CM Seminar: Imaging Viscous Flow of the Dirac Fluid in Graphene Using a Quantum Spin Magnetometer

Abstract: Hydrodynamic electron fluid has emerged as a paradigm of strongly-correlated electronic transport. In particular, the electron-hole plasma in charge-neutral graphene is predicted to realize a quantum critical fluid whose transport features a universal hydrodynamic description relevant to strongly-correlated electrons in high-Tc superconductors. This “Dirac fluid” is expected to have a shear viscosity close to a minimum bound, with an inter-particle scattering rate saturating at the Planckian time ħ/(kBT). While electrical transport measurements at finite carrier density are consistent with hydrodynamic electron flow in graphene, a clear demonstration of viscous behavior at the charge neutrality point (CNP) remains elusive. In this work, we directly image viscous Poiseuille flow of the Dirac fluid at room temperature via measurement of the associated stray magnetic field. Nanoscale magnetic imaging, performed using quantum spin magnetometers realized with nitrogen vacancy (NV) centers in diamond, reveals a parabolic Poiseuille profile for electron flow in a graphene channel near the CNP, establishing the viscous transport of the Dirac fluid. Via combined imaging-transport measurements, we obtain viscosity and scattering rates, and observe that these quantities are comparable to the universal values expected at quantum criticality. This finding establishes a nearly-ideal electron fluid in charge-neutral graphene at room temperature.

Biosketch: Mark Jen-Hao Ku is a postdoctoral associate working at Harvard University, with joint appointments at the Harvard-Smithsonian Center for Astrophysics and the University of Maryland Quantum Technology Center.  He attended the University of British Columbia, where he received B.Sc in Mathematics and Physics and M.Sc in Physics.  He obtained Ph.D in Physics at the Massachusetts Institute of Technology, where he used ultracold matter formed by atomic gases for quantum simulation of high-temperature superfluids.  At Harvard, he uses nitrogen-vacancy centers in diamond as a quantum sensor to probe condensed matter phenomena in the nanoscale, including transport of correlated electrons in graphene and spin-waves in ferromagnets.