Connection between Charge Fluctuations and the Coherent Temperature in the Heavy-Fermion System ${\mathrm{SmOs}}_4{\mathrm{Sb}}_{12}$: A ${}^{121,\hspace{0.17em}123}\mathrm{Sb}$ NQR Study

We report ${}^{121,\hspace{0.17em}123}\mathrm{Sb}$ nuclear quadrupole resonance measurements under pressure in a novel heavy fermion (HF) system ${\mathrm{SmOs}}_4{\mathrm{Sb}}_{12}$. The nuclear spin-spin relaxation rate $1/T_2$ exhibits a distinct peak near the coherent temperature of the Kondo effect. The isotope effect of ${}^{121}\mathrm{Sb}$ and ${}^{123}\mathrm{Sb}$ indicates that the peak in $1/T_2$ is electrical in origin. The connection between the peak in $1/T_2$ and the development of coherency of the Kondo effect is robust even under pressure. It is conjectured that charge fluctuation plays an important role in forming the HF state in ${\mathrm{SmOs}}_4{\mathrm{Sb}}_{12}$.

Figure 1

Temperature dependence of $(1/T_1{)}_{4f}$ for $2{\nu }_Q$ transition of ${}^{123}\mathrm{Sb}$. The inset shows the comparison of $1/(T_1{\gamma }_N^2)$ measured for ${}^{121}\mathrm{Sb}$ and ${}^{123}\mathrm{Sb}$. The NQR intensity weakens around $T_C$, disappearing at low temperatures. At ambient pressure, the intensity decreases to $\sim 20\%$ of the expected value at 1.4 K. Under pressure, on the other hand, the signal disappears without showing the peak in $1/T_1$. The cause is not clarified at the present stage. The arrows at 0.8 and 1.9 GPa indicate $T_C$ determined by the resistivity measurements.

Figure 2

Temperature dependences of $1/T_2$ for $2{\nu }_Q$ transition of ${}^{123}\mathrm{Sb}$ in ${\mathrm{SmOs}}_4{\mathrm{Sb}}_{12}$ for various pressures, ${\mathrm{PrOs}}_4{\mathrm{Sb}}_{12}$, and ${\mathrm{LaOs}}_4{\mathrm{Sb}}_{12}$. The data for ${\mathrm{SmOs}}_4{\mathrm{Sb}}_{12}$ under various pressures are shifted upward for clarity. $T_2$ was measured by fixing the width of $\pi /2$ pulse and $\pi$ pulse. The dotted curves indicate the magnetic contribution (see text).

Figure 3

Temperature dependence of (a) FWHM and (b) $1/T_2$ measured for ${}^{121}\mathrm{Sb}$ and ${}^{123}\mathrm{Sb}$ in ${\mathrm{SmOs}}_4{\mathrm{Sb}}_{12}$. The data for ${}^{121}\mathrm{Sb}$ are normalized to the data for ${}^{123}\mathrm{Sb}$. The respective factors are 0.8 for FWHM and 0.46 for $1/T_2$, irrespective of pressure. The inset shows the spectra for $2{\nu }_Q$ transition of ${}^{123}\mathrm{Sb}$ at 170 and 70 K. All the lines are guides for the eye.

Figure 4

(a) Pressure-temperature phase diagram of ${\mathrm{SmOs}}_4{\mathrm{Sb}}_{12}$. Temperatures showing the peak in $1/T_2$ (closed squares) are consistent with $T^{*}$ determined by the resistivity (open squares) and $1/T_1$ (triangles). (b–e) Pressure dependence of the maximum value in $1/T_2$ around $T^{*}$, the exponent $n$, the residual resistivity ${\rho }_0$, and the coefficient $A$.

Figure 5

$\rho \mathrm{\text{−}}{\rho }_0$ vs temperature. $\rho$ obeys the FL form of $n=2$ at ambient pressure and 2.15 GPa, but $n=1.75$ is observed at 0.72 GPa.