Graphene near charge neutrality is expected to behave like a quantum-critical, relativistic
plasma—the “Dirac fluid”—in which massless electrons and holes collide at a rapid rate. We
used on-chip terahertz spectroscopy to measure the frequency-dependent optical
conductivity of clean, micrometer-scale graphene at electron temperatures between 77 and
300 kelvin. At charge neutrality, we observed the quantum-critical scattering rate
characteristic of the Dirac fluid. At higher doping, we detected two distinct current-carrying …

Graphene near charge neutrality is expected to behave like a quantum-critical, relativistic
plasma-the``Dirac fluid''-in which massless electrons and holes rapidly collide at a rate
proportional to temperature. We measure the frequency-dependent optical conductivity of
clean micron-scale graphene encapsulated in hexagonal Boron Nitride at electron
temperatures between 77 and 300 K using on-chip terahertz spectroscopy. At charge
neutrality, we observe the quantum-critical scattering rate characteristic of the Dirac fluid. At …