Using controlled disorder to probe the interplay between charge order and superconductivity in NbSe₂

The interplay between superconductivity and charge-density wave (CDW) in 2H-NbSe₂ is not fully understood despite decades of study. Artificially introduced disorder can tip the delicate balance between two competing long-range orders, and reveal the underlying interactions that give rise to them. Here we introduce disorder by electron irradiation and measure in-plane resistivity, Hall resistivity, X-ray scattering, and London penetration depth. With increasing disorder, the superconducting transition temperature, T_c, varies non-monotonically, whereas the CDW transition temperature, T_CDW, monotonically decreases and becomes unresolvable above a critical irradiation dose where T_c drops sharply. Our results imply that the CDW order initially competes with superconductivity, but eventually assists it. We argue that at the transition where the long-range CDW order disappears, the cooperation with superconductivity is dramatically suppressed. X-ray scattering and Hall resistivity measurements reveal that the short-range CDW survives above the transition. Superconductivity persists to much higher dose levels, consistent with fully gapped superconductivity and moderate interband pairing.