Elastic tricriticality at the collapse of magnetic order in a valence fluctuating Eu-based compound

Spin and charge fluctuations are key for understanding the properties of materials with strongly correlated electrons, but their coupling to the atomic crystal lattice is often assumed to be weak and perturbative. In this work, we demonstrate that at tricritical points (TCPs), that are associated with electronic correlations, a coupling to crystal elasticity is generically nonperturbative. It will modify the tricritical universality class leading to elastic tricriticality , characterized by a compressibility that diverges in a universal manner with the correlation length. To test this scenario experimentally, we use single crystals of EuPd 2 (Si 1− x Ge x ) 2 where, depending on x and pressure, magnetic order collapses due to strong valence fluctuations. First, we confirm the presence of a TCP for x ≈ 0.1 by using complementary probes, in particular, measurements of relative length changes under control of temperature and He-gas pressure. Second, by studying the elastic response, we uncover clear signatures of elastic tricriticality, notably a drastically enhanced compressibility.