arXiv subjects

Yonatan Dubi

Publications and source records attributed to Yonatan Dubi.

At least 37 records · Page 2Linked to original sources

arXiv2020

Environment-Assisted Quantum Photo-Protection in Interacting Multi-Exciton Transfer Complexes

Transport of excitons through transfer networks is a unique example where the interplay between quantum coherence and environment-induced dephasing leads to non-trivial effects. One exciting example is the "environment-assisted quantum transport", (ENAQT), where, counter intuitively, the environment interrupts quantum transport in a way that enhances the transfer efficiency. Here we show that the interplay between dephasing and quantum coherence can lead to an additional function in exciton transport - photo-protection, that is the reduction of current upon increased excitation. We start by demonstrating that even under weak coupling conditions, weak excitations can lead to multiple-exciton generation, thus going beyond the common single-exciton approximation. We then consider the transport of two excitons, and show that a interplay between dephasing and exciton-exciton interactions leads to a reduction in current, i.e. to photo-protection. The effect may be related to actual photo-protection observed in photosynthetic systems.

physics.bio-ph↗

arXiv2020

Effects of disorder and interactions on environment assisted quantum transport

Understanding the interplay between disorder, environment and interactions is key to elucidating the transport properties of open quantum systems, from excitons in photosynthetic networks to qubits in ion traps. This interplay is studied here theoretically in the context of environment-assisted quantum transport (ENAQT), a unique situation in open system where an environment-induced dephasing can, counter-intuitively, enhance transport. First, we show a surprising situation where the particle current grows with increasing disorder, even without dephasing. Then, we suggest a specific mechanism for ENAQT (which we dub population uniformization) and demonstrate that it can explain the persistence of ENAQT deep into the disorder-induced localization regime. Finally, we show that repulsive interactions are detrimental to ENAQT, and lead to an environment-hampered quantum transport. Our predictions can readily be tested within the scope of particle current experimental capabilities.

quant-ph↗

arXiv2020

Environment assisted and environment hampered efficiency at maximum power in a molecular photo cell

The molecular photo cell, i.e., a single molecule donor-acceptor complex, beside being technologically important, is a paradigmatic example of a many-body system operating in strong non-equilibrium. The quantum transport and the photo-voltaic energy conversion efficiency of the photocell, attached to two external leads, are investigated within the open quantum system approach by solving the Lindblad master equation. The interplay of the vibrational degrees of freedom corresponding to the molecules (via the electron-phonon interaction) and the environment (via dephasing) shows its signature in the efficiency at maximum power. We find vibration assisted electron transport in the medium to strong electron-phonon coupling regime when the system does not suffer dephasing. Exposure to dephasing hampers such a vibration assisted electron transport in a specific range of dephasing rate.

cond-mat.mes-hall↗

arXiv2019

"Hot" electrons in metallic nanostructures -- non-thermal carriers or heating?

Understanding the interplay between illumination and the electron distribution in metallic nanostructures is a crucial step towards developing applications such as plasmonic photo-catalysis for green fuels, nano-scale photo-detection and more. Elucidating this interplay is challenging, as it requires taking into account all channels of energy flow in the electronic system. Here, we develop such a theory, which is based on a coupled Boltzmann-heat equations and requires only energy conservation and basic thermodynamics, where the electron distribution, and the electron and phonon (lattice) temperatures are determined {\em uniquely}. Applying this theory to realistic illuminated nanoparticle systems, we find that the electron and phonon temperatures are similar, thus justifying the (classical) single temperature models. We show that while the fraction of high-energy ``hot'' carriers compared to thermalized carriers grows substantially with illumination intensity, it remains extremely small (on the order of $10^{-8}$). Importantly, most of the absorbed illumination power goes into heating rather than generating hot carriers, thus rendering plasmonic hot carrier generation extremely inefficient. Our formulation allows for the first time a unique quantitative comparison of theory and measurements of steady-state electron distributions in metallic nanostructures.

