Tunable electron–flexural phonon interaction in graphene heterostructures

Abstract

Peculiar electron-phonon interaction characteristics underpin the ultrahigh mobility, electron hydrodynamics, superconductivity, and superfluidity observed in graphene heterostructures. Here, the Lorenz ratio (L) between the electronic thermal conductivity and the product of the electrical conductivity and temperature provides unique insight into electron-phonon interactions that is inaccessible to past graphene measurements. Here we show an unusual L peak in degenerate graphene near 60 Kelvin and decreased peak magnitude with increased mobility. When combined with ab initio calculations of the many-body electron-phonon self-energy and analytical models, this experimental observation reveals that broken reflection symmetry in graphene heterostructures can relax a restrictive selection rule to allow quasielastic electron coupling with an odd number of flexural phonons, contributing to the increase of L toward the Sommerfeld limit at an intermediate temperature sandwiched between the low-temperature hydrodynamic regime and the inelastic electron-phonon scattering regime above 120 Kelvin. In contrast to past practices of neglecting flexural phonon contributions to transport in two-dimensional materials, this work suggests that tunable electron-flexural phonon coupling can provide a handle to control quantum matter at the atomic scale, such as magic angle twisted bilayer graphene where low-energy excitations may mediate Cooper pairing of flat-band electrons.