NMR study on mixed valence insulator SmB₆

Owing to strong electronic correlations, SmB₆ is a Kondo insulator that has been studied for several decades [1] and has more recently been proposed to be a topological Kondo insulator [2]. Recent magnetic torque measurements on pristine floating-zone grown single crystals of SmB₆ found bulk quantum oscillations despite the bulk electrical insulating behavior [3], suggesting the surprising existence of a bulk Fermi surface in this unconventional insulator. We have conducted NMR measurements on the same high-quality floating zone grown single crystals of SmB₆ below 1K and for magnetic fields up to 18T for two inequivalent boron sites B1 and B2. For all magnetic fields below 18T, the Knight shift remains constant, showing behavior akin to a bulk metal, despite the bulk electric insulating character of the material. The spin-lattice relaxation rate 1/T₁ indicates the existence of localized fermion density of states at low fields. The anisotropy of the spin-lattice relaxation rate is suppressed with the increase of the magnetic field. The Korringa constant indicates the presence of ferromagnetic correlations up to 18T. We will discuss the implication of these results based on an in-gap state model [4] and the possibility of the existence of a neutral bulk Fermi surface. A portion of this work was performed at the National High Magnetic Field Laboratory, which is supported by the National Science Foundation Cooperative Agreement No. DMR-1644779 and the State of Florida. Work at Brown was supported in part by the National Science Foundation grant No. DMR-1905532.

[1] Cohen, R. L., et al. PRL, 24.8 (1970): 383.

[2] Dzero, Maxim, et al. PRL 104.10 (2010): 106408.

[3] Tan, B. S., et al. Science 349.6245 (2015): 287-290.

[4] Caldwell T, et al. PRB, 2007, 75(7): 075106.