Energy gap formation in the valence fluctuating compound CeIrSb probed by Sb NMR and NQR

Abstract

Sb NMR and NQR studies have revealed the formation of a pseudogap at the Fermi level in the density of states in a valence fluctuating compound CeIrSb. The nuclear spin-lattice relaxation rate divided by temperature, $1∕{T}_{1}T$, has a maximum around $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and decreases significantly as $1∕{T}_{1}T\ensuremath{\sim}{T}^{2}$, followed by a $1∕{T}_{1}T=\mathrm{const}$ relation at low temperature. This temperature dependence of $1∕{T}_{1}T$ is well reproduced by assuming a V-shaped energy gap with a residual density of states at the Fermi level. The size of energy gap for CeIrSb is estimated to be about $350\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, which is by 1 order of magnitude larger than those for the isostructural Kondo semiconductors CeRhSb and CeNiSn. Despite the large difference in the size of energy gap, CeIrSb, CeRhSb, and CeNiSn are indicated to be classified into the same group revealing a V-shaped gap due to $c\text{\ensuremath{-}}f$ hybridization. The temperature dependence of the Knight shift measured in a high magnetic field agrees with the formation of this pseudogap.