The electron-phonon coupling constant and the Debye temperature in polyhydrides of thorium, hexadeuteride of yttrium, and metallic hydrogen phase III

Abstract

Milestone experimental discovery of superconductivity above 200 K in highly-compressed sulphur hydride by Drozdov (Nature 525, 73 (2015)) sparked experimental and theoretical investigations of metallic hydrides. Since then, a dozen of superconducting binary and ternary polyhydrides have been discovered. For instance, there are three superconducting polyhydrides of thorium: Th4H15, ThH9, and ThH10 and four polyhydrides of yttrium: YH4, YH6, YH7, YH9. In addition to binary and ternary hydrogen-based metallic compounds, recently Eremets (arXiv:2109.11104) reported on the metallization of hydrogen, which exhibits a phase transition into metallic hydrogen phase III at P>330 GPa and T<200 K. Here we analyzed temperature-dependent resistance, R(T), in polyhydrides of thorium, hexadeuteride of yttrium and in hydrogen phase III and deduced the Debye temperature, $T$${}_{\theta }$, and the electron-phonon coupling constant, ${\lambda }_{e-ph}$, for these conductors. We found that Th4H15 phase exhibits ${\lambda }_{e-ph}$ = 0.82-0.99 which is in a very good agreement with experimental value of ${\lambda }_{e-ph}$ = 0.84 deduced from heat capacity measurements (Miller, Phys. Rev. B 14, 2795 (1976)). For P63/mmc-ThH9 phase (P = 170 GPa) we deduced ${\lambda }_{e-ph}$(170 GPa) = 1.46, which is in a reasonable agreement with ${\lambda }_{e-ph}$ reported by Semenok (Materials Today 33, 36 (2020)). Deduced ${\lambda }_{e-ph}$(170 GPa) = 1.70 for Fm-3m-ThH10 is in remarkable agreement with first-principles calculated ${\lambda }_{e-ph}$(174 GPa)=1.75 (Semenok, Materials Today 33, 36 (2020)). Deduced ${\lambda }_{e-ph}$(172 GPa) = 1.90 for Im-3m-YD6 is also in excellent agreement with first-principles calculated ${\lambda }_{e-ph}$(165 GPa)=1.80 (Troyan, Advanced Materials 33, 2006832 (2021)). And finally, we deduced $T$${}_{\theta }$(402 GPa) = 727 K for hydrogen phase III which implies that ${\lambda }_{e-ph}$(402 GPa)<1.7 in this metal.

30 pages, 12 figures

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