-Interference Effects in the Conductance of Permethylated Oligosilanes.

As silicon-based integrated circuits continue to shrink, their molecular characteristics become more pronounced. However, the structure-property relationship of silicon-based molecular junctions remains to be elucidated. Here, an intuitive explanation of the effects of backbone dihedral angles on transport properties in permethylated oligosilanes is presented employing the Ladder C model Hamiltonian combined with nonequilibrium Green's function formalism. Backbone dihedral angles modulate quantum interference (QI), resulting in the change of the transmission coefficient at the Fermi energy (EF) by up to 6 orders of magnitude in Si4Me10. Because the types of QI (constructive or destructive) between molecular conductance orbitals (MCOs) are unchanged, the relative magnitudes of contributions from QI are critical. This quantitative aspect of QI is often neglected in previous theoretical studies. Small backbone dihedral angles lead to localized MCOs near EF and delocalized MCOs further away from EF. As a result, the constructive QI between the MCOs near EF is suppressed, while the destructive QI between other MCOs is enhanced. This insight opens an avenue to harness QI to realize ultrainsulating molecular devices.

Extreme Anomalous Conductance Enhancement in Neutral Diradical Acene-like Molecular Junctions.

We achieve, at room temperature, conductance enhancements over 2 orders of magnitude in single molecule circuits formed with polycyclic benzoquinoidal (BQn) diradicals upon increasing molecular length by ∼5 Å. We find that this extreme and atypical anti-ohmic conductance enhancement at longer molecular lengths is due to the diradical character of the molecules, which can be described as a topologically nontrivial electronic state, and results in constructive interference between the frontier molecular orbitals. The distinct feature of the compounds studied here as molecular wires is that they are characterized by moderate diradical character in the neutral state, allowing for robust and facile measurements of their transport properties. We adapt the 1D-SSH model, originally developed to examine electronic topological order in linear carbon chains, to the polycyclic systems studied here and find that it captures the anti-ohmic trends in this molecular series. Specifically, our model reveals that the mechanism of conductance enhancement with length in polycyclic systems is constructive quantum interference between the frontier orbitals with nontrivial topology, which is present in acene-like, but not in linear, molecular systems. Importantly, we use our model to predict and experimentally validate that anti-ohmic trends can be engineered through synthetic adjustments of the diradical character of the acene-like molecules. Overall, we achieve extreme anti-ohmic enhancement and mechanistic insight into electronic transport in a class of materials that we identify here as promising candidates for creating highly conductive and tunable nanoscale wires.

Bias switching in single-molecule junctions through destructive quantum interference

Highly nonlinear current-voltage (I-V) characteristics in single-molecule junctions based on destructive quantum interference (DQI) effects are key to realizing bias-controlled molecular switches. Using first-principles quantum transport calculations, we investigated the charge transport properties of 1,4-diphenyl-2,3-dioxa-7-tellurabicyclo(DPDT) coupled to gold electrodes via cyanide (-CN), isocyanide (-NC) and pyridyl (-PY) anchoring groups, respectively. We demonstrated that there are DQI effects between LUMO and LUMO+1 of the opposite phase in the three junctions. It was revealed that the p orbital of the tellurium atom can localize HOMO orbitals on central units of molecular systems, and weaken the amplitude of LUMO relative to LUMO+1. This eliminates the constructive quantum interference (CQI) between HOMO and LUMO, but enhances the DQI between LUMO and LUMO+1 near the Fermi energy level of gold electrodes (EF). As a result, the three molecular junctions dominated by two LUMOs exhibit extremely high nonlinear I-V characteristics, contributing to a current at a high bias (±0.8 V) >170 times larger than that at a low bias (±0.2 V). Our findings provide a potentially stable and low-loss bias switching in single-molecule junctions.

Thermoelectric Properties of Porphyrin Nano Rings: A Theoretical and Modelling Investigation.

