Spin-Orbit Enhanced Superconductivity in Bernal Bilayer Graphene

Creators: Zhang, Yiran; Polski, Robert; Thomson, Alex; Lantagne-Hurtubise, Étienne; Lewandowski, Cyprian; Zhou, Haoxin; Watanabe, Kenji; Taniguchi, Takashi; Alicea, Jason; Nadj-Perge, Stevan

Abstract

In the presence of a large perpendicular electric field, Bernal-stacked bilayer graphene (BLG) features several broken-symmetry metallic phases as well as magnetic-field-induced superconductivity. The superconducting state is quite fragile, however, appearing only in a narrow window of density and with a maximum critical temperature T꜀ ≈ 30~mK. Here, we show that placing monolayer tungsten diselenide (WSe₂) on BLG promotes Cooper pairing to an extraordinary degree: superconductivity appears at zero magnetic field, exhibits an order of magnitude enhancement in T꜀, and occurs over a density range that is wider by a factor of eight. By mapping quantum oscillations in BLG-WSe₂ as a function of electric field and doping, we establish that superconductivity emerges throughout a region whose normal state is polarized, with two out of four spin-valley flavours predominantly populated. In-plane magnetic field measurements further reveal a striking dependence of the critical field on doping, with the Chandrasekhar-Clogston (Pauli) limit roughly obeyed on one end of the superconducting dome yet sharply violated on the other. Moreover, the superconductivity arises only for perpendicular electric fields that push BLG hole wavefunctions towards WSe₂ -- suggesting that proximity-induced (Ising) spin-orbit coupling plays a key role in enhancing the pairing. Our results pave the way for engineering robust, highly tunable, and ultra-clean graphene-based superconductors.

Additional Information

Attribution 4.0 International (CC BY 4.0). We thank Andrea Young and Allan Macdonald for fruitful discussions. This work has been primarily supported by NSF-CAREER award (DMR-1753306), and Office of Naval Research (grant no. N142112635), and Army Research Office under Grant Award W911NF17-1-0323. Nanofabrication efforts have been in part supported by Department of Energy DOE-QIS program (DE-SC0019166). S.N-P. acknowledges support from the Sloan Foundation (grant no. FG-2020-13716). J.A. and S.N.-P. also acknowledge support of the Institute for Quantum Information and Matter, an NSF Physics Frontiers Center with support of the Gordon and Betty Moore Foundation through Grant GBMF1250. C.L. and E.L.H. acknowledge support from the Gordon and Betty Moore Foundation's EPiQS Initiative, grant GBMF8682. Author Contribution: Y.Z. and S.N.-P. designed the experiment. Y.Z., R.P. and H.Z. performed the measurements, fabricated the devices, and analyzed the data. A.T., E.L.-H. and C.L. developed theoretical models and performed calculations supervised by J.A. K.W. and T.T. provided hBN crystals. S.N.-P. supervised the project. Y.Z., A.T., E.L.-H., C.L., H.Z., R.P., J.A., and S.N.-P. wrote the manuscript with the input of other authors. Data availability: The data supporting the findings of this study are available from the corresponding authors on reasonable request. Code availability: All code used in modeling in this study is available from the corresponding authors on reasonable request. The authors declare no competing interests.

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