arXiv subjects

Dvira Segal

Publications and source records attributed to Dvira Segal.

At least 91 records · Page 5Linked to original sources

arXiv2018

Phononic heat transport in molecular junctions: quantum effects and vibrational mismatch

Problems of heat transport are ubiquitous to various technologies such as power generation, cooling, electronics, and thermoelectrics. In this paper we advocate for the application of the quantum self-consistent reservoir method, which is based on the generalized quantum Langevin equation, to study phononic thermal conduction in molecular junctions. The method emulates phonon-phonon scattering processes while taking into account quantum effects and far-from-equilibrium (large temperature difference) conditions. We test the applicability of the method by simulating the thermal conductance of molecular junctions with one-dimensional molecules sandwiched between solid surfaces. Our results satisfy the expected behavior of the thermal conductance in anharmonic chains as a function of length, phonon scattering rate and temperature, thus validating the computational scheme. Moreover, we examine the effects of vibrational mismatch between the solids' phonon spectra on the heat transfer characteristics in molecular junctions. Here, we reveal the dual role of vibrational anharmonicity: It raises the resistance of the junction due to multiple scattering processes, yet it promotes energy transport across a vibrational mismatch by enabling phonon recombination and decay processes.

cond-mat.mes-hall↗

arXiv2018

On the relationship between the mean first-passage time and the steady state transfer rate in classical chains

Understanding excitation and charge transfer in disordered media is a significant challenge in chemistry, biophysics and material science. We study two experimentally-relevant measures for carriers transfer in finite-size chains, the trapping mean first-passage time (MFPT) and the steady state transfer time (SSTT). We discuss the relationship between these measures, and derive analytic formulae for one-dimensional chains. We exemplify the behavior of these timescales in different motifs: donor-bridge-acceptor systems, biased chains, and alternating and stacked co-polymers. We find that the MFPT and the SSTT may administer different, complementary information on the system, jointly reporting on molecular length and energetics. Under constraints such as fixed donor-acceptor energy bias, we show that the MFPT and the SSTT are optimized (minimized) under fundamentally different internal potential profiles. This study brings insights on the behavior of the MFPT and the SSTT, and suggests that it is beneficial to perform both transient and steady state measurements on a conducing network so as to gather a more complete picture of its properties.

cond-mat.mes-hall↗

arXiv2018

Quantum energy exchange and refrigeration: A full-counting statistics approach

We formulate a full-counting statistics description to study energy exchange in multi-terminal junctions. Our approach applies to quantum systems that are coupled either additively or non-additively (cooperatively) to multiple reservoirs. We derive a Markovian Redfield-type equation for the counting-field dependent reduced density operator. Under the secular approximation, we confirm that the cumulant generating function satisfies the heat exchange fluctuation theorem. Our treatment thus respects the second law of thermodynamics. We exemplify our formalism on a multi-terminal two-level quantum system, and apply it to realize the smallest quantum absorption refrigerator, operating through engineered reservoirs, and achievable only through a cooperative bath interaction model.

cond-mat.mes-hall↗

arXiv2018

On the validity of the thermodynamic uncertainty relation in quantum systems

We examine the so-called thermodynamic uncertainty relation (TUR), a cost-precision trade-off relationship in transport systems. Based on the fluctuation symmetry, we derive a condition on the validity of the TUR for general nonequilibrium (classical and quantum) systems. We find that the first non-zero contribution to the TUR beyond equilibrium, given in terms of nonlinear transport coefficients, can be positive or negative, thus affirming or violating the TUR depending on the details of the system. We exemplify our results for noninteracting quantum systems by deriving the thermodynamic uncertainty relation in the language of the transmission function. We demonstrate that quantum coherent systems that do not follow a population Markovian master equation, e.g. by supporting high order tunneling processes or relying on coherences, violate the TUR.

cond-mat.mes-hall↗

arXiv2018

Current fluctuations in quantum absorption refrigerators

Absorption refrigerators transfer thermal energy from a cold bath to a hot bath without input power by utilizing heat from an additional "work" reservoir. Particularly interesting is a three-level design for a quantum absorption refrigerator, which can be optimized to reach the maximal (Carnot) cooling efficiency. Previous studies of three-level chillers focused on the behavior of the averaged cooling current. Here, we go beyond that and study the full counting statistics of heat exchange in a three-level chiller model. We explain how to obtain the complete cumulant generating function of the refrigerator in steady state, then derive a partial cumulant generating function, which yields closed-form expressions for both the averaged cooling current and its noise. Our analytical results and simulations are beneficial for the design of nanoscale engines and cooling systems far from equilibrium, with their performance optimized according to different criteria, efficiency, power, fluctuations and dissipation.

