Observation of a small superconducting energy gap in K$_{0.7}$Fe$_{1.8}$Se$_2$ by optical spectroscopy

We report an optical spectroscopy study on the newly discovered iron-selenide superconductor K$_{0.8}$Fe$_{1.7}$Se$_2$. In the far-infrared region, there is a clear signature of the superonducting energy gap with a gap ratio 2$\Delta/k_BT_c \sim$ 1.3, far below the usual weak-coupling BCS value. The large energy gap in the electron Fermi pockets observed by angle-resolved photoemission (ARPES) technique is completely absent in the infrared probe. The complex behavior may imply the presence of both dirty and clean channels of superconductivity. Our measurements also reveal a surprisingly low charge carrier density (and superconducting condensate below $T_c$) for the compound.