NMR Study of a Valence Fluctuating Compound SmB₆

High-field suppression of in-gap states in the Kondo insulatorSmB6

“…We obtained a quadrupole splitting of Q = 570͑3͒ kHz independent of temperature, in agreement with previous works. 17,24 However, our NMR spectra are markedly different from the early low field NMR work 17 in three aspects. First, the spectra showed a wellresolved central peak anisotropy where the ͉ ±1/2͘ ↔ ͉ ϯ 1/2͘ transition of the B2 sites exhibits a measureable secondorder shift.…”

“…The most important difference is that the Knight shift K derived from the spectra shows a weak but marked temperature dependence that is not observed in Ref. 17. Apart from a difference in sample quality, the application of high fields enables us to make very accurate measurements of small Knight shifts.…”

“…In SmB 6 , the transferred hyperfine coupling A i is dominated by the Sm ion 4f multiplet states. 17,33 The measured Knight shifts are shown in Fig. 2.…”

“…Inherent to hexaboride crystals is a boron deficiency, which plays a significant role when the cation is divalent, e.g., for the semimetal EuB 6 , the insulator CaB 6 , 23 and the semiconductor SmB 6 . Previous low magnetic field NMR spin-lattice relaxation measurements 17,24 suggest that the metallic Korringa-type mechanism, with allowance for a gap in the density of states, satisfactorily describes the data above 20 K. At lower temperatures, the in-gap states were not detected but it was suggested that the relaxation mechanism is due to the Wignercrystal-like formation of localized states related to conductivity. 17 The nature of the in-gap bound states, whether intrinsic or extrinsic, is still controversial.…”

“…15 In this range, the magnetic susceptibility is only weakly temperature-dependent. 16,17 For 6 Ͻ T Ͻ 50 K, the conductivity is activated, the Hall coefficient is negative, and the susceptibility decreases slightly before reaching a roughly constant value below 20 K. 17 Below 6 K, the temperature dependence of is smaller, and below 2 -3 K, is almost constant. 7 The properties below 6 K are believed to be dominated by in-gap bound states.…”