  Sr3Ir4Sn13 is an interesting compound showing a coexistence of structural
phase transition and superconductivity. The structural phase transition at 147
K leads to the formation of a superlattice. We performed optical spectroscopy
measurements across the structural phase transition on single crystal sample of
Sr3Ir4Sn13. The optical spectroscopy study reveals an unusual temperature
induced spectral weight transfer over broad energy scale, yielding evidence for
the presence of electron correlation effect. Below the structural phase
transition temperature an energy gap-like suppression in optical conductivity
was observed, leading to the removal of partial itinerant carriers near Fermi
level. Unexpectedly, the suppression appears at much higher energy scale than
that expected for a usual charge density wave phase transition.
