  Optical conductivity (OC) can serve as a measure of correlation effects in a
wide range of condensed matter systems. We here show that the long-range tail
of the Coulomb interaction yields a universal correction to the OC in a
three-dimensional Weyl semimetal $\sigma(\Omega)=\sigma_0(\Omega)\left[
1+\frac{1}{N+1} \right]$, where of $\sigma_0(\Omega)=Ne^2_0 \Omega/(12 h v)$ is
the OC in the non-interacting system, with $v$ as the actual (renormalized)
Fermi velocity of Weyl quasiparticles at frequency $\Omega$, and $e_0$ is the
electron charge in vacuum. Such universal enhancement of OC, which depends only
on the number of Weyl nodes near the Fermi level ($N$), is a remarkable
consequence of an intriguing conspiracy among the quantum-critical nature of an
interacting Weyl liquid, marginal irrelevance of the long-range Coulomb
interaction and the violation of hyperscaling in three dimensions, and can
directly be measured in recently discovered Weyl as well as Dirac materials. By
contrast, a local density-density interaction produces a non-universal
correction to the OC, stemming from the non-renormalizable nature of the
corresponding interacting field theory.
