# Kondo-induced giant isotropic negative thermal expansion

## TL;DR

This study uncovers a giant, isotropic negative thermal expansion in Y-doped SmS driven by Kondo lattice interactions, with potential for tunable thermal properties in advanced materials.

## Contribution

It demonstrates a novel link between Kondo physics and negative thermal expansion, enabling control over the effect via chemical doping.

## Key findings

- Over 20% decrease in Sm valence at low temperatures.
- Negative thermal expansion strongly depends on Y concentration.
- Identification of Kondo lattice coupling as the mechanism.

## Abstract

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 new avenues for the design of Kondo lattice materials with tunable, giant and isotropic negative thermal expansion.