Pressure Suppression of Spin-Density-Wave Gap in the Optical Conductivity of SrFe2As2

Optical reflectance $R(\omega)$ of a pressure-induced superconductor SrFe 2 As 2 has been measured under external pressure to 6 GPa and at temperatures to 8 K. Optical conductivity $\sigma(\omega)$ has been derived from the measured $R(\omega)$. At ambient pressure, in the antiferromagnetic state below $T_{\text{N}}=198$ K, a pronounced feature develops in $\sigma(\omega)$ due to the opening of a spin density wave (SDW) gap, as already reported in the literature. With increasing pressure, the SDW gap feature in $\sigma(\omega)$ is progressively suppressed. At 4 GPa, where the sample is superconducting, the SDW gap feature in $\sigma(\omega)$ is strongly reduced than that at ambient pressure, but is still clearly observed. At 6 GPa, the SDW gap is completely suppressed. The evolution of SDW gap feature with pressure is qualitatively similar to that with chemical doping previously reported.