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Single crystal growth and optical conductivity of SrPt$_2$As$_2$ superconductors

SrPt$_2$As$_2$ single crystals with CaBe$_2$Ge$_2$-type structure were synthesized by self-melting technique. X-ray diffraction, transmission electron microscopy, electrical resistivity, specific heat and optical spectroscopy measurements were conducted to elucidate the properties of SrPt$_2$As$_2$. SrPt$_2$As$_2$ single crystals exhibit a superconducting transition at 5.2K, experiencing a structural phase transition well above room temperature (about 455K). The superconducting and structural phase transition temperatures are both reduced by 6% Iridium doping. Both pure SrPt$_2$As$_2$ and the doped single crystals are revealed to be highly metallic with rather high plasma frequencies. In particular, the optical spectroscopy measurement revealed two gap-like suppression features. We elaborated that the one at higher energy scale could be ascribed to the correlation effect, while the other one at lower energy scale is related to the structural phase transition, leading to the removal of a small portion of the Fermi surfaces.