David Gelbwaser-Klimovsky

On the operation of machines powered by quantum non-thermal baths

Diverse models of engines energised by quantum-coherent, hence non-thermal, baths allow the engine efficiency to transgress the standard thermodynamic Carnot bound. These transgressions call for an elucidation of the underlying mechanisms. Here we show that non-thermal baths may impart not only heat, but also mechanical work to a machine. The Carnot bound is inapplicable to such a hybrid machine. Intriguingly, it may exhibit dual action, concurrently as engine and refrigerator, with up to 100% efficiency. We conclude that even though a machine powered by a quantum bath may exhibit an unconventional performance, it still abides by the traditional principles of thermodynamics.

Strongly Coupled Quantum Heat Machines

Energy conversion of heat into work at the quantum level is modeled by quantum heat machines (QHMs) generally assumed to operate at weak coupling to the baths. This supposition is grounded in the separability principle between systems and allows the derivation of the evolution equation. In the weak coupling regime, the machines output is limited by the coupling strength, restricting their application. Seeking to overcome this limitation, we analyze QHMs in the virtually unexplored strong coupling regime here, where separability, as well as other standard thermodynamic assumptions, may no longer hold. We show that strongly coupled QHMs may be as efficient as their weakly coupled counterparts. In addition, we find a novel turnover behavior where their output saturates and disappears in the limit of ultrastrong coupling.

Non-equilibrium quantum heat machines

Standard heat machines (engine, heat pump, refrigerator) are composed of a system (working fluid) coupled to at least two equilibrium baths at different temperatures and periodically driven by an external device (piston or rotor) sometimes called the work reservoir. The aim of this paper is to go beyond this scheme by considering environments which are stationary but cannot be decomposed into a few baths at thermal equilibrium. Such situations are important, for example in solar cells, chemical machines in biology, various realizations of laser cooling or nanoscopic machines driven by laser radiation. We classify non-equilibrium baths depending on their thermodynamic behavior and show that the efficiency of heat machines powered by them is limited by the generalized Carnot bound.

Solar cell as a self-oscillating heat engine

Solar cells are engines converting energy supplied by the photon flux into work. All known types of macroscopic engines and turbines are also self-oscillating systems which yield a periodic motion at the expense of a usually non-periodic source of energy. The very definition of work in the formalism of quantum open systems suggests the hypothesis that the oscillating piston is a necessary ingredient of the work extraction process. This aspect of solar cell operation is absent in the existing descriptions and the main goal of this paper is to show that plasma oscillations provide the physical implementation of a piston.

Laser-induced cooling of broadband heat reservoirs

We explore, theoretically and experimentally, a method for cooling a broadband heat reservoir, via its laser-assisted collisions with two-level atoms followed by their fluorescence. This method is shown to be advantageous compared to existing laser-cooling methods in terms of its cooling efficiency, the lowest attainable temperature for broadband baths and its versatility: it can cool down any heat reservoir, provided the laser is red-detuned from the atomic resonance. It is applicable to cooling down both dense gaseous and condensed media.

A thermodynamic cycle for the solar cell

Abstract A solar cell is a heat engine, but textbook treatments are not wholly satisfactory from a thermodynamic standpoint, since they present solar cells as directly converting the energy of light into electricity, and the current in the circuit as maintained by an electrostatic potential. We propose a thermodynamic cycle in which the gas of electrons in the p phase serves as the working substance. The interface between the p and n phases acts as a self-oscillating piston that modulates the absorption of heat from the photons so that it may perform a net positive work during a complete cycle of its motion, in accordance with the laws of thermodynamics. We draw a simple hydrodynamical analogy between this model and the “putt-putt” engine of toy boats, in which the interface between the water’s liquid and gas phases serves as the piston. We point out some testable consequences of this model.

High-Voltage-Assisted Mechanical Stabilization of Single-Molecule Junctions

Resonant tunneling is an efficient mechanism for charge transport through nanoscale conductance junctions due to the relatively high currents involved. However, continuous charging and discharging cycles of the nanoconductor during resonant tunneling often lead to mechanical instability. The realization of efficient nanoscale electronic components therefore depends to a large extent on the ability to mechanically stabilize them during resonant transport. In this work, we focus on single-molecule junctions, demonstrating that their mechanical stability during resonant transport can be increased by increasing the bias voltage. This counter-intuitive effect is attributed to the energy dependence of the molecule-lead coupling densities, which promote the rate of transport-induced cooling of molecular vibrations at higher voltages. The required energy dependence is characteristic of realistic electrodes (such as graphene), which cannot be modeled within the commonly invoked wide-band approximation. Our research provides new guidelines for the design of mechanically stable molecular devices operating in the regime of resonant charge transport and demonstrates these guidelines while considering realistic features of single-molecule junctions.

Exploring Electronic Structure and Order in Polymers via Single-Particle Microresonator Spectroscopy

PEDOTnPSS, a transparent electrically conductive polymer, finds widespread use in electronic devices. While empirical efforts have increased conductivity, a detailed understanding of the coupled electronic and morphological landscapes in PEDOT:PSS has lagged due to substantial structural heterogeneity on multiple length-scales. We use an optical microresonator-based absorption spectrometer to perform single-particle measurements, providing a bottom-up examination of electronic structure and morphology ranging from single PEDOT:PSS polymers to nascent films. Using single-particle spectroscopy with complementary theoretical calculations and ultrafast spectroscopy, we demonstrate that PEDOT:PSS displays bulk-like optical response even in single polymers. We find highly ordered PEDOT assemblies with long-range ordering mediated by the insulating PSS matrix and reveal a preferential surface orientation of PEDOT nanocrystallites absent in bulk films with implications for interfacial electronic communication. Our single-particle perspective provides a unique window into the microscopic structure and electronic properties of PEDOT:PSS.

Optical Spectra of p-Doped PEDOT Nanoaggregates Provide Insight into the Material Disorder

Highly doped poly(3,4-ethylenedioxythiophene), or PEDOT, is a conductive polymer with a wide range of applications in energy conversion due to its ease of processing, optical properties, and high conductivity. The latter is influenced by processing conditions, including formulation, annealing, and solvent treatment of the polymer, which also affects the polymer arrangement. Here, we show that the analysis of the optical spectra of PEDOT domains reveals the nature and magnitude of the structural disorder in the material. In particular, the optical spectra of objects on individual domains can be used for elucidation of molecular disorder in an oligomer arrangement, which is a key factor affecting the conductivity.

Two-level masers as heat-to-work converters

Significance We propose a paradigm of heat-powered maser. In contrast to textbook knowledge, it does not require population inversion or coherent driving and hence can operate with a two-level working medium. Therefore, it is a conceptually different type of maser and, more generally, a conceptually different quantum heat machine. Its autonomous character and “free” power source make this machine technologically enticing. Heat engines, which cyclically transform heat into work, are ubiquitous in technology. Lasers and masers may be viewed as heat engines that rely on population inversion or coherence in the active medium. Here we put forward an unconventional paradigm of a remarkably simple and robust electromagnetic heat-powered engine that bears basic differences to any known maser or laser: The proposed device makes use of only one Raman transition and does not rely on population inversion or coherence in its two-level working medium. Nor does it require any coherent driving. The engine can be powered by the ambient temperature difference between the sky and the ground surface. Its autonomous character and “free” power source make this engine conceptually and technologically enticing.