physics.optics↗

arXiv2019

Eppur si riscalda -- and yet, it (just) heats up: Further Comments on "Quantifying hot carrier and thermal contributions in plasmonic photocatalysis"

Our Comment [Sivan et al., Science 2019] (as well as its longer version [Dubi, Un, & Sivan, ArXiv 2019], and the supporting theoretical studies [Dubi and Sivan, ArXiv 2018]) on recent attempts to distinguish thermal and non-thermal ("hot carrier") contributions to plasmon-assisted photocatalysis [Zhou et al., Science 2018] initiated a re-evaluation process of previous literature on the topic within the nano-plasmonics and chemistry communities. The Response of Zhou et al. attempts to defend the claims of the original paper. In this manuscript, we show that the Response presents additional data that further validates our central criticism: inaccurately measured temperatures (that are lower than the actual temperature of the catalyst) led Zhou et al. to incorrectly claim conclusive evidence of non-thermal effects. We identify flaws in the experimental setup (e.g. the use of the default settings for the thermal camera and incorrect positioning of the thermometer) that may have led Zhou et al. to make such claims. We further show that the Response contains several factual errors and does not address the technical problems we identified with the data acquisition in [Zhou et al., Science 2018]. We demonstrate that both the Response and the original paper contain additional faults, for example, in the power determination and in the normalization of the rate to the catalyst volume, and exhibit misconceptions regarding the thermo-optic response of metal nanostructures. The burden of proof required by the proposal of a novel physical mechanism has simply not been met, especially when the existing data can be modeled exquisitely by conventional theory.

physics.chem-ph↗

arXiv2019

Thermal effects - an alternative mechanism for plasmonic-assisted photo-catalysis

Recent experiments claimed that the enhancement of catalytic reaction rates occurs via the reduction of activation barriers driven by non-equilibrium (``hot'') electrons in plasmonic metal nanoparticles. These experiments place plasmonic photo-catalysis as a promising path for enhancing the efficiency of various chemical reactions. Here, we argue that what appears to be photo-catalysis is in fact thermo-catalysis, driven by the well-known plasmon-enhanced ability of illuminated metallic nanoparticles to serve as heat sources. Specifically, we point to some of the most important papers in the field, and show that a simple theory of illumination-induced heating can explain the extracted experimental data to remarkable agreement, with minimal to no fit parameters. We further show that any small temperature difference between the photocatalysis experiment and a control experiment performed under uniform external heating is effectively amplified by the exponential sensitivity of the reaction, and very likely to be interpreted incorrectly as ``hot'' electron effects.

physics.chem-ph↗

arXiv2019

Non-Monotonic Thermoelectric Currents and Energy Harvesting in Interacting Double Quantum-Dots

A theoretical study of the thermoelectric current and energy harvesting in an interacting double quantum dot system, connected to reservoirs held at different chemical potentials and temperatures is presented. Using a rate-equation approach, the current is evaluated for different energetic configurations of the double quantum dot. We discuss in detail the current-temperature gradient relations (the thermoelectric analog to current-voltage relations), and demonstrate that, due to interactions, the current is non-monotonically dependent on thermal bias. This interaction-induced non-monotonicity influences the possibility of harvesting thermal energy from the double-quantum dot, and it is shown that in some configurations, energy cannot be harvested at all. We analyze the conditions under which energy can indeed be harvested and converted to useful electrical power, and the optimal conditions for thermoelectric energy conversion.

cond-mat.mes-hall↗

arXiv2018

Vibration-Assisted and Vibration-Hampered Excitonic Quantum Transport

The interplay between exctions and vibrations is considered to be a key factor in determining the exciton transfer properties in light-harvesting exciton transfer complexes. Here we study this interplay theoretically in a model for exciton transport, composed of two chromophores coupled to an exciton source and sink in the presence of vibrations. We consider two cases, that show qualitatively distinct transport features. In the first, the vibrations are global and affect the two chromophores simultaneously. In the second case, the vibrations are localized on each chromophore. For global vibrations, the current exhibits {\sl anti-resonances} as a function of the chromophore energy difference, which are due to exciton-polaron interference. For local vibrations, on the other hand, the currents shows tunneling {\sl resonances} at multiples of the vibration energy. Counter-intuitively, both effects increase with increasing temperature. Our results demonstrate that an environment can either assist or hamper exciton transport, and are in accord with current understanding of energy transfer in natural exciton transfer complexes.

