Rare-earth polyhydrides formed under pressure are promising conventional superconductors, with the critical temperature $T_c$ in compressed ${\mathrm{LaH}}_{10}$ almost reaching room temperature. Here, we report a systematic computational investigation of the structural and superconducting properties of rare-earth (RE) polyhydrides formed under pressure across the whole lanthanide series. Analyses of the electronic and dynamical properties and electron-phonon coupling interaction for the most hydrogen-rich hydrides ${\mathrm{REH}}_n$ ($n=8,9,10$) that can be stabilized below 400 GPa show that enhanced $T_c$ correlates with a high density of H $s$ states and low number of RE $f$ states at the Fermi level. In addition to previously predicted and measured ${\mathrm{LaH}}_{10}$ and ${\mathrm{CeH}}_9$, we suggest ${\mathrm{YbH}}_{10}$ and ${\mathrm{LuH}}_8$ as additional potential high-$T_c$ superconducters. They form a “second island” of high-$T_c$ superconductivity amongst the late lanthanide polyhydrides, with an estimated $T_c$ of 102 K for ${\mathrm{YbH}}_{10}$ at 250 GPa.

Density functional theory calculations and crystal structure predictions using the particle swarm optimization method have been combined to determine stable hydrides of lead under pressure. In contrast to other group-IVa hydrides, the stoichiometry ${\mathrm{PbH}}_6$ is the first hydride to become stable, at just under 1 Mbar. For two previously studied stoichiometries, ${\mathrm{PbH}}_4$ and ${\mathrm{PbH}}_8$, energetically more favorable phases were identified to become stable around 2 Mbar. In all structures, the hydrogenic sublattices comprise negatively charged ${{\mathrm{H}}_2}^{\delta -}$ molecules. Competitive ${\mathrm{PbH}}_4$ and ${\mathrm{PbH}}_6$ structures are layered. ${\mathrm{PbH}}_6$ features ${\mathrm{H}}_2$ molecules intercalated between hcp Pb layers, the stable phase of dense pure lead, thus offering a potentially straightforward route towards synthesis. In ${\mathrm{PbH}}_8$, the Pb lattice adapts a $\beta$-Sn structure, and hydrogen atoms form quasi-one-dimensional-chains. All structures were found to be metallic and to feature superconductivity in their respective stability range, with moderately high $T_c$ in the range 60–100 K for ${\mathrm{PbH}}_4$ and ${\mathrm{PbH}}_6$ and 161–178 K for ${\mathrm{PbH}}_8$.

Two-dimensional (2D) magnetic materials are essential to developing high-performance spintronic devices. Recent experimental discoveries of several atomic thin 2D ferromagnetic materials have stimulated great interest in further exploring this fascinating class of materials. Here, combining an advanced crystal structure search method and extensive first-principles energetic and dynamic calculations, we have identified a planar ${\mathrm{CoB}}_6$ monolayer as a stable 2D ferromagnet. We show that the ferromagnetic ground state of the ${\mathrm{CoB}}_6$ monolayer remains robust in the ambient environment, and the magnetic stability and moment can be remarkably enhanced and tuned by external strain. Moreover, we propose feasible synthesis routes for the the newly predicted ${\mathrm{CoB}}_6$ monolayer, either by Co atom adsorption on the recently proposed ${\delta }_4$ boron sheet or by direct chemical growth. The present results establish a fundamental material and physics basis for synthesis and characterization of the ${\mathrm{CoB}}_6$ monolayer among the emerging 2D ferromagnetic materials.

Recently, the carbonaceous sulfur hydride was synthesized [E. Snider et al., Nature (London) 586, 373 (2020)], and the measured superconducting transition temperature ($T_c$) is about 278 K under 267 GPa, placing it as a promising candidate of the room temperature superconductor. However, the ultrahigh synthesis pressure is indeed a challenge for experimental technique, stimulating us to search other high temperature superconductors under relatively low pressure. Here we perform comprehensively structural searches of ternary alkaline (Am) boron hydrides. Our calculations indicate that the ternary ${\mathrm{AmB}}_2{\mathrm{H}}_8$ hydrides are energetically stable at about 10 GPa. Interestingly, the metallic $\mathrm{Rb}/{\mathrm{CsB}}_2{\mathrm{H}}_8$ polyhydrides are superior superconductors with high $T_c$ of $\sim 100$ K at 25 GPa. The electron-phonon coupling calculations suggest that the Am atoms are promising electron contributors, which effectively improve the $T_c$ and decrease the synthesized pressure in complex hydrides. These findings elucidate the superconductivity mechanism of ${\mathrm{AmB}}_2{\mathrm{H}}_8$ hydrides and offer crucial insights for design and synthesis of novel superconductors.