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We report that 1T-TiSe2, an archetypical layered transition metaldichalcogenide, becomes superconducting when Ta is substituted for Ti but notwhen Nb is substituted for Ti. This is unexpected because Nb and Ta should bechemically equivalent electron donors. Superconductivity emerges near x = 0.02for Ti1-xTaxSe2, while for Ti1-xNbxSe2, no superconducting transitions areobserved above 0.4 K. The equivalent chemical nature of the dopants isconfirmed by X-ray photoelectron spectroscopy. ARPES and Raman scatteringstudies show similarities and differences between the two systems, but thefundamental reasons why the Nb and Ta dopants yield such different behavior areunknown. We present a comparison of the electronic phase diagrams of manyelectron-doped 1T-TiSe2 systems, showing that they behave quite differently,which may have broad implications in the search for new superconductors. Wepropose that superconducting Ti0.8Ta0.2Se2 will be suitable for devices andother studies based on exfoliated crystal flakes.

Differences in chemical doping matter - Superconductivity in Ti1-xTaxSe2 but not in Ti1-xNbxSe2