Anomalously high value of Coulomb pseudopotential for the H5S2 superconductor

Nonadiabatic superconductivity in a Li-intercalated hexagonal boron nitride bilayer

When considering a Li-intercalated hexagonal boron nitride bilayer (Li-hBN), the vertex corrections of electron-phonon interaction cannot be omitted. This is evidenced by the very high value of the ratio λωD/εF ≈ 0.46, where λ is the electron-phonon coupling constant, ωD is the Debye frequency, and εF represents the Fermi energy. Due to nonadiabatic effects, the phonon-induced superconducting state in Li-hBN is characterized by much lower values of the critical temperature (T LOVC C ∈ {19.1, 15.5, 11.8} K, for μ* ∈ {0.1, 0.14, 0.2}, respectively) than would result from calculations not taking this effect into account (T ME C∈ {31.9, 26.9, 21} K). From the technological point of view, the low value of T C limits the possible applications of Li-hBN. The calculations were carried out under the classic Migdal-Eliashberg formalism (ME) and the Eliashberg theory with lowest-order vertex corrections (LOVC). We show that the vertex corrections of higher order (λ3) lower the value of T LOVC C by a few percent.

From LaH10 to room-temperature superconductors

Thermodynamic parameters of the LaH10 superconductor were an object of our interest. LaH10 is characterised by the highest experimentally observed value of the critical temperature: [Formula: see text] K (pa = 150 GPa) and [Formula: see text] K (pb = 190 GPa). It belongs to the group of superconductors with a strong electron-phonon coupling (λa ~ 2.2 and λb ~ 2.8). We calculated the thermodynamic parameters of this superconductor and found that the values of the order parameter, the thermodynamic critical field, and the specific heat differ significantly from the values predicted by the conventional BCS theory. Due to the specific structure of the Eliashberg function for the hydrogenated compounds, the qualitative analysis suggests that the superconductors of the LaδX1-δH10-type (LaXH-type) structure, where X ∈ {Sc, Y}, would exhibit significantly higher critical temperature than TC obtained for LaH10. In the case of LaScH we came to the following assessments: [Formula: see text] K and [Formula: see text] K, while the results for LaYH were: [Formula: see text] K and [Formula: see text] K.