cond-mat.mes-hall↗

arXiv2018

Phonon as environmental disturbance in three level system

This work investigates the effect of phonon coupling on the transfer of population and creation of coherence using variant of stimulated Raman adiabatic passage (STIRAP) known as \emph{fractional} stimulated Raman adiabatic passage (FSTIRAP). The study is based on the Liouville equation, which is solved numerically in the adiabatic limit. Although the phonon is assumed to be coupled only to the intermediate state, it is coupled to the other two states by dipolar system-environment interaction, inducing phonon coupling to the other states which are not directly in contact with the phonon. At zero temperature the STIRAP pulse protocol's efficiency of the transfer decreases exponentially with the electron-phonon coupling, until the coupling strength is strong enough to make the process fully incoherent, in which case the population transfer is $\frac{1}{3}$ in each level. For the FSTIRAP protocol we find that the transferred population to target state decreases, leaving some population on the intermediate state. Consequently, there is an increase in the magnitude of the coherences $ρ_{01}, ρ_{12}$, albeit small. Furthermore population transfer for non-zero temperature and effect of coupling strength is investigated, it is observed that while both parameters negatively influence the efficiency of transfer the former decrease the transfer exponentially, thereby equilibrating the system fast, while the latter seen to decrease the transfer monotonically, and hence equilibrates slowly.

quant-ph↗

arXiv2018

Topological quantization of energy transport in micro- and nano-mechanical lattices

Topological effects typically discussed in the context of quantum physics are emerging as one of the central paradigms of physics. Here, we demonstrate the role of topology in energy transport through dimerized micro- and nano-mechanical lattices in the classical regime, i.e., essentially "masses and springs". We show that the thermal conductance factorizes into topological and non-topological components. The former takes on three discrete values and arises due to the appearance of edge modes that prevent good contact between the heat reservoirs and the bulk, giving a length-independent reduction of the conductance. In essence, energy input at the boundary mostly stays there, an effect robust against disorder and nonlinearity. These results bridge two seemingly disconnected disciplines of physics, namely topology and thermal transport, and suggest ways to engineer thermal contacts, opening a direction to explore the ramifications of topological properties on nanoscale technology.

cond-mat.mes-hall↗

arXiv2018

Universal Origin for Environment-Assisted Quantum Transport in Exciton Transfer Networks

Environment-assisted quantum transport (ENAQT) is the possibility of an external environment to enhance transport efficiency of quantum particles. This idea has generated much excitement over recent years, especially due to the experimentally-motivated possibility of ENAQT in photo-synthetic exciton transfer complexes. Many theoretical calculations have shown ENAQT, but the explanations for its origin differ, and a universal explanation has been elusive. Here we demonstrate a universal origin for ENAQT in quantum networks with a dephasing environment, based on a relation between exciton current and occupation within a Markovian open quantum system approach. We show that ENAQT appears due to two competing processes, namely the tendency of dephasing to make the exciton population uniform, and the formation of an exciton density gradient, defined by the source and the sink. Furthermore, we find a geometric condition on the network for the appearance of ENAQT, relevant to natural and artificial systems.

quant-ph↗

arXiv2017

Optical Flip-Flops and Shift Registers from populations and coherences in multi-level systems using stimulated Raman adiabatic passage

