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

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, $Fm\overline{3}m\text{−}{\mathrm{CeH}}_{10}$ (SC-I phase) and $P{6}_3/mmc\text{−}{\mathrm{CeH}}_9$ (SC-II phase) at pressures that are much lower ($<100\mathrm{GPa}$) 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 $T_c=115\mathrm{K}$ 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 $T_c=57\mathrm{K}$ at 88 GPa, rapidly increasing to a maximum $T_c\sim 100\mathrm{K}$ 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 $P{6}_3/mmc\text{−}{\mathrm{CeH}}_9$ into a symmetry-broken modification $C2/c\text{−}{\mathrm{CeH}}_9$. 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.

Figure 1

Superconducting transitions determined by the electrical resistivity measurements in typical cells. (a) Pressurized $\mathrm{Ce}/\mathrm{AB}$ sample and four electrodes on the insulated gasket in cells H1 and H4 with bottom (left) and double-side illumination (right). The edge of cerium is marked with red dotted lines and the red arrows point to the parts with apparent changes. (b) Temperature dependence of the electrical resistance at the superconducting transitions (the warming cycle) in DACs H1-5 at selected pressures. Squares and circles indicate the apparent turning points. The scheme of the experimental assembly is shown in the inset. (c),(d) $R\text{−}T$ dependence in a magnetic field of 1–5 T at 139 GPa (cell H1, warming cycle) and 0–8 T at 137 GPa (cell H2). Dashed lines are linear fittings of the normal and transition states, and $T_c$ is defined at their intersection. The upper critical magnetic fields estimated using the Werthamer-Helfand-Hohenberg (WHH) [67] and Ginzburg-Landau (GL) [68] theories are shown in the inset. Warm and cool colors represent warming and cooling cycles, respectively in (d).

Figure 2

XRD and $P\text{−}V$ data of the $\mathrm{Ce}─\mathrm{H}$ phases at different pressures. (a) Typical XRD (0.6199 Å) of the electrical cells H2, H3, H5, and H6. Relative phase fraction among ${\mathrm{CeH}}_{10}$, ${\mathrm{CeH}}_9$, and ${\mathrm{CeH}}_4$ were estimated and presented in Fig. S3 [47, 52]. (b) Pressure dependence of the cell volume per formula unit. Experimental data in this study are represented by different point symbols. Dashed and solid lines indicate the calculated $P\text{−}V$ data and fitted experimental data from Ref. [36], respectively. More data are plotted in Fig. S2.

Figure 3

XRD and electrical measurements in the external magnetic field of the synthesized $\mathrm{Ce}─\mathrm{D}$ phases. (a) Selected XRD ($\lambda =0.6199\AA$) of cell S2. (b) Le Bail refinement of cell S2 at 117 GPa (c) $R\text{−}T$ dependence of the cell D2 in the external magnetic field of 0–7 T at 120 GPa. Circles represent the selected data for extrapolating $H_{c2}(0)$. (d) Upper critical magnetic field was extrapolated using the WHH [67] and GL [68, 69] models. Inset is the photo of sample chamber.

Figure 4

Superconducting parameters of $Fm\overline{3}m\text{−}{\mathrm{CeH}}_{10}$ and $P{6}_3/mmc\text{−}{\mathrm{CeH}}_9$. (a),(c) Experimental $T_c$ as a function of pressure for phases SC-I and SC-II. Insets show the hydrogen cages, and yellow atoms represents the lost hydrogen from $P{6}_3/mmc\text{−}{\mathrm{CeH}}_9$ to $P{6}_{3}mc\text{−}{\mathrm{CeH}}_8$. (b),(d) Calculated logarithmic averaged frequency ${\omega }_{\log }$, EPC coefficient $\lambda$ and $T_c$ at various pressures for $Fm\overline{3}m\text{−}{\mathrm{CeH}}_{10}$ and $P{6}_3/mmc\text{−}{\mathrm{CeH}}_9$. The theoretical data of $Fm\overline{3}m\text{−}{\mathrm{CeH}}_{10}$ are from Ref. [39].