Electronic interactions in Dirac fluids measured by nano-terahertz spacetime metrology

Electrons in graphene near the charge neutral point form correlated Dirac fluid, which exhibits unusual behaviors including a quantum critical scattering rate, breakdown of Wiedemann-Franz law and large magnetoresistance. Here, we harnessed electron-photon quasiparticles to quantify electronic interactions in a Dirac fluid. To investigate the collective behavior of the Dirac fluid, we experimentally visualized the worldlines of surface plasmon polariton --- trajectories of polaritonic wave packets in space and in time. Our methodology is based on a newly developed spacetime metrology, which utilizes terahertz time-domain nano-imaging. This is implemented in a custom-built cryogenic Terahertz Scanning Near-field Optical Microscopy setting. Leveraging the sub-50nm spatial resolution and femtosecond temporal resolution, we visually depict the electron-photon hybrids within a strongly interacting Dirac fluid, where the dynamics of these hybrids fundamentally stem from graphene’s fine-structure constant. Our work reveals new aspects of light-matter interaction in strongly correlated quantum systems.