Enhanced hydrodynamic transport in near magic angle twisted bilayer graphene

TL;DR

This paper investigates hydrodynamic transport phenomena in near magic angle twisted bilayer graphene using a semiclassical Boltzmann approach, highlighting conditions for strong hydrodynamics and identifying experimental signatures.

Contribution

It introduces a detailed analysis of transport properties considering twist angle-dependent Fermi velocity and scattering mechanisms, revealing conditions for observable hydrodynamic behavior.

Findings

Identification of a temperature and angle window for minimal momentum-non-conserving collisions.

Prediction of sharp doping dependence of electrical resistivity.

Observation of enhanced Wiedemann-Franz ratio and Seebeck coefficient.

Abstract

Using the semiclassical quantum Boltzmann theory and employing the Dirac model with twist angle-dependent Fermi velocity we obtain results for the electrical resistivity, the electronic thermal resistivity, the Seebeck coefficient, and the Wiedemann-Franz ratio in near magic angle twisted bilayer graphene, as functions of doping density (around the charge-neutrality-point) and modified Fermi velocity $\tilde v$. The $\tilde v$-dependence of the relevant scattering mechanisms, i.e. electron-hole Coulomb, long-ranged impurities, and acoustic gauge phonons, is considered in detail. We find a range of twist angles and temperatures, where the combined effect of momentum-non-conserving collisions (long-ranged impurities and phonons) is minimal, opening a window for the observation of strong hydrodynamic transport. Several experimental signatures are identified, such as a sharp dependence of the electric resistivity on doping density and a large enhancement of the Wiedemann-Franz ratio and the Seebeck coefficient.