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

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 $300\mathrm{K}$ and decreases significantly as $1∕T_{1}T\sim T^2$, 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 $350\mathrm{K}$, 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 $c\text{−}f$ hybridization. The temperature dependence of the Knight shift measured in a high magnetic field agrees with the formation of this pseudogap.

Figure 1

${}^{121,123}\mathrm{Sb}\text{−}\mathrm{NQR}$ spectra of (a) CeIrSb and (b) LaIrSb at $4.2\mathrm{K}$.

Figure 2

Temperature dependences of $1∕T_{1}T$ for CeIrSb (closed circles), LaIrSb (closed squares), and CeRhSb [crosses Ref. 4] in zero field. The open squares and circles represent $1∕T_{1}T$ for CeIrSb in magnetic fields of 1 and $105.5\mathrm{kOe}$, respectively. The solid line indicates the calculation by assuming an effective DOS with V-shaped gap structure at $E_F$ as drawn in the bottom left.

Figure 3

$1∕T_1$ normalized by the value at $T_m$ as a function of the normalized temperature $T∕T_m$ for CeIrSb (closed circles) and CeRhSb [crosses Ref. 4] in zero field. The open squares and circles represent the data for CeIrSb at $H=1$ and $105.5\mathrm{kOe}$, respectively. The solid curve gives the reproduction by the best-fitted V-shaped gap.

Figure 4

Temperature dependence of ${}^{121}\mathrm{Sb}$ Knight shift for CeIrSb. Inset shows the field-swept ${}^{121}\mathrm{Sb}\text{−}\mathrm{NMR}$ spectrum at $10\mathrm{K}$ measured with the fixed rf frequency of $107.3\mathrm{MHz}$.