arXiv subjects

Raúl A. Bustos-Marún

Publications and source records attributed to Raúl A. Bustos-Marún.

At least 19 recordsLinked to original sources

arXiv2024

Second-order adiabatic expansions of heat and charge currents with nonequilibrium Green's functions

Due to technological needs, nanoscale heat management, energy conversion and quantum thermodynamics have become key areas of research, putting heat pumps and nanomotors center stage. The treatment of these particular systems often requires the use of adiabatic expansions in terms of the frequency of the external driving or the velocity of some classical degree of freedom. However, due to the difficulty of getting the expressions, most works have only explored first-order terms. Despite this, adiabatic expansions have allowed the study of intriguing phenomena such as adiabatic quantum pumps and motors, or electronic friction. Here, we use nonequilibrium Green's functions, within a Schwinger-Keldysh approach, to develop second-order expressions for the energy, heat, and charge currents. We illustrate, through two simple models, how the obtained formulas produce physically consistent results, and allow for the thermodynamic study of unexplored phenomena, such as second-order monoparametric pumping.

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arXiv2024

Hysteresis and effective reciprocity breaking due to current-induced forces

Directed transport is a key concept for many ongoing applications including nanoscale heat management, current rectification, source protection, and energy harvesting. Within the context of quantum transport, we here explore the use of nonlinear effects introduced by current-induced forces (CIFs) as a practical way to effectively break charge and heat transport reciprocities. In particular, we consider a simple model consisting of a mobile quantum dot (QD) coupled to two leads, where the charge (or heat) current develops an asymmetric behavior under inversion of voltage (or temperature) bias, thereby turning the system into a quantum diode (or quantum thermal diode). Furthermore, we find multiple stable positions for the QD and we show how the extraction of useful work is possible by modulating the nonequilibrium sources along well-established hysteresis loops. Finally, we explore a particular case where the nonlinearity of the CIFs can be exploited to pump heat or charge, even for systems that preserve inversion symmetry. This counterintuitive result is attributed to a spontaneous breaking of the inversion symmetry due to the intrinsic system's dynamics.

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arXiv2023

Fluctuation-induced currents in suspended graphene nanoribbons: Adiabatic quantum pumping approach

Graphene nanoribbons (GNRs) are thin strips of graphene with unique properties due to their structure and nanometric dimensions. They stand out as basic components for the construction of different types of nanoelectromechanical systems (NEMS), including some very promising sensors and pumps. However, various phenomena, such as unintended mechanical vibrations, can induce undesired electrical currents in these devices. Here, we take a quantum mechanical approach to analyze how currents induced by fluctuations (either thermal or of some other kind) in suspended GNRs contribute to the electric current. In particular, we study the pumping current induced by the adiabatic variation of the Hamiltonian of the system when a transverse vibration (flexural mode) of a GNR suspended over a gate is excited. Our theoretical approach and results provide useful tools and rules of thumb to understand and control the charge current induced by fluctuations in GNR-based NEMS, which is important for their applications in nanoscale sensors, pumps, and energy harvesting devices.

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arXiv2022

A dynamical model for Brownian molecular motors driven by inelastic electron tunneling

In recent years, several artificial molecular motors driven and controlled by electric currents have been proposed. Similar to Brownian machines, these systems work by turning random inelastic tunneling events into a directional rotation of the molecule. Despite their importance as the ultimate component of future molecular machines, their modeling has not been sufficiently studied. Here, we develop a dynamical model to describe these systems. We illustrate the validity and usefulness of our model by applying it to a well-known molecular motor, showing that the obtained results are consistent with the available experimental data. Moreover, we demonstrate how to use our model to extract some difficult-to-access microscopic parameters. Finally, we include an analysis of the expected effects of current-induced forces (CIFs). Our analysis suggests that, although nonconservative contributions of the CIFs can be important in some scenarios, they do not seem important in the analyzed case. Despite this, the conservative contributions of CIFs could be strong enough to significantly alter the system's dynamics.