Propagation of De Broglie waves through nanomolecular junctions is greatly affected by molecular topology changes, which in turn plays a key role in determining the electronic and thermoelectric properties of source|molecule|drain junctions. The probing and realization of the constructive quantum interference (CQI) and a destructive quantum interference (DQI) are well established in this work. The critical role of quantum interference (QI) in governing and enhancing the transmission coefficient T(E), thermopower (S), power factor (P) and electronic figure of merit (ZelT) of porphyrin nanorings has been investigated using a combination of density functional theory (DFT) methods, a tight binding (Hückel) modelling (TBHM) and quantum transport theory (QTT). Remarkably, DQI not only dominates the asymmetric molecular pathways and lowering T(E), but also improves the thermoelectric properties.

Destructive quantum interference in meta-oligo(phenyleneethynylene) molecular wires with gold-graphene heterojunctions.

Quantum interference (QI) is well recognised as a significant contributing factor to the magnitude of molecular conductance values in both single-molecule and large area junctions. Numerous structure-property relationship studies have shown that para-connected oligo(phenyleneethynylene) (OPE) based molecular wires exemplify the impact of constructive quantum interference (CQI), whilst destructive quantum interference (DQI) effects are responsible for the orders of magnitude lower conductance of analogous meta-contacted OPE derivatives, despite the somewhat shorter effective tunnelling distance. Since molecular conductance is related to the value of the transmission function, evaluated at the electrode Fermi energy, T(EF), which in turn is influenced by the presence and relative energy of (anti)resonances, it follows that the relative single-molecule conductance of para- and meta-contacted OPE-type molecules is tuned both by the anchor group and the nature of the electrode materials used in the construction of molecular junctions (gold|molecule|gold vs. gold|molecule|graphene). It is shown here that whilst amine-contacted junctions show little influence of the electrode material on molecular conductance due to the similar electrode-molecule coupling through this anchor group to both types of electrodes, the weaker coupling between thiomethyl and ethynyl anchors and the graphene substrate electrode results in a relative enhancement of the DQI effect. This work highlights an additional parameter space to explore QI effects and establishes a new working model based on the electrode materials and anchor groups in modulating QI effects beyond the chemical structure of the molecular backbone.

Theoretical and modelling investigation of pendant groups effect on quantum interference for single-molecule junctions.

Quantum interference (QI) is one of the most important phenomena that affects the charge transport through single molecules. The effect of a constructive and destructive quantum interference on electronic, thermoelectric and spectroscopic properties of oligo(phenyleneethynylene) based-molecular junctions has been investigated using a combination of density functional theory (DFT) methods, tight binding (Hückel) modelling (TBHM) and quantum transport theory (QTT). Molecules with carbonyl, diphenyl, ethane and ethynylferrocene substituents show a destructive quantum interference (DQI), which enhances thermoelectric properties of these molecules making them promising materials for thermoelectric applications.

Spin-Dependent Destructive and Constructive Quantum Interference Associated with Chirality-Induced Spin Selectivity in Single Circular Helix Molecules.

Chirality-induced spin selectivity (CISS) effect in straight helical molecules has received intense studies in past decade; however, the CISS effect in circular helical molecules (CHMs) has still rarely been explored. Here, we have constructed single CHMs having chirality-induced spin-orbit coupling (SOC) and connected by two nonmagnetic leads and successfully gained the required conditions for CISS effect occurring in CHMs for the first time. Our results uncover that only when the CHMs form a closed loop and when the lattice positions are coupled asymmetrically with both leads does the CISS effect occur. More importantly, the CISS-associated spin-dependent destructive and constructive quantum interference (QI) together with their phase transition appears in CHMs. The combination of CISS effect and spin-dependent QI phenomena opens up a new door to understand the underlying physics of the CISS effect in helical molecules.