cond-mat.mes-hall↗

arXiv2018

Coherence and decoherence in quantum absorption refrigerators

Absorption refrigerators transfer thermal energy from a cold reservoir to a hot reservoir using input energy from a third, so-called work reservoir. We examine the operation of quantum absorption refrigerators when coherences between eigenstates survive in the steady state limit. In our model, the working medium comprises a discrete, four-level system. We manifest that eigenbasis quantum coherences within this system generally suppress the cooling current in the refrigerator, while minimally affecting the coefficient of performance (cooling efficiency). We rationalize the behavior of the four-level refrigerator by studying two, three-level model systems for energy transport and refrigeration. Our calculations further illuminate the shortcomings of secular quantum master equations, and the necessity of employing dynamical equations of motion that retain couplings between population and coherences.

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arXiv2017

From exhaustive simulations to key principles in DNA nanoelectronics

Charge transfer can take place along double helical DNA over distances as long as 30 nanometers. However, given the active role of the thermal environment surrounding charge carriers in DNA, physical mechanisms driving the transfer process are highly debated. Moreover, the overall potential of DNA to act as a conducting material in nanoelectronic circuits is questionable. Here, we identify key principles in DNA nanoelectronics by performing an exhaustive computational study. The electronic structure of double-stranded DNA is described with a coarse-grained model. The dynamics of the molecular system and its environment is taken into account using a quantum scattering method, mimicking incoherent, elastic and inelastic effects. By analyzing all possible sequences with 3 to 7 base pairs, we identify fundamental principles in DNA nanoelectronics: The environment crucially influences the electrical conductance of DNA, and the majority of sequences conduct via a mixed, coherent-incoherent mechanism. Likewise, the metal-molecule coupling and the gateway states play significant roles in the transport behavior. While most sequences analyzed here are exposed to be rather poor electrical conductors, we identify exceptional DNA molecules, which we predict to be excellent and robust conductors of electric current over a wide range of physical conditions.

cond-mat.mes-hall↗

arXiv2017

Qubit absorption refrigerator at strong coupling

We demonstrate that a quantum absorption refrigerator can be realized from the smallest quantum system, a qubit, by coupling it in a non-additive (strong) manner to three heat baths. This function is un-attainable for the qubit model under the weak system-bath coupling limit, when the dissipation is additive. In an optimal design, the reservoirs are engineered and characterized by a single frequency component. We obtain then closed expressions for the cooling window and refrigeration efficiency, as well as bounds for the maximal cooling efficiency and the efficiency at maximal power. Our results agree with macroscopic designs and with three-level models for quantum absorption refrigerators, which are based on the weak system-bath coupling assumption. Beyond the optimal limit, we show with analytic calculations and numerical simulations that the cooling efficiency varies in a non-universal manner with model parameters. Our work demonstrates that strongly-coupled quantum machines can exhibit function that is un-attainable under the weak system-bath coupling assumption.

cond-mat.mes-hall↗

arXiv2017

Quantum efficiency bound for continuous heat engines coupled to non-canonical reservoirs

We derive an efficiency bound for continuous quantum heat engines absorbing heat from squeezed thermal reservoirs. Our approach relies on a full-counting statistics description of nonequilibrium transport and it is not limited to the framework of irreversible thermodynamics. Our result, a generalized Carnot efficiency bound, is valid beyond the small squeezing and high temperature limit. Our findings are embodied in a prototype three-terminal quantum photoelectric engine where a qubit converts heat absorbed from a squeezed thermal reservoir into electrical power. We demonstrate that in the quantum regime the efficiency can be greatly amplified by squeezing. From the fluctuation relation we further receive other operational measures in linear response, for example, the universal maximum power efficiency bound.

cond-mat.stat-mech↗

arXiv2017

The Anderson impurity model out of equilibrium: Assessing the accuracy of simulation techniques with an exact current-occupation relation

We study the interacting, symmetrically coupled single impurity Anderson model. By employing the nonequilibrium Green's function formalism, we establish an exact relationship between the steady-state charge current flowing through the impurity (dot) and its occupation. We argue that the steady state current-occupation relation can be used to assess the consistency of simulation techniques, and identify spurious transport phenomena. We test this relation in two different model variants: First, we study the Anderson-Holstein model in the strong electron-vibration coupling limit using the polaronic quantum master equation method. We find that the current-occupation relation is violated numerically in standard calculations, with simulations bringing up incorrect transport effects. Using a numerical procedure, we resolve the problem efficiently. Second, we simulate the Anderson model with electron-electron interaction on the dot using a deterministic numerically-exact time-evolution scheme. Here, we observe that the current-occupation relation is satisfied in the steady-state limit---even before results converge to the exact limit.