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arXiv2021

Role of coherence in quantum-dot-based nanomachines within the Coulomb blockade regime

During the last decades, quantum dots within the Coulomb blockade regime of transport have been proposed as essential building blocks for a wide variety of nanomachines. This includes thermoelectric devices, quantum shuttles, quantum pumps, and even quantum motors. However, in this regime, the role of quantum mechanics is commonly limited to provide energy quantization while the working principle of the devices is ultimately the same as their classic counterparts. Here, we study quantum-dot-based nanomachines in the Coulomb blockade regime, but in a configuration where the coherent superpositions of the dots' states plays a crucial role. We show that the studied system can be used as the basis for different forms of "true" quantum machines that should only work in the presence of these coherent superpositions. We analyze the efficiency of these machines against different nonequilibrium sources (bias voltage, temperature gradient, and external driving) and the factors that limit it, including decoherence and the role of the different orders appearing in the adiabatic expansion of the charge/heat currents.

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arXiv2020

Current-induced forces in single-resonance systems

In recent years, there has been an increasing interest in nanoelectromechanical devices, current-driven quantum machines, and the mechanical effects of electric currents on nanoscale conductors. Here, we carry out a thorough study of the current-induced forces and the electronic friction of systems whose electronic effective Hamiltonian can be described by an archetypal model, a single energy level coupled to two reservoirs. Our results can help better understand the general conditions that maximize the performance of different devices modeled as a quantum dot coupled to two electronic reservoirs. Additionally, they can be useful to rationalize the role of current-induced forces in the mechanical deformation of one-dimensional conductors.

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arXiv2020

Entropy current and efficiency of quantum machines driven by nonequilibrium incoherent reservoirs

Nanotechnology has not only provided us the possibility of developing quantum machines but also noncanonical power sources able to drive them. Here we focus on studying the performance of quantum machines driven by arbitrary combinations of equilibrium reservoirs and a form of engineered reservoirs consisting of noninteracting particles but whose distribution functions are nonthermal. We provide the expressions for calculating the maximum efficiency of those machines without needing any knowledge of how the nonequilibrium reservoirs were actually made. The formulas require the calculation of a quantity that we term entropy current, which we also derive. We illustrate our methodology through a solvable toy model where heat "spontaneously" flows against the temperature gradient.

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arXiv2019

Thermodynamics and steady state of quantum motors and pumps far from equilibrium

In this article, we briefly review dynamical and thermodynamical aspects of different forms of quantum motors and quantum pumps. We then extend previous results to provide new theoretical tools for a systematic study of those phenomena at far-from-equilibrium conditions. We mainly focus on two key topics: (1) The steady-state regime of quantum motors and pumps, paying particular attention to the role of higher-order terms in the nonadiabatic expansion of the current-induced forces. (2) The thermodynamical properties of such systems, emphasizing systematic ways of studying the relationship between different energy fluxes (charge and heat currents, and mechanical power) passing through the system when beyond-first-order expansions are required. We derive a general order-by-order scheme based on energy conservation to rationalize how every order of the expansion of one form of energy flux is connected with the others. We use this approach to give a physical interpretation of the leading terms of the expansion. Finally, we illustrate the above-discussed topics in a double quantum dot within the Coulomb-blockade regime and capacitively coupled to a mechanical rotor. We find many exciting features of this system for arbitrary nonequilibrium conditions: A definite parity of the expansion coefficients with respect to the voltage or temperature biases; negative friction coefficients; and the fact that, under fixed parameters, the device can exhibit multiple steady states where it may operate as a quantum motor or as a quantum pump depending on the initial conditions.

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arXiv2018

Nonequilibrium current-induced forces caused by quantum localization: Anderson Adiabatic Quantum Motors

In recent years there has been an increasing interest in nanomachines. Among them, current-driven ones deserve special attention as quantum effects can play a significant role there. Examples of the latter are the so-called adiabatic quantum motors. In this work, we propose using Anderson's localization to induce nonequilibrium forces in adiabatic quantum motors. We study the nonequilibrium current-induced forces and the maximum efficiency of these nanomotors in terms of their respective probability distribution functions. Expressions for these distribution functions are obtained in two characteristic regimes: the steady-state and the short-time regimes. Even though both regimes have distinctive expressions for their efficiencies, we find that, under certain conditions, the probability distribution functions of their maximum efficiency are approximately the same. Finally, we provide a simple relation to estimate the minimal disorder strength that should ensure efficient nanomotors.

