arXiv subjects

Ramandeep S. Johal

Publications and source records attributed to Ramandeep S. Johal.

At least 19 recordsLinked to original sources

arXiv2026

Endoreversible Carnot machines with long-time reversible limit

The well-known Curzon-Ahlborn (CA) engine [Am. J. Phys. {\bf 43}, 22 (1975)] runs in finite time and yields a finite power output. In order to recover the standard Carnot cycle from the CA model, the limit of quasi-static operation has to be concurrent with the reversible limit, a condition strangely missing from the CA model. We augment this model by specifying the duration of the isothermal process as a monotonic decreasing function of the temperature difference between the working substance and the reservoir. As an illustration, we analyze the machine's operation in the low-dissipation (LD) regime in which the entropy production in each isothermal branch is inversely proportional to the duration of the process. The modified model, while retaining the optimal features of the CA model, also yields explicit expressions for the optimal durations of isothermal processes. We compare the optimal cycle periods of the present model and the LD model, and show their agreement in the linear response regime.

cond-mat.stat-mech↗

arXiv2026

Inertia-Driven Information Flow and Symmetry Breaking in a Nonequilibrium Two-Bead System

We investigate information-flow generation in a nonequilibrium two-bead system coupled to two heat baths. We show that the system acts as an information-flow generator in both overdamped and underdamped regimes, with the underdamped dynamics revealing a divergence of the scaled information flow along specific paths in the thermal asymmetry--inertia parameter space that is hidden in the overdamped limit. This information generation suggests a possible route toward information engines and demon-like mechanisms in nanomachines. A symmetry-perturbation analysis of the response landscape of information flow reveals a geometric structure reminiscent of a Ginzburg--Landau framework: the symmetric reference state can correspond to a minimum or maximum depending on the perturbation direction, while the flat overdamped landscape develops a finite curvature under inertia. Mass asymmetry shifts the resulting maxima, and Hessian eigenvalue and eigenvector analysis reveals level touching of principal modes and bimodality along a constant-diffusion path. These results establish a minimal framework for understanding how inertia and microscopic heterogeneity shape information landscapes in nonequilibrium systems, with potential extensions to more complex heterogeneous networks.

cond-mat.stat-mech↗

arXiv2026

Kinetic temperatures and inertial effects in a nonequilibrium bead-spring model

We investigate a nonequilibrium steady-state model consisting of two coupled beads with arbitrary masses in contact with two thermal baths at different temperatures. Using a covariance-matrix approach together with numerical simulations of the underdamped Langevin dynamics, we characterize steady-state probability distributions, heat transport, and entropy production. We show that irreversibility measures such as entropy production and heat current are invariant under an exchange of the bead masses, whereas energy-storage observables depend explicitly on the mass arrangement in a symmetrical set up. This reveals a fundamental distinction: energy observables exhibit path dependence in singular mass limits, while transport and irreversibility remain well defined. We show that kinetic temperatures provide the natural variables governing the thermodynamics of the system: their difference controls transport and entropy production, while their sum determines the mean energy via a model specific generalized equipartition relation. In the infinite-mass limit, only constitutive relations expressed in terms of kinetic temperatures remain meaningful. Thus, energy, transport, and irreversibility are unified through kinetic temperatures as the organizing variables. We also derive an effective temperature that defines an equilibrium-like canonical distribution. Finally, we analyze the notion of ergodicity and show that the time-averaged observables converge significantly faster than the ensemble averages.

cond-mat.stat-mech↗

arXiv2026

Optimization of cooling power of a thermoelectric refrigerator: A unified approach

We analyze the steady-state formalism for optimizing the cooling power of a thermoelectric refrigerator (TER), unifying the endoreversible and exoreversible approximations within one framework. Although the cooling power is non-optimizable within the endoreversible model based on Newtonian heat-transfer law, we show that the issue can be circumvented in the near-reversible regime where the external thermal conductances are large, but finite. We extend this analysis to optimize the cooling power in the presence of both internal and external irreversibilities and derive a closed-form expression for the coefficient of performance (COP) that depends on the thermoelectric figure of merit and the ratio of internal to external thermal conductances. The model reproduces the endoreversible and the exoreversible limits as special cases. We conclude that for small temperature differences, the combined irreversibilities reduce the COP to values below 1/2, which aligns with the observed performance of the single-stage TER, and can provide realistic estimates for the COP.