Quantum state engineering in a three-level system with periodical Gaussian pulses

In this paper, we propose a scheme to achieve selective population transfer in a Λ-type three-level atomic system with a train of periodical Gaussian pulses. Based on temporal constructive quantum interference between the sequential transitions and subsequent coherent accumulations, the selective population transfer can be achieved with the pulse train, without satisfying the requirement of two-photon resonance condition. It can be understood as a result of the formation of a comb-like structure of the pulse train spectrum in time domain. Numerical simulations demonstrate that the scheme is robust against the wave-form deformation of single pulse while sensitive to the pulse train repetition period. Due to the Gaussian waveform of the laser field been readily implemented in experiments, the scheme holds great practicality for quantum state engineering in quantum information processing.

Influence of Environmental Fluctuations on Quantum Interference in Naphthalene and Azulene

Both naphthalene and azulene have the same number of carbon and hydrogen atoms, but the former is an alternant hydrocarbon and the latter is a nonalternant hydrocarbon. This leads to a large difference in their electronic and transport properties. Herein, quantum transport is investigated through these two molecules and it is shown how quantum interference (QI) affects their electrical conductance. It is demonstrated that the orbital rule to predict QI breaks down in both naphthalene and azulene. The influence of environmental fluctuations on their QI and electrical conductance is also investigated. The results show that QI in azulene is more sensitive to environmental fluctuations than in naphthalene. In particular, destructive QI can be changed to constructive QI in azulene by small environmental fluctuations.

Controlling quantum interference in tetraphenyl-aza-BODIPYs

This study presents systematic theoretical investigations employing an ab initio DFT approach combined with analysis of heuristic tight-binding models. We examine the effect of using conjugated and non-conjugated bridge on the electrical transport of tetraphenyl-aza-BODIPY derivatives. This work demonstrates that, substitution a conjugated bridging atom by non-conjugated one, causes the electrical conductance to switch from constructive quantum interference CQI to destructive DQI (on/off). This demonstration of switching behaviour means that if molecules with alternating structures (i.e., non-/conjugated), can be deposited on a metal surface, then they form a basis for enhancing the thermovoltage in nanoscale thermoelectric nanotechnology devices.

Peculiar electronic properties of wider armchair graphene nanoribbons: Multiple topological end-states and “phase transitions”

In contrast to the thinner 5,6,7-AGNRs in which one pair of topological end-states is generated at a critical length Lc in a form of a metal-insulator-like “phase transition”, in wider AGNRs (with Z = 3n, 3n±1, n = ,2, 3 … zigzag rings) n ≥ 2 pairs of end-states (associated with n ≥ 2 transitions) can be produced, with very peculiar properties, such as quantum interference (QI), stemming from the interaction between (multiple) pairs of end states localized in the same narrow region. At the level of density functional theory such AGNRs at large enough lengths would be misleadingly appear as highly spin polarized with spin s = n. Our results are fully supported by experiment, wherever available. We verify that the Z = 3n-1 AGNRs are always metallic with near zero bandgap, as was originally believed, contrary to some recent suggestions, and do not strictly follow the above pattern. It is shown that in the 13-AGNRs (Z = 3n, n = 2) a unique and intense (close to the theoretical limit) constructive QI (CQI) appears, which covers a relatively wide region of lengths from about L∼ 50 Å to L∼80 Å. This peculiar L-dependent CQI is strongly related to their aromaticity pattern consisting of migrating Clar sextets. The rest of AGNRs exhibit only small kinks (for n = 2) or dips (for n = 3) in conductivity, which could be considered as weak and narrow CQI, or DQI (destructive QI), respectively.

Switching Quantum Interference in Single‐Molecule Junctions by Mechanical Tuning

AbstractThe charge transport through single‐molecule electronic devices can be controlled mechanically by changing the molecular geometrical configuration in situ, but the tunable conductance range is typically less than two orders of magnitude. Herein, we proposed a new mechanical tuning strategy to control the charge transport through the single‐molecule junctions via switching quantum interference patterns. By designing molecules with multiple anchoring groups, we switched the electron transport between the constructive quantum interference (CQI) pathway and the destructive quantum interference (DQI) pathway, and more than four orders of magnitude conductance variation can be achieved by shifting the electrodes in a range of about 0.6 nm, which is the highest conductance range ever achieved using mechanical tuning.

