Thermodynamic model of twisted bilayer graphene: Configuration entropy matters

TL;DR

This paper develops a thermodynamic model for twisted bilayer graphene, highlighting the importance of configuration entropy in understanding its properties and explaining experimental observations related to moire patterns and superlubricity.

Contribution

It introduces a novel thermodynamic framework linking configuration entropy to moire patterns in tBLG, providing physical insights and explaining experimental results.

Findings

Configuration entropy is proportional to the logarithm of the moire period to lattice constant ratio.

Configuration entropy dominates the Helmholtz free energy of tBLG.

The model explains experimental observations in superlubric contacts.

Abstract

Twisted bilayer materials have attracted tremendous attention due to their unique and novel properties. Here, we derive a thermodynamic model for twisted bilayer graphene (tBLG) within the framework of the classical statistical mechanics, based on which, the configuration entropy reflecting the number of micro-status in moire unit-cells, is directly derived from the Helmholtz free energy with a clear physical interpretation. More importantly, we show the configuration entropy of tBLG relative to the AB-stacked bilayer graphene is proportional to the logarithmic function of the ratio of moire period and the atomic lattice constant, which we found dominates the Helmholtz free energy of tBLG and can well explain experimental observations in superlubric contacts. Our work provides a theoretical foundation for studying moire effect of incommensurate contact interfaces and could facilitate twisting based applications such as superlubricity.