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

Milestone experimental discovery of superconductivity above 200 K in highly-compressed sulphur hydride by Drozdov et al (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 et al (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 analysed temperature-dependent resistance, R(T), in polyhydrides of thorium, hexadeuteride of yttrium and in hydrogen phase III and deduced the Debye temperature, T, and the electron-phonon coupling constant, e-ph, for these conductors. We found that I-43dTh4H15 exhibits e-ph = 0.82-0.99, which is in a very good agreement with experimental value of e-ph = 0.84 deduced from heat capacity measurements (Miller et al, Phys. Rev. B 14, 2795 (1976)). For P63/mmc-ThH9 (P = 170 GPa) we deduced e-ph(170 GPa) = 1.46 ± 0.01, which is in a reasonable agreement with e-ph computed by first-principles calculations ( Semenok et al. Materials Today 33, 36 (2020)). Deduced e-ph(170 GPa) = 1.70±0.04 for Fm-3m-ThH10 2 is in remarkable agreement with first-principles calculated e-ph(174 GPa) = 1.75 (Semenok et al, Materials Today 33, 36 (2020)). Deduced e-ph(172 GPa) = 1.90 ± 0.02 for Im-3m-YD6 is also in excellent agreement with first-principles calculated e-ph(165 GPa) = 1.80 (Troyan et al, Advanced Materials 33, 2006832 (2021)). And finally, we deduced T(402 GPa) = 727 ± 6 K for hydrogen phase III which implies that e-ph(402 GPa) ≤ 1.7 in this metal.