Differences in Chemical Doping Matter: Superconductivity in Ti_1–xTa_xSe₂ but Not in Ti_1–xNb_xSe₂

We report that 1T-TiSe2, an archetypical layered transition metal dichalcogenide, becomes superconducting when Ta is substituted for Ti but not when Nb is substituted for Ti. This is unexpected because Nb and Ta should be chemically equivalent electron donors. Superconductivity emerges near x = 0.02 for Ti1–xTaxSe2, while, for Ti1–xNbxSe2, no superconducting transitions are observed above 0.4 K. The equivalent chemical nature of the dopants is confirmed by X-ray photoelectron spectroscopy. ARPES and Raman scattering studies show similarities and differences between the two systems, but the fundamental reasons why the Nb and Ta dopants yield such different behavior are unknown. We present a comparison of the electronic phase diagrams of many electron-doped 1T-TiSe2 systems, showing that they behave quite differently, which may have broad implications in the search for new superconductors. We propose that superconducting Ti0.8Ta0.2Se2 will be suitable for devices and other studies based on exfoliated crystal flakes.