Kondo-Induced Giant Isotropic Negative Thermal Expansion

Negative thermal expansion is an unusual phenomenon appearing in only a handful of materials, but pursuit and mastery of the phenomenon holds great promise for applications across disciplines and industries. Here we report use of x-ray spectroscopy and diffraction to investigate the $4f$-electronic properties in Y-doped SmS and employ the Kondo volume collapse model to interpret the results. Our measurements reveal an unparalleled decrease of the bulk Sm valence by over 20% at low temperatures in the mixed-valent golden phase, which we show is caused by a strong coupling between an emergent Kondo lattice state and a large isotropic volume change. The amplitude and temperature range of the negative thermal expansion appear strongly dependent on the Y concentration and the associated chemical disorder, providing control over the observed effect. This finding opens avenues for the design of Kondo lattice materials with tunable, giant, and isotropic negative thermal expansion.

Figure 1

Temperature dependent unit cell volume of $b\text{−}\mathrm{SmS}$, $b\text{−}{\mathrm{Sm}}_{0.86}{\mathrm{Y}}_{0.14}\mathrm{S}$, $g\text{−}{\mathrm{Sm}}_{0.77}{\mathrm{Y}}_{0.23}\mathrm{S}$, and $g\text{−}{\mathrm{Sm}}_{0.67}{\mathrm{Y}}_{0.33}\mathrm{S}$.

Figure 2

(a) Temperature dependence of the $\mathrm{Sm}\text{−}{\mathrm{L}}_3$ PFY-XAS spectra for $b\text{−}\mathrm{SmS}$, $b\text{−}{\mathrm{Sm}}_{0.86}{\mathrm{Y}}_{0.14}\mathrm{S}$, $g\text{−}{\mathrm{Sm}}_{0.77}{\mathrm{Y}}_{0.23}\mathrm{S}$, and $g\text{−}{\mathrm{Sm}}_{0.67}{\mathrm{Y}}_{0.33}\mathrm{S}$, respectively. The dashed lines display the best fit to the base temperature data. (b) Temperature and doping dependence of the Sm valence.

Figure 3

(a) KVC equation of state for various reduced temperatures $T/D$. (b) and (c) Temperature dependent first-order valence $\nu$ and volume transitions at pressures $p_1<p_2$.

Figure 4

Fit of the experimental data (circles) with the two-fluid model (dashed line). The valence $v$ is given as a function of Y concentration $x$ and temperature $T$: $v(x,T)=[x{v}_{\text{imp}}+(1\text{−}x)v_{\text{lat},\mathrm{HT}}]f(T_C\text{−}T)+v_{\text{lat},\mathrm{LT}}f(T\text{−}{T}_C)$ with $v_{\text{imp}}=2.56$, $v_{\text{lat},\mathrm{LT}}=v_{\text{lat},\mathrm{HT}}=2.19$ and $f(T)=\{1+\tanh [(T/T_C)/\sigma ]\}/2$ with $\sigma \sim 0.615$.