The discovery of a superconducting phase in sulfur hydride under high pressure with a critical temperature above 200 K by Drozdov et al.  has provided a new impetus to the search for even higher Tc. The observation of a sharp drop in resistance to zero at Tc, its downward shift with magnetic field and a Meissner effect confirm superconductivity but the mechanism involved remains to be determined. Using the AILES beam line at Soleil, we provided a first optical spectroscopy study of this new superconductor. Experimental results for the optical reflectivity of H3S, under high pressure, were compared with theoretical calculations based on Eliashberg theory using DFT results for the electron-phonon spectral density. One significant features stands out: a band with a depressed reflectance in the superconducting state in the region from 450 meV to 600 meV. The shape, magnitude, and energy dependence of this band at 150 K agrees with our calculations. This is strong evidence of a conventional electron-phonon mechanism. Also, this band provides a tool for a non-invasive probe of superconductivity.
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- J.P. Carbotte, E.J. Nicol, and T. Timusk, Physical Review Letters, 121, 047002 (2018)