Optical conductivity of CrAs across the paramagnetic to antiferromagnetic transition

We report the optical spectroscopy of monopnictide crystal CrAs. This system exhibits a PM to AF transition at T_N~265K. This AF phase gradually disappears under pressure with the emergence of 2.2K superconductivity at 0.7GPa. To understand this rich phase diagram, various technics have investigated similarities of this material with the parent iron-pnictide superconductors. Here we studied the PM to AF transition by measuring the reflectivity from 300K to 5K at ambient pressure from 17meV to 3eV. As a function of incident photon energy, the reflectivity shows a gradual decay for all temperatures. The optical conductivity was extracted using Kramers–Kronig. Below 50meV, a Drude peak is clearly found only for temperatures below T_N. Extrapolation to zero frequencies of the conductivity is in agreement with previous reported DC transport measurements. In the mid-infrared range, the optical response of the AF order (T=250K) shows a rounded gap features with respect to the PM phase (T=300K): suppression in the low frequency optical conductivity, alongside a spectral weight transfer from low to high frequencies. The spectral weight transfer dip-hump structure will be compared to previous SDW gaps found in iron-based superconductors and electron doped cuprates.