Enhanced Thermopower in a Magnetic Semiconductor EuTe4 with Multiple Charge-Density-Wave Instabilities

We investigate the layered magnetic semiconductor <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:msub><a:mrow><a:mi>Eu</a:mi><a:mi>Te</a:mi></a:mrow><a:mn>4</a:mn></a:msub></a:math>, focusing on its intricate charge-density-wave (CDW) states near and above room temperature through single-crystal x-ray diffraction (XRD), magnetic, and thermoelectric measurements. The XRD measurement revealed that the CDW state inducing the polar lattice distortion persists even at 650 K, demonstrating its remarkable thermal stability. Notably, the Seebeck coefficient near room temperature reaches values exceeding 500 µV K<c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:msup><c:mi/><c:mrow><c:mo>−</c:mo><c:mn>1</c:mn></c:mrow></c:msup></c:math>. This large Seebeck coefficient, not fully captured by a simple band calculation, is comparable to those observed in heavy-electron semiconducting oxides, suggesting the importance of electron correlation and spin- and lattice instabilities. Furthermore, potentially reflecting the competition of two types of CDW state, the thermal conductivity near room temperature is as low as 0.02 W cm<e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:msup><e:mi/><e:mrow><e:mo>−</e:mo><e:mn>1</e:mn></e:mrow></e:msup></e:math> K<g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:msup><g:mi/><g:mrow><g:mo>−</g:mo><g:mn>1</g:mn></g:mrow></g:msup></g:math>. As a result, the thermoelectric figure of merit reaches 0.22 at 460 K. These findings establish <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:msub><i:mrow><i:mi>Eu</i:mi><i:mi>Te</i:mi></i:mrow><i:mn>4</i:mn></i:msub></i:math> as a compelling platform to explore novel types of thermoelectric materials with multiple electronic instability.