The discovery of a superconducting phase in sulfur hydride under high pressure with a critical temperature above 200 K has provided fresh impetus to the search for superconductors at ever higher temperatures. Although this systems displays all the hallmarks of superconductivity, the mechanism through which it arises remains to be determined. Here we provide a first optical spectroscopy study of this superconductor. Experimental results for the optical reflectivity of H 3 S, under hydrostatic pressure of 150 GPa, for several temperatures and over the range 60 to 600 meV of photon energies, are compared with theoretical calculations based on Eliashberg theory. Two significant features stand out: some remarkably strong infrared active phonons at around 160 meV, and a band with a depressed reflectance in the superconducting state in the region from 450 meV to 600 meV. In this energy range H3S becomes more reflecting with increasing temperature, a change that is traced to superconductivity originating from the electron-phonon interaction. 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. Keywordssuperconductivity; H3S; optical data; the electron-boson spectral density * pascale.roy@synchrotron-soleil.fr.† timusk@mcmaster.ca. Author contributionsThis project has been initiated and supervised by T.T., M.I.E. and P.R. Samples have been synthesized and characterized by A.D. and M.I.E. Infrared measurements and data treatment were carried by B.L., F.C., J.B.B., P.R. and T.T. The calculations were performed by E.J.N. and J.P.C. All authors contributed to the writing of the paper. Competing financial interestsThe authors declare no competing financial interests. Furthermore, the superconducting phase has been found to be H 3 S by x-ray diffraction6. Calculations based on density functional theory (DFT) suggest that superconductivity in H 3 S is caused by the electron-phonon interaction, enhanced by a combination of the light mass of hydrogen and very strong coupling to high energy modes7-11. What is lacking is an experimental verification of this mechanism. A step in that direction would be the identification of the spectrum of bosons that couple to the charge carriers to form the glue that leads to superconducting pairing.The mechanism whereby conventional metals become superconductors is well established and involves the electron-phonon interaction12,13. The current-voltage characteristics of tunneling junctions12 and optical spectroscopy14-17 have yielded detailed information on the electron-phonon spectral density α 2 F(Ω) as a function of phonon energy ħΩ. These phonon spectra were further verified by neutron scattering18.It is an experimental challenge to extend these methods to the recently discovered hydrogen sulfide under pressure of 150 GPa for several reasons. The sample size ≈ 50 μm clearly excludes ...

The optical constants of sodium chloride in a wide pressure range were determined from the analysis of the reflectance and transmittance spectra of a minute quantity of NaCl powder placed in diamond anvil cells. The so-called "reststrahlen band" dominates the far-infrared reflectance spectra shifting from 150 cm −1 to 500 cm −1 at 100 GPa. For the 0−17.5 GPa pressure range, measurements allow accurate determination of both transverse and longitudinal mode frequencies. Higher pressure measurements reveal the B1 → B2 structural transition around 30 GPa and provide frequencies for the transverse and longitudinal modes. This spectroscopic signature on a sample smaller than 100 μm using light of a wavelength close to this dimension was observed, thanks to the high brilliance synchrotron source. In addition, ab initio calculations performed for the 0−200 GPa range predict the TO and LO frequencies. They are validated by the excellent agreement with the experiment.

A new optical setup is described that allows the reflectivity at grazing incidence to be measured, including ultrathin films and two-dimensional electron systems (2DES) down to liquid-helium temperatures, by exploiting the Berreman effect and the high brilliance of infrared synchrotron radiation. This apparatus is well adapted to detect the absorption of a 2DES of nanometric thickness, namely that which forms spontaneously at the interface between a thin film of LaAlO 3 and its SrTiO 3 substrate, and to determine its Drude parameters.

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