We report on a comparative study of the narrow-band semimetals FeSb2 and its structural homologue RuSb2 by means of Sb-121,Sb-123 nuclear quadrupole (NQR) and nuclear magnetic resonance (NMR) spectroscopy. From NQR for both compounds two temperature regimes could be identified by use of (123)(1/T (1)) measurements. Above 40 K a conventional activated behavior (with Delta/k (B) a parts per thousand... 400 K for FeSb2) dominates in (123)(1/T (1)), whereas below 40 K in both systems an unconventional (123)(1/T (1)) behavior with a smooth maximum at around 10 K is observed. To analyze this behavior, we propose the presence of T-dependent in-gap states forming a narrow energy level of localized spins with S = A1/2 near the bottom of the conduction band. These states might have originated from an inherent Sb-deficiency in both compounds. This model enables us to fit the (123)(1/T (1)) data in the entire investigated temperature range (2-200 K) for FeSb2. Ab initio band structure calculations reveal more than a factor of two larger Delta value for RuSb2 as compared with FeSb2. This results in dissimilar behavior of (123)(1/T (1)) in FeSb2 and RuSb2 above 40 K evidencing the inefficiency of thermal activation of electrons over the large energy gap at T a parts per thousand currency sign 300 K in RuSb2 and dominating of quadrupole relaxation channel in RuSb2 in this temperature range caused by phonon relaxation involving two-phonon (Raman) scattering. In addition, extra wide range field-sweep NMR measurements are performed at various temperatures on FeSb2 and RuSb2. The complex broad spectra could be modeled and from the shift of the Sb-121 central transition the 3d component of the shift K (3d) (T) could be extracted.
Applied Magnetic Resonance, 2014, Vol 45, Issue 11, p. 1237-1252