Tailored Ising superconductivity in intercalated bulk NbSe2

Reducing the dimensionality of layered materials can result in properties distinct from their bulk crystals1,2,3. However, the emergent properties in atomically thin samples, in particular in metallic monolayer flakes, are often obtained at the expense of other important properties. For example, while Ising superconductivity—where the pairing of electrons with opposite out-of-plane spins from K and K′ valleys leads to an in-plane upper critical field exceeding the Pauli limit—does not occur in bulk NbSe2, it was observed in two-dimensional monolayer flakes4. However, the critical temperature was reduced as compared to bulk crystals4,5,6,7,8,9,10,11,12,13. Here we take a different route to control the superconducting properties of NbSe2 by intercalating bulk crystals with cations from ionic liquids. This produces Ising superconductivity with a similar critical temperature to the non-intercalated bulk and is more stable than in a monolayer flake. Our angle-resolved photoemission spectroscopy measurements reveal the effectively two-dimensional electronic structure, and a comparison of the experimental electronic structures between intercalated bulk NbSe2 and monolayer NbSe2 film reveals that the intercalant induces electron doping. This suggests ionic liquid cation intercalation is an effective technique for controlling both the dimensionality and the carrier concentration, allowing tailored properties exceeding both bulk crystals and monolayer samples.