Switching Quantum Interference in Single-Molecule Junctions by Mechanical Tuning.

The charge transport through single-molecule electronic devices can be controlled mechanically by changing the molecular geometrical configuration in situ, but the tunable conductance range is typically less than two orders of magnitude. Herein, we proposed a new mechanical tuning strategy to control the charge transport through the single-molecule junctions via switching quantum interference patterns. By designing molecules with multiple anchoring groups, we switched the electron transport between the constructive quantum interference (CQI) pathway and the destructive quantum interference (DQI) pathway, and more than four orders of magnitude conductance variation can be achieved by shifting the electrodes in a range of about 0.6 nm, which is the highest conductance range ever achieved using mechanical tuning.

Modulating Quantum Interference Between Destructive and Constructive States in Double N‐Substituted Single Molecule Junctions (Adv. Electron. Mater. 2/2023)

Quantum Interferences In article number 2201024, Xiangfeng Shao, Colin J. Lambert, Hao-Li Zhang, and co-workers present the significant modulation effect of double N-substitution on quantum interference (QI) in meta-phenylene ethylene oligomer molecular devices, reveal how two N-atoms synergistically modulate the molecular conductance between destructive QI and constructive QI states. The findings of this work pave a path toward utilization of heterocyclic aromatic hydrocarbons in molecular electronics.

Resurgent revivals in bosonic quantum gases: A striking signature of many-body quantum interferences

Matter wave revivals depend on a delicate interplay of constructive many-body quantum interferences in the developing dynamics of an ultracold bosonic system in an optical lattice. It is shown that the interplay between weak intersite tunneling and strong onsite interactions can lead to the quantum dynamics of a density wave displaying several features not found in the mean-field limit: occupancy oscillations, resurgent revivals, and a (anti-) synchronization of revival peaks and occupancy oscillation peaks. This implies cooperative interference effects that alternate between constructive and destructive features leading to the peak revival behaviors. These many-body quantum interference phenomena create striking features in various observables, which are accessible in experimental measurements.

Scaling of quantum interference from single molecules to molecular cages and their monolayers

The discovery of quantum interference (QI) is widely considered as an important advance in molecular electronics since it provides unique opportunities for achieving single-molecule devices with unprecedented performance. Although some pioneering studies suggested the presence of spin qubit coherence and QI in collective systems such as thin films, it remains unclear whether the QI can be transferred step-by-step from single molecules to different length scales, which hinders the application of QI in fabricating active molecular devices. Here, we found that QI can be transferred from a single molecule to their assemblies. We synthesized and investigated the charge transport through the molecular cages using 1,3-dipyridylbenzene (DPB) as a ligand block with a destructive quantum interference (DQI) effect and 2,5-dipyridylfuran (DPF) as a control building block with a constructive quantum interference (CQI) effect using both single-molecule break junction and large area junction techniques. Combined experiments and calculations revealed that both DQI and CQI had been transferred from the ligand blocks to the molecular cages and the monolayer thin film of the cages. Our work introduced QI effects from a ligand to the molecular cage comprising 732 atoms and even their monolayers, suggesting that the quantum interference could be scaled up within the phase-coherent distance.

Quantum Interference in Mixed-Valence Complexes: Tuning Electronic Coupling Through Substituent Effects.

Whilst 2- or 5-OMe groups on the bridging phenylene ring in [{Cp*(dppe)RuC≡C}2 (μ-1,3-C6 H4 )]+ have little influence on the electronic structure of this weakly coupled mixed-valence complex, a 4-OMe substituent enhances ground state electron delocalization, and increases the intensity of the IVCT transition. Vibrational frequency and TDDFT calculations (LH20t-D3(BJ), def2-SVP, COSMO (CH2 Cl2 )) on ([{Cp*(dppe)RuC≡C}2 (μ-1,3-C6 H3 -n-OMe)]+ (n=2, 4, 5) models are in excellent agreement with the experimental results. The stronger ground state coupling is attributed to the change in composition of the β-HOSO brought about by the 4-OMe group, which is ortho or para to each of the metal fragments. The intensity of the IVCT transition increases with the greater overlap of the β-HOSO and β-LUSO, whilst the relative phases of the β-HOSO and β-LUSO in the 4-OMe substituted complex are consistent with predictions of constructive quantum interference from molecular circuit rules.