In digital circuits, a Flip-Flop (FF) is a circuit element that has two stable states which can be used to store and remember state information. The state of the circuit can be changed by applying signals to the control input. FFs are the basic building blocks of sequential logic circuits, as logic gates AND,OR, NOT are the basic building block for combinational logic circuits, and are therefore necessary for any computations involving memory. Consequently, the design and implementation of FFs can be considered as a pre-requisite for memory machine design. Here we present the design of an optical FFs in an atomic multi-level system, based on the optical manipulation of populations and coherences using stimulated Raman adiabatic passage. We first demonstrate that both populations and coherences can be transferred over multistate systems. We then propose the design of toggle-FFs, Delay-FFs, and Serial-in Serial-Out (SISO) shift registers using such systems. For the design of the filp-flops we use a three level $Λ$-type system. In order to design SISO shift registers we concatenate two $Λ$-type systems and construct an "M"-type scheme, and similarly concatenating three $Λ$-type system we are able to obtain a seven level system. By concatenating we are able to use output of one three level $Λ$-type system serve as input of another three level $Λ$-type system. On top of using populations for design of logic gates, we uniquely exploit the coherences for logic machine, which provides an additional degree of freedom which can be used for the design of computing elements.

quant-ph↗

arXiv2017

Ramp Reversal Memory and Phase-Boundary Scarring in Transition Metal Oxides

Transition metal oxides (TMOs) are complex electronic systems which exhibit a multitude of collective phenomena. Two archetypal examples are VO2 and NdNiO3, which undergo a metal-insulator phase-transition (MIT), the origin of which is still under debate. Here we report the discovery of a memory effect in both systems, manifest through an increase of resistance at a specific temperature, which is set by reversing the temperature-ramp from heating to cooling during the MIT. The characteristics of this ramp-reversal memory effect do not coincide with any previously reported history or memory effects in manganites, electron-glass or magnetic systems. From a broad range of experimental features, supported by theoretical modelling, we find that the main ingredients for the effect to arise are the spatial phase-separation of metallic and insulating regions during the MIT and the coupling of lattice strain to the local critical temperature of the phase transition. We conclude that the emergent memory effect originates from phase boundaries at the reversal-temperature leaving `scars` in the underlying lattice structure, giving rise to a local increase in the transition temperature. The universality and robustness of the effect shed new light on the MIT in complex oxides.

cond-mat.mtrl-sci↗

arXiv2017

Quantum transport under AC drive from the leads : A Quantum Master Equation approach

Evaluating the time-dependent dynamics of driven open quantum systems is relevant for a theoretical description of many systems, including molecular junctions, quantum dots, cavity-QED experiments, cold atoms experiments and more. Here, we formulate a rigorous microscopic theory of an out-of-equilibrium open quantum system of non-interacting particles on a lattice weakly coupled to multiple baths and driven by periodically varying thermodynamic parameters like temperature and chemical potential of the bath. The particles can be either bosonic or fermionic and the lattice can be of any dimension and geometry. Based on Redfield quantum master equation under Born-Markov approximation, we derive a complete set of linear differential equations for equal time two-point correlation functions from which various physical observables, for example, current, can be calculated. Various interesting physical effects, such as resonance, can be directly read-off from the equations. Thus, our theory is quite general gives quite transparent and easy-to-calculate results. We validate our theory by comparing with exact numerical simulations. We apply our method to a generic open quantum system, namely a double-quantum dot coupled to leads with modulating chemical potentials. Two most important experimentally relevant insights from this are : (i) time-dependent measurements of current for symmetric oscillating voltages (with zero instantaneous voltage bias) can point to the degree of asymmetry in the system, and (ii) under certain conditions, time-dependent currents can exceed time-averaged currents by several orders of magnitude, and can therefore be detected even when the average current is below the measurement threshold.

cond-mat.mes-hall↗

arXiv2016

Large Tunable Thermophase in Superconductor -- Quantum Dot -- Superconductor Josephson Junctions