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arXiv2018

Geometric rectification for nanoscale vibrational energy harvesting

In this work, we present a mechanism that, based on quantum-mechanical principles, allows one to recover kinetic energy at the nanoscale. Our premise is that very small mechanical excitations, such as those arising from sound waves propagating through a nanoscale system or similar phenomena, can be quite generally converted into useful electrical work by applying the same principles behind conventional adiabatic quantum pumping. The proposal is potentially useful for nanoscale vibrational energy harvesting where it can have several advantages. The most important one is that it avoids the use of classical rectification mechanisms as it is based on what we call geometric rectification. We show that this geometric rectification results from applying appropriate but quite general initial conditions to damped harmonic systems coupled to electronic reservoirs. We analyze an analytically solvable example consisting of a wire suspended over permanent charges where we find the condition for maximizing the pumped charge. We also studied the effects of coupling the system to a capacitor including the effect of current-induced forces and analyzing the steady-state voltage of operation. Finally, we show how quantum effects can be used to boost the performance of the proposed device.

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arXiv2017

Real-time diagrammatic approach to current-induced forces: Application to quantum-dot based nanomotors

During the last years there has been an increasing excitement in nanomotors and particularly in current-driven nanomotors. Despite the broad variety of stimulating results found, the regime of strong Coulomb interactions has not been fully explored for this application. Here we consider nanoelectromechanical devices composed by a set of coupled quantum dots interacting with mechanical degrees of freedom taken in the adiabatic limit and weakly coupled to electronic reservoirs. We use a real-time diagrammatic approach to derive general expressions for the current-induced forces, friction coefficients, and zero-frequency force noise in the Coulomb blockade regime of transport. We prove our expressions accomplish with Onsager's reciprocity relations and the fluctuation-dissipation theorem for the energy dissipation of the mechanical modes. The obtained results are illustrated in a nanomotor consisting of a double quantum dot capacitively coupled to some rotating charges. We analyze the dynamics and performance of the motor as function of the applied voltage and loading force for trajectories encircling different triple points in the charge stability diagram.

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arXiv2017

Dynamics and decoherence in nonideal Thouless quantum motors

Different proposals for adiabatic quantum motors (AQMs) driven by DC currents have recently attracted considerable interest. However, the systems studied are often based on simplified models with highly ideal conditions where the environment is neglected. Here, we investigate the performance (dynamics, efficiency, and output power) of a prototypical AQM, the Thouless motor. To include the effect of the surroundings on this type of AQMs, we extended our previous theory of decoherence in current-induced forces (CIFs) to account for spatially distributed decoherent processes. We provide analytical expressions that account for decoherence in CIFs, friction coefficients and the self-correlation functions of the CIFs. We prove that the model is thermodynamically consistent and we find that decoherence drastically reduces the efficiency of the motor mainly due to the increase in conductance, while its effect on the output power is not much relevant. The effect of decoherence on the current-induced friction depends on the length of the system, reducing the friction for small systems while increasing it for long ones. Finally, we find that reflections of the electrons at the boundary of the system induce additional conservative forces that affect the dynamics of the motor. In particular, this results in the hysteresis of the system and a voltage dependent switching.

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arXiv2016

Spaser and optical amplification conditions in gold-coated active nanoparticles

Due to their many potential applications, there is an increasing interest in studying hybrid systems composed of optically active media and plasmonic metamaterials. In this work we focus on a particular system which consists of an optically active silica core covered by a gold shell. We find that the spaser (surface plasmon amplification by stimulated emission of radiation) conditions can be found at the poles of the scattering cross section of the system, a result that remains valid beyond the geometry studied. We explored a wide range of parameters that cover most of the usual experimental conditions in terms of the geometry of the system and the wavelength of excitation. We show that the conditions of spaser generation necessarily require full loss compensation, but the opposite is not necessarily true. Our results, which are independent of the detailed response of the active medium, provide the gain needed and the wavelength of the spasers that can be produced by a particular geometry, discussing also the possibility of turning the system into optical amplifiers and SERS (surface enhanced Raman spectroscopy) substrates with huge enhancements. We believe that our results can find numerous applications. In particular, they can be useful for experimentalists studying similar systems in both, tuning the experimental conditions and interpreting the results.

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arXiv2015

Decoherence in current induced forces: Application to adiabatic quantum motors

Current induced forces are not only related with the discrete nature of electrons but also with its quantum character. It is natural then to wonder about the effect of decoherence. Here, we develop the theory of current induced forces including dephasing processes and we apply it to study adiabatic quantum motors (AQMs). The theory is based on Büttiker's fictitious probe model which here is reformulated for this particular case. We prove that it accomplishes fluctuation-dissipation theorem. We also show that, in spite of decoherence, the total work performed by the current induced forces remains equal to the pumped charge per cycle times the voltage. We find that decoherence affects not only the current induced forces of the system but also its intrinsic friction and noise, modifying in a non trivial way the efficiency of AQMs. We apply the theory to study an AQM inspired by a classical peristaltic pump where we surprisingly find that decoherence can play a crucial role by triggering its operation. Our results can help to understand how environmentally induced dephasing affects the quantum behavior of nano-mechanical devices.

