Title: Origin of the Drude peak and of zero sound in probe brane holography,
Abstract: At zero temperature, the charge current operator appears to be conserved,
within linear response, in certain holographic probe brane models of strange
metals. At small but finite temperature, we analytically show that the weak
non-conservation of this current leads to both a collective "zero sound" mode
and a Drude peak in the electrical conductivity. This simultaneously resolves
two outstanding puzzles about probe brane theories. The nonlinear dynamics of
the current operator itself appears qualitatively different.