The Optical Study of Single Crystalline Cs_0.8(Fe_1.05Se)₂ with High Néel Temperature

We report an optical spectroscopy study on an iron-selenide single crystal of insulating Cs_0.8(Fe_1.05Se)₂ with relatively high Néel temperature. The resistivity and magnetization measurements reveal a semiconductor behavior with T_N=492 K. The sample has been examined in the temperature range from 10 K up to 500 K by using infrared measurement. The low frequency optical measurement implies an insulating response with a major absorption near 0.31 eV. The double-peak structure in the mid-infrared region and the low-frequency phonon modes support the presence of the blocked anti-ferromagnetism. The infrared measurement reveals no signature of SDW-type energy gap formation below the anti-ferromagnetic-order temperature.