cond-mat.mes-hall↗

arXiv2017

Controlling charge transport mechanisms in molecular junctions: Distilling thermally-induced hopping from coherent-resonant conduction

The electrical conductance of molecular junctions may strongly depend on the temperature, and weakly on molecular length, under two distinct mechanisms: phase-coherent resonant conduction, with charges proceeding via delocalized molecular orbitals, and incoherent thermally-assisted multi-step hopping. While in the case of coherent conduction the temperature dependence arises from the broadening of the Fermi distribution in the metal electrodes, in the latter case it corresponds to electron-vibration interaction effects on the junction. With the objective to distill the thermally-activated hopping component, thus expose intrinsic electron-vibration interaction phenomena on the junction, we suggest the design of molecular junctions with "spacers", extended anchoring groups that act to filter out phase-coherent resonant electrons. Specifically, we study the electrical conductance of fixed-gap and variable-gap junctions that include a tunneling block, with spacers at the boundaries. Using numerical simulations and analytical considerations, we demonstrate that in our design, resonant conduction is suppressed. As a result, the electrical conductance is dominated by two (rather than three) mechanisms: superexchange (deep tunneling), and multi-step thermally-induced hopping. We further exemplify our analysis on DNA junctions with an A:T block serving as a tunneling barrier. Here, we show that the electrical conductance is insensitive to the number of G:C base-pairs at the boundaries. This indicates that the tunneling-to-hopping crossover revealed in such sequences truly corresponds to the properties of the A:T barrier.

cond-mat.mes-hall↗

arXiv2016

Energy current and its statistics in the nonequilibrium spin-boson model: Majorana fermion representation

We study the statistics of thermal energy transfer in the nonequilibrium (two-bath) spin-boson model. This quantum many-body impurity system serves as a canonical model for quantum energy transport. Our method makes use of the Majorana fermion representation for the spin operators, in combination with the Keldysh nonequilibrium Green's function approach. We derive an analytical expression for the cumulant generating function of the model in the steady state limit, and show that it satisfies the Gallavotti-Cohen fluctuation symmetry. We obtain analytical expressions for the heat current and its noise, valid beyond the sequential and the co-tunnelling regimes. Our results satisfy the quantum mechanical bound for heat current in interacting nanojunctions. Results are compared with other approximate theories, as well as with a non-interacting model, a fully harmonic thermal junction.

cond-mat.mes-hall↗

arXiv2016

Thermopower of molecular junctions: Tunneling to hopping crossover in DNA

We study the electrical conductance $G$ and the thermopower $S$ of single-molecule junctions, and reveal signatures of different transport mechanisms: off-resonant tunneling, on-resonant coherent (ballistic) motion, and multi-step hopping. These mechanisms are identified by studying the behavior of $G$ and $S$ while varying molecular length and temperature. Based on a simple one-dimensional model for molecular junctions, we derive approximate expressions for the thermopower in these different regimes. Analytical results are compared to numerical simulations, performed using a variant of Büttiker's probe technique, the so-called voltage-temperature probe, which allows us to phenomenologically introduce environmentally-induced elastic and inelastic electron scattering effects, while applying both voltage and temperature biases across the junction. We further simulate the thermopower of GC-rich DNA molecules with mediating A:T blocks, and manifest the tunneling-to-hopping crossover in both the electrical conductance and the thermopower, in accord with measurements by Y. Li et al., Nature Comm. 7, 11294 (2016).

cond-mat.mes-hall↗

arXiv2016

Intermediate coherent-incoherent charge transport: DNA as a case study

We study an intermediate quantum coherent-incoherent charge transport mechanism in metal-molecule-metal junctions using Büttiker's probe technique. This tool allows us to include incoherent effects in a controlled manner, and thus to study situations in which partial decoherence affects charge transfer dynamics. Motivated by recent experiments on intermediate coherent-incoherent charge conduction in DNA molecules [L. Xiang {\it et al.}, Nature Chem. 7, 221-226 (2015)], we focus on two representative structures: alternating (GC)$_n$ and stacked G$_n$C$_n$ sequences; the latter structure is argued to support charge delocalization within G segments, and thus an intermediate coherent-incoherent conduction. We begin our analysis with a highly simplified 1-dimensional tight-binding model, while introducing environmental effects through Büttiker's probes. This minimal model allows us to gain fundamental understanding of transport mechanisms and derive analytic results for molecular resistance in different limits. We then use a more detailed ladder-model Hamiltonian to represent double-stranded DNA structures---with environmental effects captured by Büttiker's probes. We find that hopping conduction dominates in alternating sequences, while in stacked sequences charge delocalization (visualized directly through the electronic density matrix) supports significant resonant-ballistic charge dynamics reflected by an even-odd effect and a weak distance dependence for resistance. Our analysis illustrates that lessons learned from minimal models are helpful for interpreting charge dynamics in DNA.