Knee structure in the laser-intensity dependence of harmonic generation for graphene

We investigate the high-order harmonic generations (HHGs) of graphene irradiated by linearly polarized lasers with intensities in a wide range from $10^{8}$ W/cm$^2$ to $10^{13}$ W/cm$^2$. We find a striking knee structure in the laser intensity dependence of HHGs, which consists of a linear growth regime, followed by a plateau of the saturated HHGs, and then a transition to a nonlinear growth. The knee structure is rather universal for the varied harmonic orders and has been certificated by the calculations of two-band density-matrix equations as well as the \textit{ab initio} calculations of time-dependent density functional theory. Based on the two-band model, we reveal the underlying mechanisms: The behavior of linear growth can be depicted analytically by the perturbative theory of optical conductivity; While, the plateau of saturated HHGs and the transition to a nonlinear growth are caused by the quantum destructive interference and constructive interference of harmonics generated by the electrons corresponding to the lattice momentums around Dirac points and M points in Brillouin zone, respectively. In particular, we find that tuning Fermi energy can effectively alter the knee structure while the profile of the knee structure is not sensitive to the temperature. Our calculations of the third-order harmonic vs. tuning Fermi energy are compared with recent experiment showing a good agreement. Our predicted knee structure and its associated properties are observable with the current experimental techniques.

Unusual constructive quantum interference in isolated armchair graphene nanoribbons

A new and unusual type of Constructive Quantum Interference (CQI) is revealed in selected isolated armchair graphene nanoribbons (AGNRs). CQI, normally encountered in molecular junctions, occurs when two nearly isoenergetic states interfere constructively. Such almost isoenergetic states are identified here as the energy differences between two pairs of topological end states developed in “wider” AGNRs, through topological metal-insulator-like phase transition, which for 13-AGNRS occurs at a critical length L c ≈ 50 Å. At L c a sudden jump in conductivity by a factor ∼2.5 occurs, which persists up to L ≈ 80 Å and is interpreted as a signature of CQI. Such unusual CQI, in which the zigzag region effectively plays the role of “linkers”, is unique for 13-AGNRs. This is due to their aromaticity i.e., resonance between two different aromatic Clar-sextets, and the resulting symmetry and energy separation of the frontier orbitals. The experimental measurements for 13-AGNRs fully support our results. At a critical length L c a multidimensional phase transition occurs in 13-AGNRs associated with two pairs of nearly isoenergetic end-states and discontinuities in bandgaps and conductivity, which suddenly rises by a factor of ~2.5, signifying a region of constructive quantum interference (CQI). • In wider graphene nanoribbons more than one pair of topological end states occur • These end states appear at critical length L c as metal-insulator phase transitions • For 13-armchair nanoribbons only, this leads to constructive quantum interference • Which is ultimately due to resonance between two aromatic Clar sextets • The role of “linkers” is played by the zigzag ends where end-states are localized

Atom-assisted coherent control of multiple-color mechanically induced switching

We show multiple mechanically induced switching of a weak probe field through a system of cavity-optomechanics with a medium of N number of three levels (Λ-type) atoms exhibiting a strong coherent field coupling. Three distinct MIT windows develop for the probe field transmission using optomechanical coupling together with two other coupling parameters related to the two atomic transitions. Adjustment of the relative phase associated with the coupling parameters leads to multiple absorption-induced optical switching. The switching is obtained by closing one through three of the MIT windows for the probe field transmission tuning the relative phase and vice versa. Contrary to the multiple mechanical induced transparency, the optical switching phenomenon is explained regarding constructive quantum interference effect in the system.