  2H-NbSe2 is the prototype and most frequently studied of the well-known
transition metal dichalcogenide (TMDC) superconductors. Widely acknowledged to
be 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 Tcs 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 increasing 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, 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 materials parameters that influence Tc. To
illustrate its character, the superconducting phase diagram for CuxNbSe2 is
contrasted to those of FexNbSe2 and NbSe2-xSx.
