Charge-neutral electronic excitations in quantum insulators | Nature

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Experiments on quantum materials have uncovered many interesting quantum phases ranging from superconductivity to a variety of topological quantum matter including the recently observed fractional quantum anomalous Hall insulators. The findings have come in parallel with the development of approaches to probe the rich excitations inherent in such systems. In contrast to observing electrically charged excitations, the detection of charge-neutral electronic excitations in condensed matter remains difficult, although they are essential to understanding a large class of strongly correlated phases. Low-energy neutral excitations are especially important in characterizing unconventional phases featuring electron fractionalization, such as quantum spin liquids, spin ices and insulators with neutral Fermi surfaces. In this Perspective, we discuss searches for neutral fermionic, bosonic or anyonic excitations in unconventional insulators, highlighting theoretical and experimental progress in probing excitonic insulators, new quantum spin liquid candidates and emergent correlated insulators based on two-dimensional layered crystals and moiré materials. We outline the promises and challenges in probing and using quantum insulators, and discuss exciting new opportunities for future advancements offered by ideas rooted in next-generation quantum materials, devices and experimental schemes.

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S.W. acknowledges support from Gordon and Betty Moore Foundation’s EPiQS Initiative grant no. GBMF11946, AFOSR through a Young Investigator Award (grant no. FA9550-23-1-0140), ONR through a Young Investigator Award (grant no. N00014-21-1-2804), NSF through a CAREER award (grant no. DMR-1942942) and the Sloan Foundation. N.P.O., R.J.C., L.M.S. and S.W. acknowledge support from the Materials Research Science and Engineering Center programme of the NSF (grant no. DMR-2011750). N.P.O. acknowledges support from the United States Department of Energy (grant no. DE-SC0017863) (which supported the κxy experiments on α-RuCl3) and the Gordon and Betty Moore Foundation through grant no. GBMF9466. L.M.S. acknowledges support from the Gordon and Betty Moore Foundation through grant no. GBMF9064 and the David and Lucile Packard Foundation. S.W. and L.M.S. acknowledge support from the Eric and Wendy Schmidt Transformative Technology Fund at Princeton. The research on forefront electronic materials in the laboratory of R.J.C. at Princeton is supported by the Gordon and Betty Moore Foundation through grant GBMF-9066 and the US DOE division of Basic Energy Sciences (grant no. DE-FG02-98R45706). I.S. acknowledges support from the Deutsche Forschungsgemeinschaft through research grant project numbers 542614019 and 518372354. R.M. acknowledges funding by the Deutsche Forschungsgemeinschaft under grants SFB 1143 (project-id 247310070) and the cluster of excellence ct.qmat (grant no. EXC 2147, project-id 390858490).

Department of Physics, Princeton University, Princeton, NJ, USA

Sanfeng Wu & N. P. Ong

Department of Chemistry, Princeton University, Princeton, NJ, USA

Leslie M. Schoop & Robert J. Cava

Institute for Theoretical Physics, University of Leipzig, Leipzig, Germany

Inti Sodemann

Max-Planck Institute for the Physics of Complex Systems, Dresden, Germany

Roderich Moessner

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S.W. and N.P.O. initiated the work. I.S. and R.M. wrote the theory parts. S.W., N.P.O., R.J.C. and L.M.S. wrote the experimental parts. All authors discussed and contributed to the overall writing and revisions of this work.

Correspondence to Sanfeng Wu or N. P. Ong.

The authors declare no competing interests.

Nature thanks the anonymous reviewers for their contribution to the peer review of this work.

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Wu, S., Schoop, L.M., Sodemann, I. et al. Charge-neutral electronic excitations in quantum insulators. Nature (2024). https://doi.org/10.1038/s41586-024-08091-8

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Received: 29 March 2021

Accepted: 20 September 2024

Published: 13 November 2024

DOI: https://doi.org/10.1038/s41586-024-08091-8

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