Role of Vanadium-Oxide Layer in Electronic State of Sr_2VFeAsO_3-δ with Oxygen Deficiency

Iron-based superconductor Sr_2VFeAsO_3 is composed of alternate stacking of a superconducting FeAs layer and an insulating vanadium-oxide layer with a perovskite-type structure. Electronic orders stemming from the spin and orbital degrees of freedom of V 3d electrons can arise in the vanadium-oxide layer, but such orders have not been confirmed so far. Here, we systematically investigate the electronic state of Sr_2VFeAsO_3-δ with oxygen deficiency and demonstrate the phase diagram of Sr_2VFeAsO_3-δ as a function of the c-axis lattice parameter, which has turned out to be a suitable measure of the amount of oxygen deficiency. We found a magnetic and structural anomaly at ∼ 100 K with a thermal hysteresis, which is manifested with the introduction of oxygen deficiency. The presence of orthorhombic distortion was revealed below the temperature at which the anomaly appears, suggestive of V orbital ordering involving the d_xz and d_yz orbitals. It seems that substantial fluctuations associated with the orthorhombic distortion significantly influence the electronic state of the FeAs layer. Our findings indicate that the vanadium-oxide layer plays a significant role in the electronic state of Sr_2VFeAsO_3-δ.