Superfluid and Quasiparticle Dynamics of Magic-Angle Twisted Bilayer Graphene I

The phase diagram of Magic-Angle Twisted Bilayer Graphene contains several correlation-driven phases, including superconductivity. Their nature and driving mechanisms remain an outstanding open question. However, 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.

We present a model to demonstrate that the dynamical properties of gate-defined Josephson junctions in MATBG give access to intrinsic properties of MATBG. We have probed the dynamics of MATBG by biasing the gate-defined junctions with DC and AC currents. The resulting current-voltage characteristics are hysteretic and depend both on frequency and the gate voltage, with the effect of AC drive suppressed with frequency. From our numerical simulations, we attribute the frequency dependence of the response to thermal relaxation of the Joule heated electrons in the junctions and the reactive impedance of the superfluid in MATBG. Our model of junction dynamics based on these mechanisms is shown to describe well the experimental data and allows to access the electron-phonon cooling power, electronic specific heat and superfluid density of MATBG. This analysis establishes a new method to experimentally probe the intrinsic properties of superconducting 2D materials.