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  • Review Article
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Highly crystalline 2D superconductors

Abstract

Recent advances in materials fabrication have enabled the manufacturing of ordered 2D electron systems, such as heterogeneous interfaces, atomic layers grown by molecular beam epitaxy, exfoliated thin flakes and field-effect devices. These 2D electron systems are highly crystalline, and some of them, despite their single-layer thickness, exhibit a sheet resistance more than an order of magnitude lower than that of conventional amorphous or granular thin films. In this Review, we explore recent developments in the field of highly crystalline 2D superconductors and highlight the unprecedented physical properties of these systems. In particular, we explore the quantum metallic state (or possible metallic ground state), the quantum Griffiths phase observed in out-of-plane magnetic fields and the superconducting state maintained in anomalously large in-plane magnetic fields. These phenomena are examined in the context of weakened disorder and/or broken spatial inversion symmetry. We conclude with a discussion of how these unconventional properties make highly crystalline 2D systems promising platforms for the exploration of new quantum physics and high-temperature superconductors.

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Figure 1: Evolution of the thickness of 2D superconductors since 1980.
Figure 2: 2D superconductivity in deposited metallic thin films.
Figure 3: Superconductivity at oxide interfaces.
Figure 4: Superconducting atomic layers of Pb, In and FeSe grown by molecular beam epitaxy.
Figure 5: Atomically thin superconductors based on exfoliated 2D crystals.
Figure 6: Electric-field-induced superconductivity in 2D crystals.
Figure 7: Metallic ground state in ion-gated ZrNCl.
Figure 8: Quantum Griffiths singularity in a superconducting Ga crystalline thin film.
Figure 9: Superconductivity protected by spin–valley locking in ion-gated MoS2 and NbSe2 bilayers.

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Acknowledgements

The authors thank J. T. Ye, Y. Kasahara, Y. Kohama, M. Tokunaga, Y. Nakagawa and M. Onga for experimental cooperation, and Y. Nakamura, Y. Yanase and M. S. Bahramy for theoretical discussions. Y.S. was supported by the Japan Society for the Promotion of Science (JSPS) through a research fellowship for young scientists (Grant-in-Aid for JSPS Research Fellow: no. 15J07681). This work was supported by the Strategic International Collaborative Research Program (SICORP-LEMSUPER) of the Japan Science and Technology Agency, Grant-in-Aid for Specially Promoted Research (no. 25000003) from JSPS and Grant-in-Aid for Scientific Research on Innovative Areas (no. 22103004) from MEXT of Japan.

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Saito, Y., Nojima, T. & Iwasa, Y. Highly crystalline 2D superconductors. Nat Rev Mater 2, 16094 (2017). https://doi.org/10.1038/natrevmats.2016.94

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