Distinct Roles of Hydrogen in Superconducting and Ferromagnetic Phases of CoZr$_{2}$H$_{x}$

Abstract

Hydrogenation offers a versatile route to tuning the physical properties of intermetallic compounds. In this study, we synthesized CoZr$_{2}$H$_{x}$ with different hydrogen contents and found that hydrogen is incorporated in two distinct concentration regimes separated by a wide composition gap: a low-concentration hydrogenated superconducting phase ($x$ = 0-0.054) and a high-concentration hydrogenated ferromagnetic phase ($x$ = 2.786). Hydrogen plays fundamentally different roles in the two concentration regimes. In the high hydrogen concentration phase, the Zr-H interactions substantially modify the metallic bands crossing the Fermi level, leading to the emergence of ferromagnetism. In contrast, in the low hydrogen concentration phase, hydrogen behaves as a nonmagnetic impurity without altering the electronic band structure. Despite the nearly identical Debye temperatures across the low-concentration series, the superconducting transition temperature ($T_{\mathrm{c}}$) is progressively suppressed with increasing hydrogen content.The observed $T_{\mathrm{c}}$ suppression is quantitatively described by the Abrikosov-Gor'kov pair-breaking theory, indicating that the superconducting gap of CoZr$_{2}$ is anisotropic or multigap rather than a fully isotropic $s$-wave symmetry.