Title: Thermal diffusivity and chaos in metals without quasiparticles,
Abstract: We study the thermal diffusivity $D_T$ in models of metals without
quasiparticle excitations (`strange metals'). The many-body quantum chaos and
transport properties of such metals can be efficiently described by a
holographic representation in a gravitational theory in an emergent curved
spacetime with an additional spatial dimension. We find that at generic
infra-red fixed points $D_T$ is always related to parameters characterizing
many-body quantum chaos: the butterfly velocity $v_B$, and Lyapunov time
$\tau_L$ through $D_T \sim v_B^2 \tau_L$. The relationship holds independently
of the charge density, periodic potential strength or magnetic field at the
fixed point. The generality of this result follows from the observation that
the thermal conductivity of strange metals depends only on the metric near the
horizon of a black hole in the emergent spacetime, and is otherwise insensitive
to the profile of any matter fields.