Wiedemann-Franz law in graphene in the presence of a weak magnetic field

TL;DR

This study extends a Boltzmann-transport model to analyze how a weak magnetic field affects the Wiedemann-Franz law in monolayer and bilayer graphene, revealing magnetic field influences on the Lorenz ratio's peak and sign.

Contribution

It introduces a magnetic field into the existing model to explain the Lorenz ratio behavior and compares monolayer and bilayer graphene under these conditions.

Findings

Magnetic field enhances the Lorenz ratio peak.

Magnetic field has little effect on the peak position.

Transverse Lorenz ratio can be positive or negative depending on parameters.

Abstract

The experimental work [J. Crossno et al., Science 351, 1058 (2016)], which reported the violation of the Wiedemann-Franz law in monolayer graphene characterized by a sharp peak of the Lorenz ratio at a finite temperature, has not been fully explained. Our previous work [Y.-T. Tu and S. Das Sarma, Phys. Rev. B 107, 085401 (2023)] provided a possible explanation through a Boltzmann-transport model with bipolar diffusion and an energy gap possibly induced by the substrate. In this paper, we extend our calculation to include a weak magnetic field perpendicular to the graphene layer, which is experimentally relevant, and may shed light on the possible violation or not of the Wiedemann-Franz law. We find that the magnetic field enhances the size of the peak of the Lorenz ratio but has little effect on its position, and that the transverse component of the Lorenz ratio can be either positive or negative depending on the parameter regime. In addition, we do the same calculation for bilayer graphene in the presence of a magnetic field and show the qualitative similarity with monolayer graphene. Our work should motivate magnetic-field-dependent experiments elucidating the nature of the charge carriers in graphene layers.