Bosonic Working Media in a Frustrated Rhombi Chain: Otto and Stirling Cycles from Flat Bands, Caging, and Flux Control

TL;DR

This paper shows that flat-band engineering in a bosonic rhombi chain can significantly enhance quantum heat engine performance by controlling spectral properties with magnetic flux, affecting work output and efficiency.

Contribution

It introduces a method to optimize quantum heat engines using flux-tuned flat-band formation and geometric frustration in a Bose-Hubbard model.

Findings

Otto cycle's work output and efficiency are greatly increased near the caging regime.

Spectral restructuring suppresses heat to the cold reservoir, boosting work extraction.

Stirling cycle benefits from entropy variations, allowing broader work extraction at lower efficiency.

Abstract

We demonstrate that flat-band engineering provides a direct route to control and optimize the thermodynamic performance of quantum heat engines. We consider noninteracting bosons on a rhombi-chain lattice described by a Bose-Hubbard model in the noninteracting limit, where a magnetic flux serves as a tunable parameter that continuously reshapes the single-particle spectrum. By driving the system toward the fully frustrated Aharonov-Bohm caging regime, the band structure transitions from dispersive to completely flat, strongly modifying the thermal occupation of the modes. We show that this flux-induced spectral restructuring has clear and measurable thermodynamic consequences. In particular, the Otto cycle exhibits a significant enhancement of both work output and efficiency when operating near the caging regime. We identify the underlying mechanism as a pronounced suppression of heat released to the cold reservoir, rather than an increase in absorbed heat, revealing that flat-band formation is an effective strategy to increase work extraction. In contrast, the Stirling cycle is governed by entropy variations along isothermal, flux-driven processes, leading to greater work extraction over a broader parameter range but at lower efficiency. These results establish geometric frustration and Aharonov-Bohm caging as thermodynamic resources and show that spectral engineering via synthetic gauge fields offers a viable, experimentally accessible pathway to tailor the performance of bosonic quantum thermal machines.