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Tailored Ising superconductivity in intercalated bulk NbSe2

Abstract

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.

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Fig. 1: Schematic for tuning the interlayer interaction via ionic intercalation and evidence for successful intercalation of organic cations into NbSe2 crystal.
Fig. 2: Electronic structure of intercalated NbSe2 compared with unintercalated bulk sample and monolayer film, showing layer-decoupled electronic structure and electron doping for the intercalated sample.
Fig. 3: Tailored Ising superconductivity in the intercalated sample.
Fig. 4: Sample stability and experimental results on intercalated few-layer flake samples.

Data availability

Source data are available with this paper. Other data that support the findings of this study are available from the corresponding author upon reasonable request.

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Acknowledgements

The work is mainly supported by the Ministry of Science and Technology of China (Grant Nos. 2020YFA0308800, 2021YFA1400100, 2016YFA0301004) and National Natural Science Foundation of China (Grant No. 11725418). Y.W. is supported by the Fundamental Research Funds for the Central Universities (buctrc202212) and National Natural Science Foundation of China (Grant Nos. 21975140, 51991343). Y.X. is supported by the National Natural Science Foundation of China (Grant Nos. 12025405, 11874035) and the Ministry of Science and Technology of China (Grant Nos. 2018YFA0307100, 2018YFA0305603). S.J. and P.Y. are supported by the Ministry of Science and Technology of China (Grant No. 2021YFE0107900). P.Y. is also supported by the NSFC (Grant Nos. 52025024 and 51872155) and the Beijing Nature Science Foundation (Grant No. Z200007). Y.X., W.D. and P.Y. are supported by the Basic Science Center Program of NSFC (Grant No. 51788104).

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Contributions

S.Z. and P.Y. conceived the research project. Haoxiong Zhang, A.R., C.G., L.L., R.F. and Y.W. grew, intercalated and characterized the samples. A.R., Haoxiong Zhang, Kenan Zhang, C.B., Hongyun Zhang and W.Y. performed the ARPES measurements and analysed the ARPES data. Haoxiong Zhang, S.S. and P.Y. performed the transport measurements. Kun Zhao., S.J., Xi Chen and Q.-K.X. grew the monolayer NbSe2 MBE film for ARPES measurements. Haoxiong Zhang, Xin Cong and P.T. performed the Raman measurements and analysed the Raman data. Z.L., S.X., Y.X. and W.D. performed the first-principles calculations. Haoxiong Zhang, P.Y. and S.Z. wrote the manuscript, and all authors commented on the manuscript.

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Correspondence to Shuyun Zhou.

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Supplementary Figs. 1–10, Discussion and Tables 1–2.

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Zhang, H., Rousuli, A., Zhang, K. et al. Tailored Ising superconductivity in intercalated bulk NbSe2. Nat. Phys. 18, 1425–1430 (2022). https://doi.org/10.1038/s41567-022-01778-7

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