Angle-controlled ^11 B-NMR studies of Kondo insulator SmB _6

In SmB _6 , the prototypical material of valence fluctuations and Kondo semiconductors, there remain several unsolved problems in the bulk properties at low temperatures. We have studied a single crystalline sample of SmB _6 by ^11 B-nuclear magnetic resonance (NMR) measurements with the precise control of the field direction in the ( 1̅ 1 0 ) plane. The temperature dependence of the nuclear quadrupole frequency ν _Q and the full width at half maximum (FWHM), estimated from obtained spectra indicates that there are two distinct low-temperature regions in SmB _6 : the first is the region down to approximately 10 K, where a decrease in the FWHM and an increase in ν _Q with lowering temperature are understood in the framework of the evolution of the hybridization effects between 4 f and conduction electrons accompanied with an intermediate valence state. Below 10 K, the second region, ν _Q and the FWHM undergo a sharp change in their temperature dependence. Through both electrical and magnetic interactions, the present results suggest that the low-temperature state below 10 K is dominated by additional physical properties other than the hybridization and the valence fluctuations.