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

Abstract

A milestone experimental discovery of superconductivity above 200 K in highly compressed sulfur 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, and 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 analyzed temperature-dependent resistance, R(T), in polyhydrides of thorium, hexadeuteride of yttrium, and in hydrogen phase III and deduced the Debye temperature, Tθ, and the electron–phonon coupling constant, λe−ph, for these conductors. We found that I-43d-Th4H15 exhibits λe−ph = 0.82–0.99, which is in very good agreement with the experimental value of λe−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 λe−ph(170 GPa) = 1.46 ± 0.01, which is in reasonable agreement with λe−ph computed by first-principles calculations [Semenok et al. Mater. Today 33, 36 (2020)]. Deduced λe−ph(170 GPa) = 1.70 ± 0.04 for Fm-3m-ThH10 is in remarkable agreement with first-principles calculated λe−ph(174 GPa) = 1.75 [Semenok et al., Mater. Today 33, 36 (2020)]. Deduced λe−ph(172 GPa) = 1.90 ± 0.02 for Im-3m-YD6 is also in excellent agreement with first-principles calculated λe−ph(165 GPa) = 1.80 [Troyan et al., Adv. Mater. 33, 2006832 (2021)]. Finally, we deduced Tθ(402 GPa) = 727 ± 6 K for hydrogen phase III, which implies that λe−ph(402 GPa) ≤ 1.7 in this metal.