Temperature-dependent optical properties of self-doped superconducting Fe-pnictide, Sr₂VO₃FeAs

We performed an infrared spectroscopic study on a single crystal of Sr₂VO₃FeAs grown by a self-flux method. This layered material system consists of two alternative layers of [SrVO₃]^-1 and [SrFeAs]⁺¹. Since the typical size of single crystalline Sr₂VO₃FeAs samples is 200 [Formula: see text] 200 [Formula: see text] 10 [Formula: see text]m³ an optical study on this material is challenging. We observed an additional interband transition around 1000 cm^-1, which is absent in other doped Ba-122 Fe-pnictides. The origin of this additional transition is not clearly known yet. We also observed a hidden Fermi liquid behavior. Interestingly, we observed a Fano line-shaped phonon which appears near 555 cm^-1 below 200 K and shows a strong blue-shift when the temperature is lowered. The amplitude, width, and asymmetric Fano parameter of this phonon show anomalies at 150 K, which are probably related to an electronic phase observed below 155 K recently by an NMR study (Ok et al 2017 Nat. Commun. 8 2167). Our finding may help to understand the electronic phase observed previously in the same material.