  The interplay of electronic correlations, multi-orbital excitations, and
strong spin-orbit coupling is a fertile ground for new states of matter in
quantum materials. Here, we report on a confocal Raman scattering study of
momentum-resolved charge dynamics from a thin film of semimetallic perovskite
$\mathbf{SrIrO_3}$. We demonstrate that the charge dynamics, characterized by a
broad continuum, is well described in terms of the marginal Fermi liquid
phenomenology. In addition, over a wide temperature regime, the inverse
scattering time is for all momenta close to the Planckian limit
$\mathbf{\tau^{-1}_{\hbar}=k_{\rm B} T/\hbar}$. Thus, $\mathbf{SrIrO_3}$ is a
semimetallic multi-band system that is as correlated as, for example, the
cuprate superconductors. The usual challenge to resolve the charge dynamics in
multi-band systems with very different mobilities is circumvented by taking
advantage of the momentum space selectivity of polarized electronic Raman
scattering. The Raman responses of both hole- and electron-pockets display an
electronic continuum extending far beyond 1000\icm ($\sim$125 meV), much larger
than allowed by the phase space for creating particle-hole pairs in a regular
Fermi liquid. Analyzing this response in the framework of a memory function
formalism, we are able to extract the frequency dependent scattering rate and
mass enhancement factor of both types of charge carriers, which in turn allows
us to determine the carrier-dependent mobilities and electrical resistivities.
The results are well consistent with transport measurement and demonstrate the
potential of this approach to investigate the charge dynamics in multi-band
systems.
