High-Temperature Superconducting Phases in Cerium Superhydride with a Tc up to 115 K below a Pressure of 1 Megabar

The discoveries of high-temperature superconductivity in H3S and LaH10 have excited the search for superconductivity in compressed hydrides, finally leading to the first discovery of a room-temperature superconductor in a carbonaceous sulfur hydride. In contrast to rapidly expanding theoretical studies, high-pressure experiments on hydride superconductors are expensive and technically challenging. Here, we experimentally discovered superconductivity in two new phases, Fm (3) over barm-CaH10 (SC-I phase) and P6(3)/mmc-CeH9 (SC-II phase) at pressures that are much lower (<100 GPa) than those needed to stabilize other polyhydride superconductors. Superconductivity was evidenced by a sharp drop of the electrical resistance to zero and decreased critical temperature in deuterated samples and in external magnetic field. SC-I has T-c = 115 K at 95 GPa, showing an expected decrease in further compression due to the decrease of the electron-phonon coupling (EPC) coefficient lambda (from 2.0 at 100 GPa to 0.8 at 200 GPa). SC-II has T-c = 57 K at 88 GPa, rapidly increasing to a maximum T-c similar to 100 K at 130 GPa, and then decreasing in further compression. According to the theoretical calculation, this is due to a maximum of lambda at the phase transition from P6(3)/mmc-CeH9 into a symmetry-broken modification C2/c-CeH9. The pressure-temperature conditions of synthesis affect the actual hydrogen content and the actual value of T-c. Anomalously low pressures of stability of cerium superhydrides make them appealing for studies of superhydrides and for designing new superhydrides with stability at even lower pressures.