Optical conductivity of FeTe1−xSex

Abstract

The ab-plane optical conductivity of two single crystals belonging to the chalcogenide family, FeTe0.91 and FeTe0.7Se0.3, has been studied in the normal phase between 16 and 450 K. Several differences with respect to both conventional metals and other exotic superconductors, like the cuprates, have been found both in the undoped and in the doped material. In the former compound, the transition to an antiferromagnetic (AF) state at 67 K causes the opening of a pseudogap at ω0 = 270 ± 25 cm−1 and a decrease by 70% in the free carrier density, accompanied by a strong reduction in the carrier scattering rate. The multiband analysis of the low-energy optical conductivity requires at least two Drude components, in addition to a mid-infrared band. Above 350 K, a transfer of spectral weight between those terms produces a cusp anomaly in the spectral weight. This indicates a well-defined transition, rather than a crossover, to a less-metallic regime. Finally, in FeTe0.7Se0.3 the far-infrared conductivity decreases with T, in agreement with the semiconducting behavior of the resistivity, and precursor phenomena of superconductivity appear above Tc.