Superconducting Parameters of Covalently Bounded Superhydride BiH2

The discovery of superconductivity in compressed H3S sparked experimental and theoretical studies of compressed hydrides, in which room-temperature superconductivity is expected to exist. The search is conducted for three classes of hydrides. The first class is covalently bonded hydrides (which is represented by H3S, H3P, and recently discovered BiH2 [42]. The second class is clathrate-type hydrides, among which superhydrides LaH10, YH6, CaH6, and a number of others have been discovered. The third class is molecular hydrides, represented by BaH12 and BiH4. One of the most prominent achievements of the study by Guo et al. [42] on BiH2 is that the self-field critical current density J_c( sf,T ) was measured over a wide temperature range. Thorough analysis of measured J_c( sf,T ) showed that the BiH2 exhibits two-band s-wave superconductivity (the amplitudes of the ground state superconducting gaps for both bands Δ _L( 0 ) and Δ _S( 0 ) were obtained, as well as the London penetration depth λ ( 0 ) ). In this paper, I have analyzed J_c( sf,T ) experimental data measured in [42] (apart the J_c( sf,T ) ) in covalently bounded hydride BiH2. As a result, the Debye Θ _D and Fermi T_F temperatures, as well as the electron–phonon coupling constant λ _e - ph , were determined. The latter differs significantly from that obtained using first-principles calculations [42]. The difference can be explained by the presence of the dispersed BiH4 phase in the BiH2 matrix, while first-principles calculations were performed assuming an ideal/defect-free single-phase BiH2 lattice. The analysis of the upper critical field, B_c2( T ) , confirmed two-band superconductivity of the BiH2 samples. The revealed level of nonadiabaticity Θ _D / Θ _D T_F . -0em T_F = 0.057 in BiH2 is typical of cuprates, iron-based superconductors, MgB2, and other superhydrides.