Physics news on the Internet: August 2023

The discovery of superconductivity [1] at a critical temperature reaching Tc ˆ 203 K in the pressure range of 100 to 250 GPa (in diamond anvils) in the H3S system stimulated a flood of experimental studies of high-temperature superconductivity of hydrides at megabar pressures (see reviews [2, 3]). A theoretical analysis immediately confirmed that these record Tc values are due to traditional electron±phonon interaction, and a good description of the experimental situation is due to the Eliashberg±McMillan theory in the limit of a sufficiently strong electron±phonon interaction [4, 5]. Moreover, detailed calculations for a whole number of transition metal hydrides under pressure [4] resulted in the occurrence of a rather large number of such systems with record Tc values. In some cases, these predictions found a magnificent confirmation, in particular, record temperature values Tc ˆ 250ÿ260 K were reached experimentally in the LaH10 system [6, 7]. These studies were of great importance, mainly because they clearly demonstrated the absence of substantial restrictions on Tc in the framework of the electron±phonon mechanism of Cooper paring, where it was conventionally accepted thatTc cannot exceed 30±40K.After the appearance of papers [6, 7], it became clear that the discovery of room temperature superconductivity, which had been only the dream of a few theoreticians over many years [8, 9], was not too far off. And now this barrier has been crossed, for in recent paper [10] superconductivity with Tc ˆ 287:7 1:2 K (i.e., nearly ‡15 C) has been obtained in the CÿHÿS system at a pressure of 267 10 GPa. The authors took advantage of the fact that hydrogen sulfide H2S mixes well with methane CH4. Such a mixture (with an additional injection of H2) underwent high-pressure photochemical synthesis (using laser radiation) and was examined at pressures from 100 to 300 GPa. Fairly convincing data were obtained on a sufficiently narrow superconducting transition (from resistive measurements) with Tc from 175 to 287 K upon pressure variation from 175 to 267 GPa that were confirmed by measurements of diamagnetic response (Meissner effect) at pressures from 175 to 200 GPa and by direct (resistive) measurements of the upper critical magnetic field (Tc lowering to 9 T under the action of the external magnetic field) near Tc. These measurements showed that the system under studywas a conventional second-order superconductor and theHc2 values atT ˆ 0 could reach 62 or 87 T (depending on the applied extrapolation to T ˆ 0). Unfortunately, the authors have not yet exactly determined the structure of the investigated superconducting phase CÿHÿS because of the difficulties in X-ray measurements in light-atom systems (smallness of the X-ray scattering cross section). It is believed that this problem will be solved in the nearest future in a combination of direct experiments and modern methods of theoretical simulation of high-pressure stable structures [4]. It is practically certain that the limit Tc ˆ ‡15 can be overcome in future experiments with hydrides under high pressure and, perhaps, also in the case of experimentally obtaining metallic hydrogen.