Vibrational effects in laser-driven molecular wires

Pathways for charge transport through material interfaces

Modeling charge transport across material interfaces is important for understanding the limitations of electronic devices such as transistors, electrochemical cells, sensors, and batteries. However, modeling the entire structure and full dimensionality of an interface can be computationally demanding. In this study, we investigate the validity of an efficient reduced one-dimensional Hamiltonian for calculating charge transport along interfaces by comparing to a two-dimensional model that accounts for additional charge transport pathways. We find that the one-dimensional model successfully predicts the qualitative trend of charge transmission probability among Pt/Fe₂O₃ and Ag/Fe₂O₃ interfaces. However, the two-dimensional model provides additional information on possible pathways that are not perpendicular to the interface direction. These charge transport pathways are directed along the lowest potential energy profile of the interface that correlates with the crystal structure of the constituting materials. However, the two-dimensional paths are longer and take more scattering time. Therefore, the one-dimensional model may hold sufficient information for qualitative estimation of charge transport through some material interfaces.

Extending the hierarchical quantum master equation approach to low temperatures and realistic band structures

The hierarchical quantum master equation (HQME) approach is an accurate method to describe quantum transport in interacting nanosystems. It generalizes perturbative master equation approaches by including higher-order contributions as well as non-Markovian memory and allows for the systematic convergence to the numerically exact result. As the HQME method relies on a decomposition of the bath correlation function in terms of exponentials, however, its application to systems at low temperatures coupled to baths with complexer band structures has been a challenge. In this publication, we outline an extension of the HQME approach, which uses re-summation over poles and can be applied to calculate transient currents at a numerical cost that is independent of temperature and band structure of the baths. We demonstrate the performance of the extended HQME approach for noninteracting tight-binding model systems of increasing complexity as well as for the spinless Anderson-Holstein model.

Boosting thermoelectric efficiency using time-dependent control

Thermoelectric efficiency is defined as the ratio of power delivered to the load of a device to the rate of heat flow from the source. Till date, it has been studied in presence of thermodynamic constraints set by the Onsager reciprocal relation and the second law of thermodynamics that severely bottleneck the thermoelectric efficiency. In this study, we propose a pathway to bypass these constraints using a time-dependent control and present a theoretical framework to study dynamic thermoelectric transport in the far from equilibrium regime. The presence of a control yields the sought after substantial efficiency enhancement and importantly a significant amount of power supplied by the control is utilised to convert the wasted-heat energy into useful-electric energy. Our findings are robust against nonlinear interactions and suggest that external time-dependent forcing, which can be incorporated with existing devices, provides a beneficial scheme to boost thermoelectric efficiency.

Coherent light-driven electron transport through polycyclic aromatic hydrocarbon: laser frequency, field intensity, and polarization angle dependence

Laser frequency, field intensity, and polarization angle dependence of photon assisted tunneling in molecular junctions.

Single-molecule phenyl-acetylene-macrocycle-based optoelectronic switch functioning as a quantum-interference-effect transistor

This work proposes a new type of optoelectronic switch, the phenyl-acetylene-macrocycle-based single-molecule transistor, which utilizes photon-assisted tunneling and destructive quantum interference. The analysis uses single-particle Green's functions along with Floquet theory. Without the optical field, phenyl-acetylene-macrocycle exhibits a wide range of strong antiresonance between its frontier orbitals. The simulations show large on-off ratios (over 10(4)) and measurable currents (~10(-11) A) enabled by photon-assisted tunneling in a weak optical field (~2 × 10(5) V/cm) and at a small source-drain voltage (~0.05 V). Field amplitude power scaling laws and a range of field intensities are given for operating one- and two-photon assisted tunneling in phenyl-acetylene-macrocycle-based single-molecule transistors. This development opens up a new direction for creating molecular switches.

Quantum interference and decoherence in single-molecule junctions: how vibrations induce electrical current

Quantum interference and decoherence in single-molecule junctions is analyzed employing a nonequilibrium Green's function approach. Electrons tunneling through quasidegenerate states of a molecular junction exhibit interference effects. We show that electronic-vibrational coupling, inherent to any molecular junction, strongly quenches such interference effects. This decoherence mechanism may cause significantly larger electrical currents and is particularly pronounced if the junction is vibrationally highly excited, e.g., due to inelastic processes in the resonant transport regime.

