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 (Tc) is 7.3 K, a Tc that is substantially higher than those seen for the majority of TMDCs, where Tc values between 2 and 4 K are the norm. Here we report the intercalation of Cu into 2H-NbSe2 to make CuxNbSe2. As is typically found when chemically altering an optimal superconductor, Tc decreases with an increase in x, but the way that Tc 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 Tc 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 Tc. To illustrate its character, the superconducting phase diagram for CuxNbSe2 is contrasted with those of FexNbSe2 and NbSe2–xSx.