# Charge, lattice and magnetism across the valence crossover in EuIr$_2$Si$_2$ single crystals

## TL;DR

This study investigates the structural, thermal, magnetic, and electronic properties of EuIr$_2$Si$_2$, revealing valence fluctuations, lattice softening, and intermediate valence behavior, with findings that challenge existing fluctuation models.

## Contribution

It provides a comprehensive experimental analysis of EuIr$_2$Si$_2$, highlighting its valence fluctuations and showing limitations of current fluctuation models in describing its properties.

## Key findings

- Large thermal expansion indicating lattice softening.
- Peak in specific heat, susceptibility, and resistivity derivative around 65-75 K.
- Moderately renormalized electron mass with Fermi-liquid behavior.

## Abstract

We present a detailed study of the temperature evolution of the crystal structure, specific heat, magnetic susceptibility and resistivity of single crystals of the paradigmatic valence-fluctuating compound EuIr$_2$Si$_2$. A comparison to stable-valent isostructural compounds EuCo$_2$Si$_2$ (with Eu$^{3+}$), and EuRh$_2$Si$_2$, (with Eu$^{2+}$) reveals an anomalously large thermal expansion indicative of the lattice softening associated to valence fluctuations. A marked broad peak at temperatures around 65-75 K is observed in specific heat, susceptibility and the derivative of resistivity, as thermal energy becomes large enough to excite Eu into a divalent state, which localizes one f electron and increases scattering of conduction electrons. In addition, the intermediate valence at low temperatures manifests in a moderately renormalized electron mass, with enhanced values of the Sommerfeld coefficient in the specific heat and a Fermi-liquid-like dependence of resistivity at low temperatures. The high residual magnetic susceptibility is mainly ascribed to a Van Vleck contribution. Although the intermediate/fluctuating valence duality is to some extent represented in the interconfiguration fluctuation model commonly used to analyze data on valence-fluctuating systems, we show that this model cannot describe the different physical properties of EuIr$_2$Si$_2$ with a single set of parameters.