The unusual suppression of superconducting transition temperature in double-doping 2H-NbSe2

2H-NbSe2 is one of the most widely researched transition metal dichalcogenide (TMD) superconductors, which undergoes charge-density wave (CDW) transition at T-CDW about 33 K and superconducting transition at T-c of 7.3 K. To explore the relation between its superconductivity and Fermi surface nesting, we have combined S substitution with Cu intercalation in 2H-NbSe2 to make Cu(x)NbSe(2-y)Sy (0 <= x = y <= 0.1). Upon systematic substitution of S and intercalation of Cu ions into 2H-NbSe2, we found that when the Cu and S contents (x = y >= 0.06) increase, the T-c decreases in Cu(x)NbSe(2-y)Sy. At higher x and y values, T-c keeps a constant value near 2 K, which is not commonly observed for a layered TMD. For comparison, we found that the simultaneous substitution of Nb by Cu and Se by S in Cu(x)Nb(1-x)Se(2-y)Sy (0 <= x = y <= 0.06) lowered the T-c substantially faster. We constructed superconducting phase diagrams for our double-doping compounds in contrast with the related single-ion doping systems.