S.*

The discovery of a superconducting phase in sulfur hydride under high pressure with a critical temperature above 200 K by Drozdov et al. [1] has provided a new impetus to the search for even higher T_c. The observation of a sharp drop in resistance to zero at T_c, its downward shift with magnetic field and a Meissner effect confirm superconductivity but the mechanism involved remains to be determined. Using the AILES beamline at Soleil, we provide a first optical spectroscopy study of this new superconductor[2]. Experimental results for the optical reflectivity of H₃S, under high pressure, for several temperatures and over the range 60 to 600 meV of photon energies, are compared with theoretical calculations based on Eliashberg theory using DFT results for the electron-phonon spectral density. Two significant features stand out: some remarkably strong infrared active phonons at approximately 160 meV and 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 provides strong evidence of a conventional mechanism. However, the unusually strong optical phonon suggests a contribution of electronic degrees of freedom.

1. A.P. Drozdov, M.I. Eremets, I.A. Troyan, V. Ksenofontov, and S.I. Shylin, Nature 525, 73–76 (2015).
2. F.Capitani, B. Langerome, J.-B. Brubach, P. Roy, A. Drozdov, M.I. Eremets, E. J. Nicol, J. P. Carbotte, and T. Timusk, Nature Physics 15, 859 (2017)