Effect of paramagnetic impurities on superconductivity in polyhydrides: s-wave order parameter in Nd-doped LaH10

Abstract Polyhydrides are a novel class of superconducting materials with extremely high critical parameters, which is very promising for applications. On the other hand, complete experimental study of the magnetic phase diagram for the best so far known superconductor, lanthanum decahydride LaH10, encounters a serious complication because of the large upper critical magnetic field HC2(0), exceeding 120–160 T. Partial replacement of La atoms by magnetic Nd atoms results in a decrease of the upper critical field, which makes it attainable for existing pulse magnets. We found that addition of neodymium leads to significant suppression of superconductivity in LaH10: each atomic % of Nd causes decrease in TC by 10–11 K. Using strong pulsed magnetic fields up to 68 T, we constructed the magnetic phase diagram of the ternary (La,Nd)H10 superhydride, which appears to be surprisingly linear with HC2 ∝ |T – TC|. The pronounced suppression of superconductivity in LaH10 by magnetic Nd atoms and the robustness of TC with respect to nonmagnetic impurities (e.g., Y, Al, C) under Anderson’s theorem indicate the isotropic (s‑wave) character of conventional electron-phonon pairing in the synthesized superhydrides.