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 system displays all of 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 H₃S, 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.

In this paper we provide an accurate high-pressure structural and optical study of the MAPbI3 hybrid perovskite. Structural data show the presence of a phase transition toward an orthorhombic structure around 0.3 GPa followed by full amorphization of the system above 3 GPa. After releasing the pressure, the system keeps the high-pressure orthorhombic phase. The occurrence of these structural transitions is further confirmed by pressure induced variations of the photoluminescence signal at high pressure. These variations clearly indicate that the bandgap value and the electronic structure of MAPI change across the phase transition.

Ensuring hydrostatic conditions in high pressure experiments is of utmost importance for obtaining reproducible and comparable dataset. Here, we investigate the degree of hydrostaticity of four pressure-transmitting media (PTM), three alkali halides and polyethylene, commonly used in infrared (IR) spectroscopy using multiple rubies as pressure sensors within a diamond anvil cell (DAC) pressure cavity. We demonstrate that the pressure standard deviation is always below 0.3 GPa at pressures below 10 GPa and that sodium chloride (NaCl) provides the best quasi-hydrostatic conditions up to 21 GPa. In the 10–20 GPa pressure range, NaCl, cesium iodide (CsI), and polyethylene perform significantly better than 4:1 ethanol–methanol or silicone oil – widely used PTM in high pressure experiments. These results will enable a more reliable and quantitative comparison between different experiments where the PTM investigated here are used.