Things that make TiSe$_2$ superconducting

The unusual charge density wave phase in TiSe$_2$ is accompanied by superconductivity when electron dopants like copper and palladium are intercalated between the layers of this transition-metal dichalcogenide. But when nominally one-electron donors like tantalum and niobium are brought in to replace titanium, Ti$_{1-x}$Ta$_x$Se$_2$ is superconducting and Ti$_{1-x}$Nb$_x$Se$_2$ not. We investigated by angle-resolved photoemission (ARPES) the origins of this behavior by comparing the electronic band structure of pristine TiSe$_2$ and the two doped compounds. We question whether the effect can be attributed to the differences in electron doping only.