Spectroscopic evidence of a new energy scale for superconductivity in H3S

Abstract

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 ...

Superconducting Hydrogen Sulfide

“…This prevents an immediate practical application of hydrogen sulfide, but nonetheless it represents a big leap toward the realization of metallic hydrogen and its room‐temperature superconductivity at high pressure. In particular, a recent optical reflectivity study showed convincing evidence that hydrogen sulfide is a conventional superconductor, with the superconductivity originating from electron–phonon interaction . Theoretical calculation based on the Midgal–Eliashberg theory (an extension of the BCS theory to strong coupling regime) also showed a very good agreement with the experiment in the T c and its temperature dependence.…”

Strong particle-hole asymmetry in a 200 Kelvin superconductor

“…The (1 + λ) mass enhancement shows up in the imaginary part, where it translates to an increased slope at the low-frequency linear regime as well as a strong reduction in the magnitude. These results do not resolve the origin of the feature studied by Capitani et al 35 Summary. We have demonstrated the significant impact of strong electron-phonon coupling on the electron spectral density and band renormalization of the high-temperature superconductor H 3 S, pointing out its relationship with the sharp and asymmetric DOS peak near the Fermi level caused by two neighboring vHs.…”

“…While a general study of the effect of vHs awaits further developments, something can be said about the far infrared where interband transitions are negligible. Capitani et al 35 have reported the observed and modeled reflectivity in H 3 S at high pressure, observing structure identified with an energy scale of 160 meV, suggested to be a phonon with a very strong infrared coupling that is not evident from current theory. Given the paucity of experiments that are possible at high pressure, this experiment assumes unusually strong impact.…”

Spectroscopic signatures of phonons in high pressure superconducting hydrides

“…34). Very recently, Capitani et al 35 measured the reflectance of SH 3 at 155 GPa in a diamond anvil cell (DAC) and so probed the electron-phonon interaction in the system establishing a new high energy scale for its superconductivity. These experiments leave no doubt that we are dealing with conventional electron-phonon superconductivity.…”