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arXiv2026

Virtual temperatures as a key quantifier for passive states in quantum thermodynamic processes

We analyze the role of virtual temperatures for passive quantum states through the lens of majorization theory. A mean temperature over the virtual temperatures of adjacent energy levels is defined to compare the passive states of the system resulting from isoenergetic and isoentropic transformations. The role of the minimum and the maximum (min-max) values of the virtual temperatures in determining the direction of heat flow between the system and the environment is argued based on majorization relations. We characterize the intermediate passive states in a quantum Otto engine using these virtual temperatures and derive an upper bound for the Otto efficiency that can be expressed in terms of the min-max virtual temperatures of the working medium. An explicit example of the coupled-spins system is worked out. Moreover, virtual temperatures serve to draw interesting parallels between the quantum thermodynamic processes and their classical counterparts. Thus, virtual temperature emerges as a key operational quantity linking passivity and majorization to the optimal performance of quantum thermal machines.

quant-ph↗

arXiv2025

Ergotropic advantage in a measurement-fueled quantum heat engine

This paper investigates a coupled two-qubits heat engine fueled by generalized measurements of the spin components and using a single heat reservoir as sink. Our model extends the proposal of Yi and coworkers [Phys. Rev. E {\bf 96}, 022108 (2017)] where the role of a hot reservoir in a four-stroke cycle was replaced by a quantum measurement apparatus, the other steps being two quantum adiabatic strokes and thermalization with a cold reservoir. We propose a five-stroke cycle, where an ergotropy extracting stroke is introduced following the measurement stroke, and study the effect of measurements of different spin components on the performance of the machine. For measurements along z-z directions, we find two possible occupation distributions that yield an active state and the ergotropic stroke improves the performance of the engine over the four-stroke cycle. Further, the three-stroke engine ( {without the adiabatic strokes}) yields the same performance as the five-stroke engine. For arbitrary working medium and non-selective measurements, we prove that the total work output of a five-stroke engine is equal to the sum of the work outputs of the corresponding four-stroke and three-stroke engines. For measurement directions other than z-z, there may be many possible orderings of the post-measurement probabilities that yield an active state. However, as we illustrate, for specific cases (e.g. x-x), a definite ordering may be obtained with the projective measurements. Thus, we find that the five-stroke engine exploiting ergotropy outperforms both its four-stroke as well as three-stroke counterparts.

quant-ph↗

arXiv2025

Nonequilibrium steady states in bead-spring models: Entropy production and probability distributions

We study non-equilibrium models comprising of beads connected by springs. The system is coupled to two thermal baths kept at different temperatures. We derive the steady state probability distributions of positions of the bead for the one-bead system in the underdamped case. We employ the recently proposed technique of an effective temperature, along with numerical simulations to solve the Langevin equations and obtain their corresponding probability distributions. It is observed that the marginal probability distributions in the position are independent of mass. We also obtain theoretically and numerically the rate of entropy production for the one-bead system. The probability distribution of the positions in the two-beads system are obtained theoretically and numerically, both in the underdamped and overdamped case. Lastly, we discuss the notion of ergodicity and have tested the convergence of the time-averaging and the ensemble-averaging protocols.

cond-mat.stat-mech↗

arXiv2025

Optimal performance of thermoelectric devices with small external irreversibility

In the thermodynamic analysis of thermoelectric devices, typical irreversibilities are for the processes of finite-rate heat transfer, heat leak and Joule heating. Approximate analyses often focus on either internal or external irreversibility, obtaining well-known expressions for the efficiency at maximum power (EMP), such as the Curzon-Ahlborn value for endoreversible model and the Schmiedl-Seifert form for exoreversible model. Within the Constant Properties model, we simultaneously incorporate internal as well as external irreversibilities. We employ the approximation of a symmetric and small external irreversibility (SEI), allowing a tractable expression for EMP that depends on three parameters i) the ratio of internal to external thermal conductance ii) the figure of merit of the thermoelectric material and iii) the ratio of hot and cold reservoir temperatures. We study limiting forms of this EMP and compare our framework with the exact model as well as with other irreversible models in finite-time thermodynamics, such as the minimally nonlinear model. In particular, we argue that the TEG in endoreversible approximation can be mapped to the mesoscopic model of Feynman's ratchet in the high temperatures regime, thus providing an alternative to the viewpoint in literature where the TEG in linear regime is mapped to an exoreversible case. Finally, extending our study to the thermoelectric refrigerator under similar assumptions as for the generator, we analyze the efficiency at the maximum cooling power.

