Transport in inhomogeneous quantum critical fluids and in the Dirac fluid in graphene

We develop a general hydrodynamic framework for computing direct current, thermal, and electric transport in a strongly interacting finite-temperature quantum system near a Lorentz-invariant quantum critical point. Our framework is nonperturbative in the strength of long-wavelength fluctuations in the background-charge density of the electronic fluid and requires the rate of electron-electron scattering to be faster than the rate of electron-impurity scattering. We use this formalism to compute transport coefficients in the Dirac fluid in clean samples of graphene near the charge neutrality point, and find results insensitive to long-range Coulomb interactions. Numerical results are compared to recent experimental data on thermal and electrical conductivity in the Dirac fluid in graphene and a substantially improved quantitative agreement over existing hydrodynamic theories is found. We comment on the interplay between the Dirac fluid and acoustic and optical phonons, and qualitatively explain the experimentally observed effects. Our work paves the way for quantitative contact between experimentally realized condensed matter systems and the wide body of high-energy inspired theories on transport in interacting many-body quantum systems.