arXiv subjects

Alejandro Jenkins

Publications and source records attributed to Alejandro Jenkins.

At least 19 recordsLinked to original sources

arXiv2026

Portrait of the mathematician as a young man: Revisiting Trinity's Tayler picture

A 17th-century oil painting by an unknown artist, once owned by the Tayler family and now in the collection of Trinity College, Cambridge, is currently identified as a portrait of a young Isaac Barrow. The sitter is shown pointing to a proposition in Barrow's 1655 edition of Euclid's Elements, but the portrait bears little resemblance to other depictions of Barrow. Moreover, Barrow is unlikely to have posed with that book, which appeared in print eight months after he had left England on a four-year tour of the Continent. Plausible alternatives are that the portrait is of Francis Willughby or Isaac Newton, both of whom resembled the man pictured and may be characterized as disciples of Barrow. If the Tayler were Newton's portrait, it could shed light on the patronage that allowed him to rise from undergraduate servant ("subsizar") to Lucasian Professor of Mathematics in only five and half years.

physics.hist-ph↗

arXiv2025

Non-Equilibrium Thermodynamics of Black-Hole Coronae: QPOs, Turbulence, and Jets

The variability of X-rays observed from accreting black hole systems, including quasi-periodic oscillations (QPOs), suggests a complex nonlinear dynamics in the corona. Here, we propose a new theoretical framework for this variability, based on non-equilibrium thermodynamics. In this model, coronal variability arises from feedback between a macroscopic oscillation of the plasma and the rate at which it is cooled by the inverse Compton scattering of soft photons from the disc. The "pair thermostat'' mechanism then allows the corona to act as a heat engine that extracts work cyclically from the underlying thermal disequilibrium between the low-entropy heating from the black hole and the high-entropy cooling by soft photons from the disk, in close analogy to the well-known $\kappa$-mechanism for pulsating stars. This coronal self-oscillation may explain QPOs without invoking an external periodic driving. Moreover, we argue that this mechanism can generate coronal turbulence and jets.

astro-ph.HE↗

arXiv2025

Open quantum dynamics of Josephson charge pumps

We investigate the macroscopic dynamics of Josephson charge pumps in the light of Alicki et al.'s theoretical description of the Josephson junction as an open quantum system described by a Markovian master equation. Once the electrostatic interaction between the terminals is taken into account via nonlinear capacitive terms in the Hamiltonian, we find that the resulting description of pumping is physically reasonable and in good qualitative agreement with experimental observations. We comment on how this approach relates to other theoretical treatments of quantum pumps based on time-dependent potentials or scattering amplitudes. We also highlight the significance of our results in the broader context of the dynamics of charge pumping by active systems.

quant-ph↗

arXiv2023

The irreversible relaxation of inflation

Based on the results of a previous analysis of the Markovian master equation for the irreversible evolution of an open system embedded in de Sitter space, we include in the cosmological Friedmann equations a contribution from the presence of a physical bath at temperature $T_{\rm dS} = h / 2 \pi$, where $h$ is the Hubble parameter. We show that this provides a mechanism for the irreversible relaxation of the cosmological constant and a graceful exit to inflation, without need for subsequent reheating. Thermal particle production during inflation gives adiabatic, Gaussian, and approximately scale-invariant cosmological perturbations. We thus obtain the main features of inflation without any inflaton potential. To clarify the thermodynamic interpretation of these results, we consider the analogy of this irreversible relaxation to superfluorescence in quantum optics.

gr-qc↗

arXiv2023

Quantum thermodynamics of de Sitter space

We consider the local physics of an open quantum system embedded in an expanding three-dimensional space $\mathbf x$, evolving in cosmological time $t$, weakly coupled to a massless quantum field. We derive the corresponding Markovian master equation for the system's nonunitary evolution and show that, for a de Sitter space with Hubble parameter $h = $ const., the background fields act as a physical heat bath with temperature $T_{\rm dS} = h / 2 \pi$. The energy density of this bath obeys the Stefan-Boltzmann law $\rho_{\rm dS} \propto h^4$. We comment on how these results clarify the thermodynamics of de Sitter space and support previous arguments for its instability in the infrared. The cosmological implications are considered in an accompanying letter.

