Abstract
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-Tc materials with Tc’s near room temperature values. These findings confirm earlier expectations that hydrides may have very high Tc’s due to the fact that light H atoms have very high vibrational frequencies, leading to high Tc 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-Tc superhydrides. These new superhydride “room-temperature” 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.
Highlights
INTRODUCTIONWhich is usually attributed to Gilman and Ashcroft. Originally, Ashcroft conjectured that certain compounds of hydrogen with other elements in the periodic table may form metallic alloys, or, in other words, sustain a metallic hydrogen sublattice doped with well selected elements
The attainment of room-temperature superconductivity is a longstanding challenge related historically to the metallic phase of hydrogen.1–3 The ambient-pressure molecular crystals of hydrogen are stable only at low temperature, and a metallic atomic-like hydrogen phase was proposed by Wigner and Huntington to occur under high pressure conditions.4 The pressure of metallization, estimated by Wigner and Huntington, of about 20 GPa, proved to be incorrect, and it was realized later that the actual metallization pressure should be around 500 GPa.5 Metallic hydrogen is predicted to have a very high superconducting Tc,6 which naturally arises from the Bardeen– Cooper–Schrieffer (BCS)7 electron-phonon coupling mechanism involving hydrogen’s high vibrational frequencies
Ashcroft conjectured that certain compounds of hydrogen with other elements in the periodic table may form metallic alloys, or, in other words, sustain a metallic hydrogen sublattice doped with well selected elements
Summary
Which is usually attributed to Gilman and Ashcroft. Originally, Ashcroft conjectured that certain compounds of hydrogen with other elements in the periodic table may form metallic alloys, or, in other words, sustain a metallic hydrogen sublattice doped with well selected elements. Ashcroft conjectured that certain compounds of hydrogen with other elements in the periodic table may form metallic alloys, or, in other words, sustain a metallic hydrogen sublattice doped with well selected elements. Such “doped” compounds of hydrogen may have a stability range at pressures much lower than the 500 GPa required to stabilize pure atomic metallic hydrogen.. While Ashcroft’s idea did not work well for the compounds he proposed (silane, SiH417 and similar group IV hydrides), more recent predictions by Li et al. have stimulated experimental work by Drozdov et al. and theoretical work by Duan et al., which found high Tc superconductivity in H3S below 200 GPa at Tc ∼ 200 K. For more extensive reviews on superhydrides and conventional hydrides, we redirect the reader to other sources.
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