Effect of valence on thermopower

Sketch of temperature gradient, valence gradient and thermal voltage during a thermopower measurement in a Eu sample.

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.

The thermopower of two Eu materials are well described by fits taking into account a valence-induced thermopower contribution.

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.

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