Magnetic resonance on correlated semimetals: The case of U ₂Ru₂Sn, CeRu₄Sn₆ and FeSb ₂

A comparative magnetic resonance study on correlated semiconductors is presented. Nuclear spin-lattice relaxation measurements provide an excellent method to gain local information about the gap formation in this new class of materials. In contrast to U2Ru2Sn (Δ/k_B 230 K) in CeRu ₄Sn₆ the gap is slightly reduced (Δ/k_B 200 K) and correlations form out of a residual density of states in the gap. For FeSb₂ there is revived interest after classifying this system as the second Fe containing Kondo insulator beside FeSi. Surprisingly, FeSb ₂ shows a colossal Seebeck coefficient at low temperatures. Using ^121,123Sb nuclei as a local probe, our NMR/NQR investigations strongly support the gap scenario. The spin-lattice relaxation rate ¹²³(1/T₁) in the entire investigated temperature range 2-200 K is perfectly fitted with rectangle DOS model with narrow impurity in-gap band. These in-gap states might originate the high thermopower observed. The obtained gap value is (Δ/k_B 473 K).