Title: Using controlled disorder to probe the interplay between charge order and superconductivity in NbSe2,
Abstract: The interplay between superconductivity and charge density waves (CDW) in
$H$-NbSe2 is not fully understood despite decades of study. Artificially
introduced disorder can tip the delicate balance between two competing forms of
long-range order, and reveal the underlying interactions that give rise to
them. Here we introduce disorders by electron irradiation and measure in-plane
resistivity, Hall resistivity, X-ray scattering, and London penetration depth.
With increasing disorder, $T_{\textrm{c}}$ varies nonmonotonically, whereas
$T_{\textrm{CDW}}$ monotonically decreases and becomes unresolvable above a
critical irradiation dose where $T_{\textrm{c}}$ drops sharply. Our results
imply that 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.