Effect of valence on thermopower

The thermopower of a material measures the electrical field induced by a temperature gradient in that material. The most important thermopower contributions are the diffusion thermopower due to charge carrier diffusion and the phonon drag contribution as phonons “drag” charge carriers along.

A special contribution is expected in materials with a temperature-dependent valence. In simple words, the temperature gradient is accompanied by a valence gradient. This gives rise to a thermopower contribution due to the variation of the number of available charge carriers or, more precisely, the change of the chemical potential along the sample. This thermopower contribution is approximately proportional to the valence change with temperature.

We studied the thermopower for two materials with a strongly temperature-dependent Eu valence, namely EuIr₂Si₂ and EuNi₂P₂. In both materials, we observed large thermopower values that could not be explained by simple hybridization models. Instead, we were able to show, that the thermopower of these two materials can be understood considering the temperature-dependence of the Eu valence. These findings corroborate the relevance of the valence fluctuations for understanding the properties of EuIr₂Si₂ and EuNi₂P₂ at intermediate temperatures.