High-Temperature Superconducting Phases in Cerium Superhydride with a <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>T</mml:mi></mml:mrow><mml:mrow><mml:mi>c</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math> up to 115 K below a Pressure of 1 Megabar
Wuhao Chen, Dmitrii V. Semenok, Xiaoli Huang, Haiyun Shu, Xin Li, Defang Duan, Tian Cui, Artem R. Oganov
Abstract
The discoveries of high-temperature superconductivity in ${\mathrm{H}}_{3}\mathrm{S}$ and ${\mathrm{LaH}}_{10}$ 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, $Fm\overline{3}m\text{\ensuremath{-}}{\mathrm{CeH}}_{10}$ (SC-I phase) and $P{6}_{3}/mmc\text{\ensuremath{-}}{\mathrm{CeH}}_{9}$ (SC-II phase) at pressures that are much lower ($<100\text{ }\text{ }\mathrm{GPa}$) 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 ${T}_{c}=115\text{ }\text{ }\mathrm{K}$ at 95 GPa, showing an expected decrease in further compression due to the decrease of the electron-phonon coupling (EPC) coefficient $\ensuremath{\lambda}$ (from 2.0 at 100 GPa to 0.8 at 200 GPa). SC-II has ${T}_{c}=57\text{ }\text{ }\mathrm{K}$ at 88 GPa, rapidly increasing to a maximum ${T}_{c}\ensuremath{\sim}100\text{ }\text{ }\mathrm{K}$ at 130 GPa, and then decreasing in further compression. According to the theoretical calculation, this is due to a maximum of $\ensuremath{\lambda}$ at the phase transition from $P{6}_{3}/mmc\text{\ensuremath{-}}{\mathrm{CeH}}_{9}$ into a symmetry-broken modification $C2/c\text{\ensuremath{-}}{\mathrm{CeH}}_{9}$. 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.