Heat and Charge Hydrodynamics

Speaker:

Andrea Cepellotti
University of California at Berkeley, and Lawrence Berkeley National Laboratory

Abstract:

The microscopic description of non-equilibrium properties has recently come within reach of ab-initio methods, with the first-principles Boltzmann transport equation being a prominent tool for the accurate description of transport properties. In this talk, we’ll first discuss heat diffusion in 2D materials, where the scattering dynamics is dominated by momentum conserving – normal – processes, as opposed to momentum dissipating – Umklapp – processes. Under these circumstances, heat flux is not lost at every scattering event, but is instead shuttled by scattering through multiple phonon states, coupling them. Asa consequence, exotic phenomena arise, such as ultra-high thermal conductivities and second sound (a wave-like temperature propagation), which we rationalized introducing a gas of collective phonon excitations called ‘relaxons’. Defined as the eigenvectors of the scattering matrix, relaxons allow for a simple - yet exact - interpretation of thermal conductivity in terms of a kinetic gas theory, revising the relevant time and length scale of heat flux dissipation. Additionally, this formalism provides a novel interpretation of surface scattering as a friction effect, and allows the derivation of Navier-Stokes-like equations as a correction to Fourier’s law. Next, we’ll discuss the charge hydrodynamic transport that emerges from electron-electron interactions in doped graphene. After writing an electronic Boltzmann equation with finite-temperature many-body corrections, we solve it exactly and characterize the thermal properties of electrons. In particular, we discuss the electronic compressibility, the mobility and the electronic viscosity, highlighting how electrons in doped graphene behave as a compressible non-Newtonian fluid.

• 10:30 Refreshments
• 10:45–12:00 Talk