Return of the phonons: Planckian scattering emerges from vibronic dynamics

Strange metals exhibit universal linear-in-temperature resistivity described by the Planckian scattering rate, the origin of which was not previously understood. It was thought that phonons were disqualified as a prime agent, in spite of their predominance in many other resistivity contexts. The problem seems to have been not with phonons, but how they were treated. Here we show that Planckian resistivity can in fact emerge from the \textit{nonperturbative} scattering dynamics of thermal lattice vibrations and \textit{coherent} charge carriers by a wave-on-wave approach. This becomes apparent when describing the lattice vibrations in the coherent state representation, and treating charge carriers as quantum wave packets negotiating the resulting dynamic disorder field formed by lattice vibrations, the deformation potential. Under this coherent and nonperturbative scattering dynamics, we find that carrier velocities are suppressed, the quasiparticle picture breaks down and a new phase called the vibronic fluid emerges. A competition between the static and dynamic aspects of the random acoustic deformation potential is set up, leading to a previously conjectured quantum bound of diffusion $\hbar/m^{*}$ with universal properties. We successfully obtain the $T$-linear resistivity of three different strange metals using their experimental parameters in our numerical simulations. We explain the underlying mechanism for the Planckian scattering based on the phenomenological model of Thouless scaling theory. Furthermore, we provide explanations for the violation of the Mott-Ioffe-Regel limit as well as the emergence of the displaced Drude peak in strange metals.