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arXiv2025

Microscopic Legendre Transform, Canonical Ensemble and Jaynes' Maximum Entropy Principle

Legendre transform between thermodynamic quantities such as the Helmholtz free energy and entropy plays a key role in the formulation of the canonical ensemble. In the standard treatment, the transform exchanges the independent variable from the system's internal energy to its conjugate variable -- the inverse temperature of the heat reservoir. In this article, we formulate a microscopic version of the transform between the free energy and Shannon entropy of the system, where the conjugate variables are the microstate probabilities and the energies (scaled by the inverse temperature). The present approach gives a non-conventional perspective on the connection between information-theoretic measure of entropy and thermodynamic entropy. We focus on the exact differential property of Shannon entropy, utilizing it to derive central relations within the canonical ensemble. Thermodynamics of a system in contact with the heat reservoir is discussed in this framework. Other approaches, in particular, Jaynes' maximum entropy principle is compared with the present approach.

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arXiv2024

A visual proof of entropy production during thermalization with a heat reservoir

In this note, the equilibrium curve of a thermodynamic system is used to depict entropy production in the process of thermalization with a reservoir. For the given initial and final equilibrium states of the system, the entropy production is reduced when work is also extracted during thermalization. The case of maximum work extraction corresponds to a reversible process. For less than optimal work extraction, the lost available work is shown to be directly proportional to the entropy produced.

cond-mat.stat-mech↗

arXiv2024

Clausius' theorem and the Second law in the process of isoenergetic thermalization

Isoenergetic thermalization amongst $n$ bodies is a well-known irreversible process, bringing the bodies to a common temperature $T_F$ and leading to a rise in the total entropy of the bodies. We express this change in entropy using the Clausius formula over a reversible path connecting $T_F$ with $T_f$ which corresponds to the entropy-preserving temperature of the initial nonequilibrium state. Under the assumption of positive heat capacities of the bodies, the Second law inequality simply follows from the fact that $T_F > T_f$. We extend this approach to the continuum case of an unequally heated rod, illustrating with the special case of the rod with constant heat capacity and a linear temperature profile. An interpolating profile between the discrete and the continuum models is studied whereby $T_f$, given by the geometric mean temperature over $n$ elements, is shown to approach the identric mean of the lowest and the highest temperatures as $n$ becomes large. We also discuss the case of negative heat capacity in a two-body set up where isoenergetic thermalization may be forbidden by the Second law. However, the alternate scheme in which first work is performed reversibly on the system and then an equivalent amount of energy is extracted in the form of heat, brings the system to an energy-preserving common temperature.

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arXiv2024

Energy-gap modulation and majorization in three-level quantum Otto engine

A three-level quantum system having two energy gaps presents a nontrivial working medium for a quantum heat engine. Our focus lies in understanding the constraints on the ability to modulate these gaps relative to the changes in probability distributions at the two given heat reservoirs. It is seen that an Otto engine in the quasistatic limit is feasible if at least one energy gap shrinks during the first quantum adiabatic stage. We analyze operating conditions under different variations of the gaps, revealing that a definite majorization relation between the hot and cold distributions serves as a sufficient criterion for the engine when both gaps are shrinking. Further, Otto efficiency is enhanced in case majorization holds. On the other hand, majorization becomes a necessary condition when only one of the gaps is shrinking. In the special case where one gap remains fixed, majorization is both necessary and sufficient for engine operation. For an $n$-level system, we note that a well defined change in energy gaps aligns with the majorization relation, thus characterizing the operation of the engine.

quant-ph↗

arXiv2022

Spins-based Quantum Otto Engines and Majorisation

The concept of majorisation is explored as a tool to characterize the performance of a quantum Otto engine in the quasi-static regime. For a working substance in the form of a spin of arbitrary magnitude, majorisation yields a necessary and sufficient condition for the operation of the Otto engine, provided the canonical distribution of the working medium at the hot reservoir is majorised by its canonical distribution at the cold reservoir. For the case of a spin-1/2 interacting with an arbitrary spin via isotropic Heisenberg exchange interaction, we derive sufficient criteria for positive work extraction using the majorisation relation. Finally, local thermodynamics of spins as well as an upper bound on the quantum Otto efficiency is analyzed using the majorisation relation.

