Quasiparticle and superfluid dynamics in Magic-Angle Graphene

TL;DR

This study uses a novel RF-biased Josephson junction to probe quasiparticle and superfluid dynamics in magic-angle twisted bilayer graphene, shedding light on its superconducting mechanisms and pairing symmetry.

Contribution

It introduces an innovative method to measure thermodynamic properties in 2D superconductors, revealing insights into pairing symmetry and electron-phonon interactions in MATBG.

Findings

Evidence for anisotropic or nodal pairing state

Estimated electron-phonon coupling strength

Mapped superfluid stiffness across the phase diagram

Abstract

Magic-Angle Twisted Bilayer Graphene shows a wide range of correlated phases which are electrostatically tunable. Despite a growing knowledge of the material, there is yet no consensus on the microscopic mechanisms driving its superconducting phase. In particular, elucidating the symmetry and formation mechanism of the superconducting phase may provide key insights for the understanding of unconventional, strongly coupled and topological superconductivity. A major obstacle to progress in this direction is that key thermodynamic properties, such as specific heat, electron-phonon coupling and superfluid stiffness, are extremely challenging to measure due to the 2D nature of the material and its relatively low energy scales. Here, we use a gate-defined, radio frequency-biased, Josephson junction to probe the electronic dynamics of magic-angle twisted bilayer graphene (MATBG). We reveal both the electronic quasiparticle dynamics, driven by their thermalization through phonon scattering, as well as the condensate dynamics, driven by the inertia of Cooper pairs. From these properties we recover the evolution of thermalization rates, and the superfluid stiffness across the phase diagram. Our findings favor an anisotropic or nodal pairing state and allow to estimate the strength of electron-phonon coupling. These results contribute to understanding the underlying mechanisms of superconductivity in MATBG while establishing an easy-to-implement method for characterizing thermal and superfluid properties of superconducting 2D materials.