S-Shaped Suppression of the Superconducting Transition Temperature in Cu-Intercalated NbSe₂

2H-NbSe₂ is the prototype and most frequently studied of the well-known transition metal dichalcogenide (TMDC) superconductors. As 2H-NbSe₂ is widely acknowledged as a conventional superconductor, its transition temperature to the superconducting state (T_c) is 7.3 K, a T_c that is substantially higher than those seen for the majority of TMDCs, where T_c values between 2 and 4 K are the norm. Here we report the intercalation of Cu into 2H-NbSe₂ to make Cu_xNbSe₂. As is typically found when chemically altering an optimal superconductor, T_c decreases with an increase in x, but the way that T_c is suppressed in this case is unusual: an S-shaped character is observed, with an inflection point near x = 0.03 and, at higher x values, a leveling off of the T_c near 3 K, down to the usual value for a layered TMDC. Electronic characterization reveals corresponding S-like behavior for many of the parameters of the materials that influence T_c.