Theories of Planckian dissipation in strange metals

Metallic non-Fermi liquid phases above putative quantum critical points in a wide variety of materials with strongly correlated electrons display linear-in-temperature resistivity ("strange metallic" behavior) over a wide range of temperatures, down to temperatures approaching absolute zero. Furthermore, when expressed in terms of a Drude formula using the electron effective mass obtained from nearby Fermi liquid phases with well-defined quasiparticles, the temperature dependence of the resistivity implies a "universal" electron transport lifetime τ_tr ~ α h/(2π k_BT), where α ~ 1 across different materials with very different electron-electron interaction strengths. I will describe two types of exactly solvable effective field theories, constructed along the lines of the Sachdev-Ye-Kitaev models, that can describe such universal behavior. The first kind realizes a "marginal Fermi liquid" phase as a quantum critical point between metallic Fermi liquid phases involving "small" and "large" carrier densitites. The second kind is an exotic state of matter that is very far from having well-defined quasiparticles, which lacks a sharp Fermi surface but nonetheless has a strong momentum dependence of the electron spectral function that significes a "remnant" Fermi surface.