Ratchet transport and periodic structures in parameter space

This work analyzes the parameter space of a discrete ratchet model and gives direct connections between chaotic domains and a family of isoperiodic stable structures with the ratchet current. The isoperiodic structures, where larger currents are usually observed inside, appear along preferred direction in the parameter space giving a guide to follow the current. Currents in parameter space provide a direct measure of the momentum asymmetry of the multistable and chaotic attractors times the size of the corresponding basin of attraction. Transport structures are shown to exist in the parameter space of the Langevin equation with an external oscillating force.

Molecular wires acting as quantum heat ratchets

We explore heat transfer in molecular junctions between two leads in the absence of a finite net thermal bias. The application of an unbiased time-periodic temperature modulation of the leads entails a dynamical breaking of reflection symmetry, such that a directed heat current may emerge (ratchet effect). In particular, we consider two cases of adiabatically slow driving, namely, (i) periodic temperature modulation of only one lead and (ii) temperature modulation of both leads with an ac driving that contains a second harmonic, thus, generating harmonic mixing. Both scenarios yield sizable directed heat currents, which should be detectable with present techniques. Adding a static thermal bias allows one to compute the heat current-thermal load characteristics, which includes the ratchet effect of negative thermal bias with positive-valued heat flow against the thermal bias, up to the thermal stop load. The ratchet heat flow in turn generates also an electric current. An applied electric stop voltage, yielding effective zero electric current flow, then mimics a solely heat-ratchet-induced thermopower ("ratchet Seebeck effect"), although no net thermal bias is acting. Moreover, we find that the relative phase between the two harmonics in scenario (ii) enables steering the net heat current into a direction of choice.

Vibrational nonequilibrium effects in the conductance of single molecules with multiple electronic states

Vibrational nonequilibrium effects in charge transport through single-molecule junctions are investigated. Focusing on molecular bridges with multiple electronic states, it is shown that electronic-vibrational coupling triggers a variety of vibronic emission and absorption processes, which influence the conductance properties and mechanical stability of single-molecule junctions profoundly. Employing a master equation and a nonequilibrium Green's function approach, these processes are analyzed in detail for a generic model of a molecular junction and for benzenedibutanethiolate bound to gold electrodes.

Measurement of electronic transport through 1G0 gold contacts under laser irradiation

Metal quantum point contacts (MQPCs) with dimensions comparable to the de Broglie wavelength of conducting electrons reveal ballistic transport of electrons and quantized conductance in units of G(0) = 2e(2)/h. We measure the transport properties of 1G(0) Au contacts under laser irradiation. The observed enhancement of conductance appears to be wavelength-dependent, while thermal effects on conductance are determined to be negligible. For wavelengths that are not absorbed by Au, the results are consistent with a photoassisted transport mechanism in which conductance depends both on the electronic structure of the leads and on the interaction of the transporting electrons with oscillating electric fields originating from excitation of local plasmons. For wavelengths absorbed by Au, photoinduced mechanism is suggested to be the dominant transport mechanism. The results demonstrate optical control of ballistic transport in MQPCs and are also important for future interpretation of light effects on the conductance of single-molecule junctions.

Raman scattering from nonequilibrium molecular conduction junctions

Raman scattering is a potentially important probe of structure, dynamics, and thermal properties of single-molecule conduction junctions. We combine a nonequilibrium Green's function description of the junction with a generalized scattering theory of the Raman process, which provides the first theoretical description of Raman scattering from such systems. The voltage dependence of the Raman flux shows a characteristic behavior at the conductance threshold resulting from (a) partial populations in the ground and excited molecular levels that give rise to two scattering pathways as well as interference between them and (b) junction heating that affects the Raman intensities. Comparing "effective temperatures" obtained from Raman scattering and heat balance serves to establish the integrity of this concept for nonequilibrium junctions.

