Maximal quantum chaos of the critical Fermi surface

We investigate the many-body quantum chaos of non-Fermi liquid states with Fermi surfaces in two spatial dimensions by computing their out-of-time-order correlation functions. Using a recently proposed large $N$ theory for the critical Fermi surface, and the ladder identity of Gu and Kitaev, we show that the chaos Lyapunov exponent takes the maximal value of $2 \pi k_B T/\hbar$, where $T$ is the absolute temperature. We also examine a phenomenological model in which the chaos exponent becomes smaller than the maximal value precisely when quasiparticles are restored.