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 4$f$-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.

中文翻译：

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近藤引起的巨大各向同性负热膨胀

负热膨胀是仅在少数几种材料中出现的不寻常现象，但是对这种现象的追求和掌握对于跨学科和跨行业的应用具有广阔的前景。在这里我们报告使用X射线光谱学和衍射来研究这4种$F$掺杂的SmS中的电子性质，并使用Kondo体积塌陷模型来解释结果。我们的测量结果显示，大量Sm价降低了20多个$％$在混合价金相中处于低温时，我们证明这是由于出现的近藤晶格状态与较大的各向同性体积变化之间的强耦合引起的。负热膨胀的幅度和温度范围似乎强烈取决于Y浓度和相关的化学紊乱，从而可以控制所观察到的效果。这一发现为具有可调谐的，巨大的和各向同性的负热膨胀的近藤晶格材料设计开辟了道路。