quant-ph↗

arXiv2021

Thermoelectric generator in endoreversible approximation: the effect of heat-transfer law under finite physical dimensions constraint

We revisit the optimal performance of a thermoelectric generator within the endoreversible approximation, while imposing a finite physical dimensions constraint (FPDC) in the form of a fixed total area of the heat exchangers. Our analysis is based on the linear-irreversible law for heat transfer between the reservoir and the working medium, in contrast to Newton's law usually assumed in literature. The optimization of power output is performed with respect to the thermoelectric current as well as the fractional area of the heat exchangers. We describe two alternate designs for allocating optimal areas to the heat exchangers. Interestingly, for each design, the use of linear-irreversible law yields the efficiency at maximum power in the well-known form, $2η_{\rm C}^{}/ (4-η_{\rm C}^{})$, earlier obtained for the case of thermoelectric generator under exoreversible approximation, i.e. assuming only the internal irreversibility due to Joule heating. On the other hand, the use of Newton's law yields Curzon-Ahlborn efficiency.

cond-mat.stat-mech↗

arXiv2021

Coupled autonomous thermal machines and efficiency at maximum power

We show that coupled autonomous thermal machines, in the presence of three heat reservoirs and following a global linear-irreversible description, provide a unified framework to accommodate the variety of expressions for the efficiency at maximum power (EMP). The efficiency is expressible in terms of the Carnot efficiency of the global set up if the intermediate reservoir temperature is an algebraic mean of the hot and cold temperatures. We give an explanation of the universal properties of EMP near equilibrium in terms of the properties of symmetric algebraic means. For the case of broken time reversal symmetry, a universal second order coefficient of 6/49 is predicted in the series expansion of EMP, analogous to the 1/8 coefficient in the time-reversal symmetric case.

cond-mat.stat-mech↗

arXiv2021

Quantum heat engines with complex working media, complete Otto cycles and heuristics

Quantum thermal machines make use of non-classical thermodynamic resources, one of which is interactions between elements of the quantum working medium. In this paper, we examine the performance of a quasi-static quantum Otto engine based on two spins of arbitrary magnitudes subject to an external magnetic field and coupled via an isotropic Heisenberg exchange interaction. It has been earlier shown that the said interaction provides an enhancement of cycle efficiency for two spin-1/2 particles, with an upper bound which is tighter than the Carnot efficiency. However, the necessary conditions governing engine performance and the relevant upper bound for efficiency are unknown for the general case of arbitrary spin magnitudes. Analyzing extreme-case scenarios, we formulate heuristics to infer the necessary conditions for an engine with uncoupled as well as coupled spins model. These conditions lead us to a connection between performance of quantum heat engines and the notion of majorization. Further, the study of complete Otto cycles inherent in the average cycle also yields interesting insights into the average performance.

quant-ph↗

arXiv2020

Constraints on Onsager coefficients from quasi-static and reversible operations

The performance of a generic, cyclic heat engine between two heat reservoirs is discussed within a linear-irreversible framework. The Onsager reciprocal relation is derived as a consequence of the equivalence between quasi-static and reversible operations, under the tight-coupling condition. When the latter condition is relaxed, it is possible to achieve reversible cycle in a finite duration. Onsager reciprocity must be violated when either the quasi-static cycle is not reversible, or the reversible cycle is not quasi-static.

cond-mat.stat-mech↗

arXiv2020

Irreversible thermodynamics of thermoelectric devices: From local framework to global description

Thermoelectricity is traditionally explained via Onsager's irreversible, flux-force framework. The coupled flows of heat and electric charge are modelled as steady-state flows, driven by the thermodynamic forces defined in terms of the gradients of local, intensive parameters like temperature and electrochemical potential. A thermoelectric generator is a device with a finite extension, and its performance is measured in terms of total power output and total entropy generation. These global quantities are naturally expressed in terms of discrete or global forces derived from their local counterparts. We analyze the thermodynamics of thermoelectricity in terms of global flux-force relations. These relations clearly show the additional quadratic dependence of the driver flux on global forces, corresponding to the process of Joule heating. We discuss the global kinetic coefficients defined by these flux-force relations and prove that the equality of the global cross-coefficients is derived from a similar property of the local coefficients. Finally, we clarify the differences between the global framework for thermoelectric energy conversion and the recently proposed minimally nonlinear irreversible thermodynamic model.

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