In spite of extended efforts, detecting thermoelectric effects in superconductors have proven to be a challenging task, due to the inherent superconducting particle-hole symmetry. Here we present a theoretical study of an experimentally attainable Superconductor -- Quantum Dot -- Superconductor (SC-QD-SC) Josephson Junction with highly tunable thermoelectric properties and a thermal response that is far larger than previous suggestions. The QD energy level between the SCs breaks particle-hole symmetry in a gradual manner, allowing, in the presence of a temperature gradient, for gate controlled appearance of a superconducting thermo-phase. This thermo-phase increases up to a maximal value of $π/2$ after which thermovoltage is expected to develop. Using time-independent Keldysh countour Green's functions we derive the exact thermo-phase and thermal response on the junction. These are shown to be orders of magnitude larger than usual SC tunnel junctions with a sharp dependence on gate voltage.

cond-mat.mes-hall↗

arXiv2015

Crossover behavior of the thermal conductance and Kramers' transition rate theory

Kramers' theory frames chemical reaction rates in solution as reactants overcoming a barrier in the presence of friction and noise. For weak coupling to the solution, the reaction rate is limited by the rate at which the solution can restore equilibrium after a subset of reactants have surmounted the barrier to become products. For strong coupling, there are always sufficiently energetic reactants. However, the solution returns many of the intermediate states back to the reactants before the product fully forms. Here, we demonstrate that the thermal conductance displays an analogous physical response to the friction and noise that drive the heat current through a material or structure. A crossover behavior emerges where the thermal reservoirs dominate the conductance at the extremes and only in the intermediate region are the intrinsic properties of the lattice manifest. Not only does this shed new light on Kramers' classic turnover problem, this result is significant for the design of devices for thermal management and other applications, as well as the proper simulation of transport at the nanoscale.

cond-mat.stat-mech↗

arXiv2015

Interplay between Dephasing and Geometry and Directed Heat Flow in Exciton Transfer Complexes

The striking efficiency of energy transfer in natural photosynthetic systems and the recent evidence of long-lived quantum coherence in biological light harvesting complexes has triggered much excitement, due to the evocative possibility that these systems - essential to practically all life on earth -- use quantum mechanical effects to achieve optimal functionality. A large body of theoretical work has addressed the role of local environments in determining the transport properties of excitons in photosynthetic networks and the survival of quantum coherence in a classical environment. Nonetheless, understanding the connection between quantum coherence, exciton network geometry and energy transfer efficiency remains a challenge. Here we address this connection from the perspective of heat transfer within the exciton network. Using a non-equilibrium open quantum system approach and focusing on the Fenna-Matthews-Olson complex, we demonstrate that finite local dephasing can be beneficial to the overall power output. The mechanism for this enhancement of power output is identified as a gentle balance between quantum and classical contributions to the local heat flow, such that the total heat flow is directed along the shortest paths and dissipation is minimized. Strongly related to the spatial network structure of the exciton transfer complex, this mechanism elucidates how energy flows in photosyntetic excitonic complexes.

cond-mat.mes-hall↗

arXiv2015

Enhanced Thermoelectric Performance in Hybrid Nanoparticle--Single Molecule Junctions

It was recently suggested that molecular junctions would be excellent elements for efficient and high-power thermoelectric energy conversion devices. However, experimental measurements of thermoelectric conversion in molecular junctions have indicated rather poor efficiency, raising the question of whether it is indeed possible to design a setup for molecular junctions that will exhibit enhanced thermoelectric performance. Here we suggest that hybrid single-molecule nanoparticle junctions can serve as efficient thermoelectric converters. The introduction of a semiconducting nanoparticle introduces new tuning capabilities, which are absent in conventional metal-molecule-metal junctions. Using a generic model for the molecule and nanoparticle with realistic parameters, we demonstrate that the thermopower can be of the order of hundreds of microvolts per degree Kelvin, and that the thermoelectric figure of merit can reach values close to one, an improvement of four orders of magnitude improvement over existing measurements. This favorable performance persists over a wide range of experimentally relevant parameters, and is robust against disorder (in the form of surface-attached molecules) and against electron decoherence at the nanoparticle molecule interface.

cond-mat.mes-hall↗