The search for a Fermi surface in the absence of a conventional Fermi liquid has thus far yielded very few potential candidates. Among promising materials are spin-frustrated Mott insulators near the insulator–metal transition, where theory predicts a Fermi surface associated with neutral low-energy excitations. Here we reveal another route to experimentally realize a Fermi surface in the absence of a Fermi liquid by the experimental study of a Kondo insulator SmB6 positioned close to the insulator–metal transition. We present experimental signatures down to low temperatures (≪1 K) associated with a Fermi surface in the bulk, including a sizeable linear specific heat coefficient, and on the application of a finite magnetic field, bulk magnetic quantum oscillations, finite quantum oscillatory entropy, and substantial enhancement in thermal conductivity well below the charge gap energy scale. Thus, the weight of evidence indicates that despite an extreme instance of Fermi liquid breakdown in Kondo insulating SmB6, a Fermi surface arises from novel itinerant low-energy excitations that couple to magnetic fields, but not weak DC electric fields.
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M.H., Y.-T.H., G.R.-G., J.B., M.K.K., G.H.Z. and S.E.S. acknowledge support from the Royal Society, the Winton Programme for the Physics of Sustainability, EPSRC (studentship and grant number EP/P024947/1) and the European Research Council under the European Unions Seventh Framework Programme (grant number FP/2007-2013)/ERC Grant Agreement number 337425. S.E.S. acknowledges support from the Leverhulme Trust by way of the award of a Philip Leverhulme Prize. Work done by W.H.T. and R.W.H. was funded by NSERC of Canada. Q.R.Z., B.Z. and L.B. acknowledge support from DOE-BES through award DE-SC0002613. X.C. and M.S. acknowledge support from Corpus Christi College, Cambridge and EPSRC. M.C.H. and G.B. would like to acknowledge financial support from the EPSRC, UK through Grants EP/M028771/1 and EP/L014963/1. Work done by S.N. and T.S. was supported by a Grant-in-Aid for Scientific Research on Innovative Areas ‘J-Physics’ (15H05883) and KAKENHI (15H03682) from MEXT. M.K.C. and N.H. acknowledge support from the US Department of Energy, Office of Science, BESMSE Science of 100 Tesla program. S.Y. acknowledges support from Grant-in-Aid for Scientific Research JP16K05447. G.G.L. acknowledges support from EPSRC grant EP/K012894/1. A portion of this work was performed at the National High Magnetic Field Laboratory, which is supported by National Science Foundation Cooperative Agreement No. DMR-1157490, the State of Florida and the DOE. We acknowledge discussions with many colleagues, including M. Aronson, G. Baskaran, P.-Y. Chang, D. Chowdhury, P. Coleman, N. R. Cooper, M. P. M. Dean, O. Erten, J. Flouquet, J. Knolle, N. J. Laurita, P. A. Lee, P. B. Littlewood, V. F. Mitrović, J. E. Moore, T. P. Murphy, M. Norman, C. Pépin, S. Sachdev, T. Senthil, Q. Si, A. M. Tsvelik and C. Varma. We are grateful for the experimental support provided by the NHMFL, Tallahassee, including J. Billings, R. Carrier, E. S. Choi, B. L. Dalton, D. Freeman, L. J. Gordon, M. Hicks, S. A. Maier, J. N. Piotrowski, J. A. Powell and E. Stiers.
The authors declare no competing financial interests.
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Hartstein, M., Toews, W., Hsu, YT. et al. Fermi surface in the absence of a Fermi liquid in the Kondo insulator SmB6. Nat. Phys. 14, 166–172 (2018). https://doi.org/10.1038/nphys4295
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