Femtosecond dynamics and laser control of charge transport in trans-polyacetylene

The induction of dc electronic transport in rigid and flexible trans-polyacetylene oligomers according to the omega versus 2omega coherent control scenario is investigated using a quantum-classical mean field approximation. The approach involves running a large ensemble of mixed quantum-classical trajectories under the influence of omega+2omega laser fields and choosing the initial conditions by sampling the ground-state Wigner distribution function for the nuclei. The vibronic couplings are shown to change the mean single-particle spectrum, introduce ultrafast decoherence, and enhance intramolecular vibrational and electronic relaxation. Nevertheless, even in the presence of significant couplings, limited coherent control of the electronic dynamics is still viable, the most promising route involving the use of femtosecond pulses with a duration that is comparable to the electronic dephasing time. The simulations offer a realistic description of the behavior of a simple coherent control scenario in a complex system and provide a detailed account of the femtosecond photoinduced vibronic dynamics of a conjugated polymer.

Laser-induced currents along molecular wire junctions

The treatment of the previous paper is extended to molecular wires. Specifically, the effect of electron-vibrational interactions on the electronic transport induced by femtosecond omega+2omega laser fields along unbiased molecular nanojunctions is investigated. For this, the photoinduced vibronic dynamics of trans-polyacetylene oligomers coupled to macroscopic metallic leads is followed in a mean-field mixed quantum-classical approximation. A reduced description of the dynamics is obtained by introducing projective lead-molecule couplings and deriving an effective Schrodinger equation satisfied by the orbitals in the molecular region. Two possible rectification mechanisms are identified and investigated. The first one relies on near-resonance photon-absorption and is shown to be fragile to the ultrafast electronic decoherence processes introduced by the wire's vibrations. The second one employs the dynamic Stark effect and is demonstrated to be highly efficient and robust to electron-vibrational interactions.

Photoinduced removal of the franck-condon blockade in single-electron inelastic charge transmission

A new mechanism of charge transmission through a metal-molecule-metal junction is suggested that is based on optical driving of electronic transitions in the neutral and singly charged molecular state. The effects of strong electron vibrational coupling, intramolecular vibrational energy redistribution, and molecular de-excitation caused by electron-hole pair formation in the leads are taken into account. It is shown that current suppression due to the Franck-Condon blockade can be overcome by opening new transmission channels via photoexcitation.

Charge transmission through a molecular wire: The role of terminal sites for the current-voltage behavior

The current-voltage and the conductance-voltage characteristics are analyzed for a particular type of molecular wire embedded between two electrodes. The wire is characterized by internal molecular units where the lowest occupied molecular orbital (LUMO) levels are positioned much above the Fermi energy of the electrodes, as well as above the LUMO levels of the terminal wire units. The latter act as specific intermediate donor and acceptor sites which in turn control the current formation via the superexchange and sequential electron transfer mechanisms. According to the chosen wire structure, intramolecular multiphonon processes may block the superexchange component of the interelectrode current, resulting in a negative differential resistance of the molecular wire. A pronounced current rectification appears if (i) the superexchange component dominates the electron transfer between the terminal sites and if (ii) the multiphonon suppression of distant superexchange charge hopping events between those sites is nonsymmetric.

The influence of ultrafast laser pulses on electron transfer in molecular wires studied by a non-Markovian density-matrix approach

New features of molecular wires can be observed when they are irradiated by laser fields. These effects can be achieved by periodically oscillating fields but also by short laser pulses. The theoretical foundation used for these investigations is a density-matrix formalism where the full system is partitioned into a relevant part and a thermal fermionic bath. The derivation of a quantum master equation, either based on a time-convolutionless or time-convolution projection-operator approach, incorporates the interaction with time-dependent laser fields nonperturbatively and is valid at low temperatures for weak system-bath coupling. From the population dynamics the electrical current through the molecular wire is determined. This theory including further extensions is used for the determination of electron transport through molecular wires. As examples, we show computations of coherent destruction of tunneling in asymmetric periodically driven quantum systems, alternating currents and the suppression of the directed current by using a short laser pulse.