Crystal-electric-field effects and quadrupole fluctuations in ${\text{Ce}}_3{\text{Au}}_3{\text{Sb}}_4$ detected by Sb NQR

We report ${}^{121,123}\text{S}\text{b}$ nuclear quadrupole resonance (NQR) studies on single crystals of the narrow-gap semiconductor ${\text{Ce}}_3{\text{Au}}_3{\text{Sb}}_4$. Five NQR lines from the two Sb isotopes were successfully identified. The temperature dependence of the nuclear quadrupole frequency $\left({\nu }_Q\right)$, as well as the static magnetic susceptibility $\left(\chi \right)$, is well explained by crystal-electric-field effects. The nuclear spin-lattice relaxation rates $\left(T_1^{-1}\right)$ of both ${}^{121}\text{S}\text{b}$ and ${}^{123}\text{S}\text{b}$ increase rapidly with decreasing temperature. The ratio of $T_1^{-1}$ for the two Sb isotopes is constant at high temperature but it decreases at low temperatures, indicating the role of quadrupole fluctuations of the Ce ions. The possible origin of the large specific heat at low temperatures is discussed basing on our results.

Figure 1

(Color online) Magnetic susceptibility $\chi$ measured at 0.1 T as a function of temperature. Magnetization $M$ at 2 K also is shown in the inset. Dotted lines are results from CEF calculations, and their powder averages are shown as thick solid lines. The CEF level scheme is drawn with corresponding eigenstates, in which $a=0.7561$, $b=0.6844$, ${\Delta }_1\sim 25\text{K}$, and ${\Delta }_2\sim 150\text{K}$.

Figure 2

(Color online) (a) Sb-NQR spectra obtained at $T=240\text{K}$ in zero field. Spectra of both Sb isotopes are equally spaced as expected in uniaxial symmetry. At this temperature, ${\nu }_Q$ is 27.50 MHz and 45.32 MHz for ${}^{121}\text{S}\text{b}$ and ${}^{123}\text{S}\text{b}$, respectively. (b) ${\nu }_Q$ of ${}^{123}\text{S}\text{b}$ as a function of temperature. Results for ${\text{La}}_3{\text{Au}}_3{\text{Sb}}_4$ which were scaled down by $\sim 2\mathrm{\%}$ for comparison, are shown together with the expected phonon contribution ${\nu }_Q^{\text{phonon}}$. Inset: difference between ${\text{Ce}}_3{\text{Au}}_3{\text{Sb}}_4$ data and ${\nu }_Q^{\text{phonon}}$. This difference is well explained by the expectation value of the quadrupole moment $\left|⟨O_2^0⟩\right|$ which was scaled to data.

Figure 3

(Color online) (a) $T_1^{-1}$ as a function of temperature for both ${}^{121}\text{S}\text{b}$ and ${}^{123}\text{S}\text{b}$. Both Sb nuclei show the same $1/T$ dependence in the high-temperature region. At low temperatures, below ${\Delta }_2\sim 150\text{K}$, the temperature dependence of ${{}^{123}T}_1^{-1}$ becomes distinguished from that of ${{}^{121}T}_1^{-1}$. Inset: ${{}^{123}T}_1^{-1}$ measured at $2{\nu }_Q$ transition in ${\text{La}}_3{\text{Au}}_3{\text{Sb}}_4$ decreases rapidly with lowering temperature. (b) Temperature-dependent ratio between ${{}^{121}T}_1^{-1}$ and ${{}^{123}T}_1^{-1}$. This ratio deviates from the value expected for magnetic fluctuations, revealing a significant contribution from quadrupole fluctuations at low temperatures.