Soft superconductivity in covalent bismuth dihydride BiH₂ under extreme conditions

Strong magnetic fields provide a unique environment for investigating the fundamental properties of superconducting materials, especially for hydride superconductors with large upper critical fields. Following this idea, we have investigated the effect of pulsed magnetic fields on covalent bismuth dihydride (BiH₂), successfully synthesized under pressure up to 211 GPa. The electrical resistance measurements indicate that the superconducting phase P2₁/m-BiH₂ exhibits the highest superconducting critical temperature (Tc) of 70 K among MH₂-type hydride apart from H₂S. The electrical transport experiments under both pulsed (up to 50 T) and steady magnetic fields (up to 16 T) for P2₁/m- and $C2/m$-BiH₂ indicate that the upper critical fields μ₀ Hc2(0) = 12--16 T are unusually low, much lower than that of clathrate-like metal polyhydrides with similar Tc. This is due to the unexpectedly high Fermi velocity in BiH₂, about 1.1 × 10⁶ m/s, which allows to classify BiH₂ as a 'soft' molecular superconducting hydride with relatively weak vortex pinning. Measurements of the current-voltage characteristics in the pulsed mode make it possible to experimentally establish the temperature dependence of the critical current density (the maximum Jc(0) = 10 kA/mm²), which indicates the presence of two s-wave superconducting gaps in BiH₂ at 172--176 GPa: ΔL(0) = 6.9 ± 1.2 meV and ΔS(0) ~ 1.5 meV.