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arXiv2015

Plasmonic Graded-Chains as Deep-Subwavelength Light Concentrators

We have studied the plasmonic properties of aperiodic arrays of identical nanoparticles (NPs) formed by two opposite and equal graded-chains (a chain where interactions change gradually). We found that these arrays concentrate the external electromagnetic fields even in the long wavelength limit. The phenomenon was understood by identifying the system with an effective cavity where plasmonics excitations are trapped between effective band edges, resulting from the change of passband with NP's position. Dependence of excitation concentration on several system's parameter was also assessed. This includes, different gradings as well as NP's couplings, damping, and resonant frequencies. In the spirit of the scaling laws in condensed matter physics, we developed a theory that allows us to rationalize all these system's parameters into universal curves. The theory is quite general and can also be used on many other situations (different arrays for example). Additionally, we also provided an analytical solution, in the tight-binding limit, for the plasmonic response of homogeneous linear chains of NPs illuminated by a plane wave. Our results can find applications on sensing, near field imaging, plasmon-enhanced photodetectors, as well as to increase solar cell efficiency.

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arXiv2014

Generalized multi-terminal decoherent transport: Recursive algorithms and applications to SASER and giant magnetoresistance

Decoherent transport in mesoscopic and nanoscopic systems can be formulated in terms of the D'Amato-Pastawski (DP) model. This generalizes the Landauer-Büttiker picture by considering a distribution of local decoherent processes. However, its generalization for multi-terminal setups is lacking. We first review the original two-terminal DP model for decoherent transport. Then, we extend it to a matrix formulation capable of dealing with multi-terminal problems. We also introduce recursive algorithms to evaluate the Green's functions for general banded Hamiltonians as well as local density of states, effective conductances and voltage profiles. We finally illustrate the method by analyzing two problems of current relevance. 1) Assessing the role of decoherence in a model for phonon lasers (SASER). 2) Obtaining the classical limit of Giant Magnetoresistance from a spin-dependent Hamiltonian. The presented methods should pave the way for computationally demanding calculations of transport through nanodevices, bridging the gap between fully coherent quantum schemes and semiclassical ones.

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arXiv2014

Tailoring optical fields emitted by nanometric sources

Here we study a simple way of controlling the emitted fields of sub-wavelength nanometric sources. The system consists of arrays of nanoparticles (NPs) embedded in optical active media. The key concept is the careful tuning of NP's damping factors, which changes the eigenmode's decay rates of the whole array. This, at long time, leads to a locking of relative phases and frequencies of individual localized-surfaces-plasmons (LSPs) and, thus, controlls the emitted field. The amplitude of the LSP's oscillations can be kept constant by embedding the system in optical active media. In the case of full loss compensation, this implies that, not only the relative phases, but also the amplitudes of the LSPs remain fixed, leading us, additionally, to interpret the process as a new example of synchronization. The proposed approach can be used as a general way of controlling and designing the electromagnetic fields emitted by nanometric sources, which can find applications in optoelectronic, nanoscale lithography and probing microscopy.

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arXiv2012

Excitation-Transfer Plasmonic Nanosensors based on Dynamical Phase Transitions

Dynamical Phase transitions (DPTs) describe the abrupt change in the dynamical properties of open systems when a single control parameter is slightly modified. Recently we found that this phenomenon is also present in a simple model of a linear array of metallic nanoparticles (NPs) in the form of a localized-delocalized DPT. In this work we show how to take advantage of DPTs in order to design a new kind of plasmonic sensor which should own some unique characteristics. For example, if it were used as plasmon ruler it would not follow the so called universal plasmon ruler equation [Nano Letters 2007, 7, 2080-2088], exhibiting instead an on-off switching feature. This basically means that a signal should only be observed when the control/measured parameter, i.e. a distance in the case of plasmon rulers, has a very precise and pre-determined value. Here, we demonstrate their feasibility and unique characteristics, showing that they combine high sensitivity with this on-off switching feature in terms of different distances and local dielectric constants. This property has the potentiality to be used in the design of new plasmonic devices such as plasmonic circuits activated only under certain environmental conditions.

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