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arXiv2016

Giant photon gain in large-scale quantum dot circuit-QED systems

Motivated by recent experiments on the generation of coherent light in engineered hybrid quantum systems, we investigate gain in a microwave photonic cavity coupled to quantum dot structures, and develop concrete directions for achieving a giant amplification in photon transmission. We propose two architectures for scaling up the electronic gain medium: (i) $N$ double quantum dot systems (N-DQD), (ii) $M$ quantum dots arranged in series akin to a quantum cascade laser setup. In both setups, the fermionic reservoirs are voltage biased, and the quantum dots are coupled to a single-mode cavity. Optical amplification is explained based on a sum rule for the transmission function, and it is determined by an intricate competition between two different processes: charge density response in the gain medium, and cavity losses to input and output ports. The same design principle is also responsible for the corresponding giant amplification in other photonic observables, mean photon number and emission spectrum, thereby realizing a quantum device that behaves as a giant microwave amplifier.

quant-ph↗

arXiv2016

Effects of vibrational anharmonicity on molecular electronic conduction and thermoelectric efficiency

We study inelastic vibration-assisted charge transfer effects in two-site molecular junctions, focusing on signatures of vibrational anharmonicity on the electrical characteristics and the thermoelectric response of the junction. We consider three types of oscillators: harmonic, anharmonic-Morse allowing bond dissociation, and harmonic-quartic, mimicking a confinement potential. Using a quantum master equation method which is perturbative in the electron-vibration interaction we find that the (inelastic) electrical and thermal conductances can be largely affected by the nature of the vibrational potential. In contrast, the Seebeck coefficient, the thermoelectric figure-of-merit, and the thermoelectric efficiency beyond linear response, conceal this information, showing a rather weak sensitivity to vibrational anharmonicity. Our work illustrates that anharmonic (many-body) effects, consequential to the current-voltage characteristics, are of little effect for the thermoelectric performance.

cond-mat.stat-mech↗

arXiv2016

Tunable photonic cavity coupled to a voltage-biased double quantum dot system: Diagrammatic NEGF approach

We investigate gain in microwave photonic cavities coupled to voltage-biased double quantum dot systems with an arbitrary strong dot-lead coupling and with a Holstein-like light-matter interaction, by adapting the diagrammatic Keldysh nonequilibrium Green's function approach. We compute out-of-equilibrium properties of the cavity : its transmission, phase response, mean photon number, power spectrum, and spectral function. We show that by the careful engineering of these hybrid light-matter systems, one can achieve a significant amplification of the optical signal with the voltage-biased electronic system serving as a gain medium. We also study the steady state current across the device, identifying elastic and inelastic tunnelling processes which involve the cavity mode. Our results show how recent advances in quantum electronics can be exploited to build hybrid light-matter systems that behave as single-atom amplifiers and photon source devices. The diagrammatic Keldysh approach is primarily discussed for a cavity-coupled double quantum dot architecture, but it is generalizable to other hybrid light-matter systems.

cond-mat.mes-hall↗

arXiv2015

Inelastic effects in molecular transport junctions: The probe technique at high bias

We extend the Landauer-Büttiker probe formalism for conductances to the high bias regime, and study the effects of environmentally-induced elastic and inelastic scattering on charge current in single molecule junctions, focusing on high-bias effects. The probe technique phenomenologically incorporates incoherent elastic and inelastic effects to the fully coherent case, mimicking a rich physical environment at trivial cost. We further identify environmentally-induced mechanisms which generate an asymmetry in the current, manifested as a weak diode behavior. This rectifying behavior, found in two types of molecular junction models, is absent in the coherent-elastic limit, and is only active in the case with incoherent-inelastic scattering. Our work illustrates that in the low bias - linear response regime, the commonly used "dephasing probe" (mimicking only elastic decoherence effects) operates nearly indistinguishably from a "voltage probe" (admitting inelastic-dissipative effects). However, these probes realize fundamentally distinct $I$-$V$ characteristics at high biases, reflecting the central roles of dissipation and inelastic scattering processes on molecular electronic transport far-from-equilibrium.

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