Flat-band thermodynamics reveals enhanced performance across Otto, Carnot, and Stirling cycles

TL;DR

This paper analyzes the thermodynamic performance of magic-angle twisted bilayer graphene (MATBG) across various quantum cycles, revealing its superior heat engine capabilities and unique reversible modes compared to other graphene systems.

Contribution

It provides a comprehensive thermodynamic phase diagram of MATBG under multiple quantum cycles using the continuum eight-band model, highlighting its enhanced performance and novel reversible modes.

Findings

MATBG outperforms other graphene systems as a heat engine in QSC.

High efficiency with reduced work output in QOC for MATBG.

Identification of a highly reversible Joule pump mode in QSC and QOC.

Abstract

Magic-angle twisted bilayer graphene (MATBG) exhibits remarkable electronic properties under external magnetic fields, notably the emergence of flat Landau levels. In this study, we present a comprehensive analysis of MATBG's operational phase diagram under three distinct quantum thermodynamic cycles, i.e., Quantum Otto Cycle (QOC), Quantum Carnot Cycle (QCC), and Quantum Stirling Cycle (QSC). Employing the continuum eight-band model, we evaluate the thermodynamic performance of MATBG across multiple operational modes: heat engine, refrigerator, cold pump, and Joule pump, and benchmark it against other graphene systems such as monolayer graphene, AB-Bernal stacked bilayer graphene, and non-magic-angle twisted bilayer graphene. Our findings reveal that MATBG demonstrates superior heat engine performance in QSC, while achieving high efficiency albeit with reduced work output in QOC. Even though the performance of MATBG as a cold pump or refrigerator is modest in QOC and QSC, it shows notable improvement as a refrigerator in QCC. Additionally, we identify a highly reversible Joule pump mode in both QSC and QOC under strict adiabaticity, underscoring the unique thermodynamic behavior of MATBG.