Superconductivity in La and Y hydrides: Remaining questions to experiment and theory

Recent reports of the superconductivity in hydrides of two different families (covalent lattice, as in SH3 and clathrate-type H-cages containing La and Y atoms, as in LaH10 and YH6) have revealed new families of high-T. materials with T-c's near room temperature values. These findings confirm earlier expectations that hydrides may have very high T-c's due to the fact that light H atoms have very high vibrational frequencies, leading to high T-c values within the conventional Bardeen-Cooper-Schrieffer phonon mechanism of superconductivity. However, as is pointed out by Ashcroft, it is important to have the metallic hydrogen alloyed with the elements added to it. This concept of a metallic alloy containing a high concentration of metal-like hydrogen atoms has been instrumental in finding new high-T-c superhydrides. These new superhydride roomtemperature superconductors are stabilized only at very high pressures above 100 GPa, making the experimental search for their superconducting properties very difficult. We will review the current experimental and theoretical results for LaH10-x and YH6-x superhydrides. (C) 2020 Author(s).