gr-qc↗

arXiv2023

The Josephson junction as a quantum engine

We treat the Cooper pairs in the superconducting electrodes of a Josephson junction (JJ) as an open system, coupled via Andreev scattering to external baths of electrons. The disequilibrium between the baths generates the direct-current bias applied to the JJ. In the weak-coupling limit we obtain a Markovian master equation that provides a simple dynamical description consistent with the main features of the JJ, including the form of the current-voltage characteristic, its hysteresis, and the appearance under periodic voltage driving of discrete Shapiro steps. For small dissipation, our model also exhibits a self-oscillation of the JJ's electrical dipole with frequency $\Omega = 2 e V / \hbar$ around mean voltage $V$. This self-oscillation, associated with "hidden attractors" of the nonlinear equations of motion, explains the observed production of monochromatic radiation with frequency $\Omega$ and its harmonics. We argue that this picture of the JJ as a quantum engine resolves open questions about the Josephson effect as an irreversible process and could open new perspectives in quantum thermodynamics and in the theory of dynamical systems.

quant-ph↗

arXiv2021

The problem of engines in statistical physics

Engines are open systems that can generate work cyclically, at the expense of an external disequilibrium. They are ubiquitous in nature and technology, but the course of mathematical physics over the last 300 years has tended to make their dynamics in time a theoretical blind spot. This has hampered the usefulness of statistical mechanics applied to active systems, including living matter. We argue that recent advances in the theory of open quantum systems, coupled with renewed interest in understanding how active forces result from positive feedback between different macroscopic degrees of freedom in the presence of dissipation, point to a more realistic description of autonomous engines. We propose a general conceptualization of an engine that helps clarify the distinction between its heat and work outputs. Based on this, we show how the external loading force and the thermal noise may be incorporated into the relevant equations of motion. This modifies the usual Fokker-Planck and Langevin equations, offering a thermodynamically complete formulation of the irreversible dynamics of simple oscillating and rotating engines.

cond-mat.stat-mech↗

arXiv2021

Quantum thermodynamics of coronal heating

Using the quantum Markovian master equation, we show that convection in the stellar photosphere generates plasma waves by an irreversible process akin to Zeldovich superradiance and sonic booms. In the Sun, this mechanism is most efficient in quiet regions with small magnetic fields. Energy is mostly carried by megahertz Alfven waves that scatter elastically until they reach a height at which they can dissipate via mode conversion. This gives the right power flux for coronal heating and may account for chromospheric evaporation leading to impulsive heat transport in the corona.

astro-ph.SR↗

arXiv2020

Dynamical theory for the battery's electromotive force

We propose a dynamical theory of how the chemical energy stored in a battery generates the electromotive force (emf). In this picture, the battery's half-cell acts as an engine, cyclically extracting work from its underlying chemical disequilibrium. We show that the double layer at the electrode-electrolyte interface can exhibit a rapid self-oscillation that pumps an electric current, thus accounting for the persistent conversion of chemical energy into electrical work equal to the emf times the separated charge. We suggest a connection between this mechanism and the slow self-oscillations observed in various electrochemical cells, including batteries, as well as the enhancement of the current observed when ultrasound is applied to the half-cell. Finally, we propose more direct experimental tests of the predictions of this dynamical theory.

physics.chem-ph↗

arXiv2020

Leaking elastic capacitor as model for active matter

We introduce the "leaking elastic capacitor" (LEC) model, a nonconservative dynamical system that combines simple electrical and mechanical degrees of freedom. We show that an LEC connected to an external voltage source can be destabilized (Hopf bifurcation) due to positive feedback between the mechanical separation of the plates and their electrical charging. Numerical simulation finds regimes in which the LEC exhibits a limit cycle (regular self-oscillation) or strange attractors (chaos). The LEC acts as an autonomous engine, cyclically performing work at the expense of the constant voltage source. We show that this mechanical work can be used to pump current, generating an electromotive force without any time-varying magnetic flux and in a thermodynamically irreversible way. We consider how this mechanism can sustain electromechanical waves propagating along flexible plates. We argue that the LEC model can offer a qualitatively new and more realistic description of important properties of active systems with electrical double layers in condensed-matter physics, chemistry, and biology.

physics.class-ph↗

arXiv2019

Quantum Theory of Triboelectricity

We propose a microphysical theory of the triboelectric effect by which mechanical rubbing separates charges across the interface between two materials. Surface electrons are treated as an open system coupled to two baths, corresponding to the bulks. Extending Zel'dovich's theory of bosonic superradiance, we show that motion-induced population inversion can generate an electromotive force. We argue that this is consistent with the basic phenomenology of triboelectrification and triboluminescence as irreversible processes, and we suggest how to carry out more precise experimental tests.

