Strange metal behavior is ubiquitous to correlated materials ranging from cuprate superconductors to bilayer graphene. There is increasing recognition that it arises from physics beyond the quantum fluctuations of a Landau order parameter which, in q
uantum critical heavy fermion antiferromagnets, may be realized as critical Kondo entanglement of spin and charge. The dynamics of the associated electronic delocalization transition could be ideally probed by optical conductivity, but experiments in the corresponding frequency and temperature ranges have remained elusive. We present terahertz time-domain transmission spectroscopy on molecular beam epitaxy-grown thin films of YbRh$_2$Si$_2$, a model strange metal compound. We observe frequency over temperature scaling of the optical conductivity as a hallmark of beyond-Landau quantum criticality. Our discovery implicates critical charge fluctuations as playing a central role in the strange metal behavior, thereby elucidating one of the longstanding mysteries of correlated quantum matter.
Recent theoretical studies of topologically nontrivial electronic states in Kondo insulators have pointed to the importance of spin-orbit coupling (SOC) for stabilizing these states. However, systematic experimental studies that tune the SOC paramete
r $lambda_{rm{SOC}}$ in Kondo insulators remain elusive. The main reason is that variations of (chemical) pressure or doping strongly influence the Kondo coupling $J_{text{K}}$ and the chemical potential $mu$ -- both essential parameters determining the ground state of the material -- and thus possible $lambda_{rm{SOC}}$ tuning effects have remained unnoticed. Here we present the successful growth of the substitution series Ce$_3$Bi$_4$(Pt$_{1-x}$Pd$_x$)$_3$ ($0 le x le 1$) of the archetypal (noncentrosymmetric) Kondo insulator Ce$_3$Bi$_4$Pt$_3$. The Pt-Pd substitution is isostructural, isoelectronic, and isosize, and therefore likely to leave $J_{text{K}}$ and $mu$ essentially unchanged. By contrast, the large mass difference between the $5d$ element Pt and the $4d$ element Pd leads to a large difference in $lambda_{rm{SOC}}$, which thus is the dominating tuning parameter in the series. Surprisingly, with increasing $x$ (decreasing $lambda_{rm{SOC}}$), we observe a Kondo insulator to semimetal transition, demonstrating an unprecedented drastic influence of the SOC. The fully substituted end compound Ce$_3$Bi$_4$Pd$_3$ shows thermodynamic signatures of a recently predicted Weyl-Kondo semimetal.