cond-mat.mes-hall↗

arXiv2019

A dynamic picture of energy conversion in photovoltaic devices

Studies of emerging photovoltaics, such as organic and perovskite solar cells, have recently shown that the separation of photo-generated charge carriers is correlated with non-thermal, coherent oscillations within the illuminated device. We consider this experimental evidence in light of results from the theory of open quantum systems that point to the need for a self-oscillating internal capacitor, acting as a microscopic piston, to explain how an illuminated solar cell operates as an autonomous heat engine. We propose a picture of work extraction by photovoltaic devices that supersedes the quasi-static descriptions prevalent in the literature. Finally, we argue that such a dialogue between condensed matter physics and quantum thermodynamics may offer a guide for the design of new energy transducers.

physics.app-ph↗

arXiv2018

A physical approach to dissipation-induced instabilities

Self-oscillatory and self-rotatory process driven by non-conservative forces have usually been treated as applications of the concepts of Hopf bifurcation and limit cycle in the theory of differential equations, or as instability problems in feedback control. Here we explore a complimentary approach, based on physical considerations of work extraction and thermodynamic irreversibility. From this perspective, the fact that a system can be destabilized by dissipation does not appear as a mathematical paradox, but rather as a straightforward consequence of the dissipative medium's motion. We apply this analysis to various mechanical and hydrodynamical systems of interest and show how it clarifies questions on which the literature remains contentious, such as the conditions for the appearance of non-conservative positional forces, or the roles of viscosity and turbulence in the raising of ocean waves by the wind.

physics.class-ph↗

arXiv2017

Irreversibility of mechanical and hydrodynamic instabilities

The literature on dynamical systems has, for the most part, considered self-oscillators (i.e., systems capable of generating and maintaining a periodic motion at the expense of an external energy source with no corresponding periodicity) either as applications of the concepts of limit cycle and Hopf bifurcation in the theory of differential equations, or else as instability problems in feedback control systems. Here we outline a complementary approach, based on physical considerations of work extraction and thermodynamic irreversibility. We illustrate the power of this method with two concrete examples: the mechanical instability of rotors that spin at super-critical speeds, and the hydrodynamic Kelvin-Helmholtz instability of the interface between fluid layers with different tangential velocities. Our treatment clarifies the necessary role of frictional or viscous dissipation (and therefore of irreversibility), while revealing an underlying unity to the physics of many irreversible processes that generate mechanical work and an autonomous temporal structure (periodic, quasi-periodic, or chaotic) in the presence of an out-of-equilibrium background.

physics.class-ph↗

arXiv2017

Interaction of a quantum field with a rotating heat bath

The linear coupling of a rotating heat bath to a quantum field is studied in the framework of the Markovian master equation for the field's non-unitary time evolution. The bath's rotation induces population inversion for the field's low-energy modes. For bosons, this leads to superradiance, an irreversible process in which some of the bath's kinetic energy is extracted by spontaneous and stimulated emission. We find the energy and entropy balance for such systems and apply our results to the theory of black hole radiation. We also comment on how this relates to classical self-oscillations, including shear flow instabilities in hydrodynamics.

quant-ph↗

arXiv2016

A thermodynamic cycle for the solar cell

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.

cond-mat.stat-mech↗

arXiv2015

Chandler wobble: Stochastic and deterministic dynamics

We propose a model of the Earth's torqueless precession, the "Chandler wobble", as a self-oscillation driven by positive feedback between the wobble and the centrifugal deformation of the portion of the Earth's mass contained in circulating fluids. The wobble may thus run like a heat engine, extracting energy from heat-powered geophysical circulations whose natural periods would otherwise be unrelated to the wobble's observed period of about fourteen months. This can explain, more plausibly than previous models based on stochastic perturbations or forced resonance, how the wobble is maintained against viscous dissipation. The self-oscillation is a deterministic process, but stochastic variations in the magnitude and distribution of the circulations may turn off the positive feedback (a Hopf bifurcation), accounting for the occasional extinctions, followed by random phase jumps, seen in the data. This model may have implications for broader questions about the relation between stochastic and deterministic dynamics in complex systems, and the statistical analysis thereof.

physics.geo-ph↗

arXiv2014

On the Title of Moriarty's 'Dynamics of an Asteroid'

We propose an explanation of the title of Prof. James Moriarty's treatise_Dynamics of an Asteroid_, a scientific work mentioned by Sherlock Holmes in_The Valley of Fear_ and prominently featured in Guy Ritchie's 2011 film_Sherlock Holmes: A Game of Shadows_. Our views on the subject differ from those expressed in Isaac Asimov's "The Ultimate Crime".

physics.pop-ph↗