Correlation between pseudo-gap electronic structures and thermoelectric properties in Fe_2 VAl intermetallic compounds

heusler型Fe_2VAl化合物在理论上被预测在费米能级上有一个明显的伪间隙，实验证明了电子输运性质的剧烈变化和热电功率的大幅增强，这是由其化学计量偏离组成的结果。本研究的目的是利用正、逆光电子能谱技术实验阐明heusler型fe_2val相关合金的电子结构与热电性能之间的关系，从而为热电材料的开发提供新的指导。本文研究了Fe_2VAl、非化学计量（Fe_<2/3>V_<1/3>）_<100-y>Al_y和Fe_<2-x>V_<1+x>Al的电子结构，并取代了Fe_2VAl_<1-z>Si_z。实验首次证明了赝隙的存在和赝隙中费米能级的相对位置与其热电性质密切相关。对于Fe_<2/3>V_<1/3>)_<100-y>Al_y和Fe_2VAl_<1-z>Si_z，而对于Fe_<2-x>V_<1+x>Al，则不能用刚性带模型定性地解释费米能级附近电子结构和热电性质随组成的变化。这是由于与Al和Si的sp价态相比，过渡金属d态对费米能级附近的电子结构造成了很大的改变。这些中隙态是低温下不寻常的输运性质的原因。因此，控制费米能级附近的d态对于进一步发展这些heusler型合金作为热电材料具有重要意义。我们认为用第四元代替费米能级定位和用微观结构控制代替低导热系数是发展的有效方法。

The Heusler-type Fe_2VAl compound has been theoretically predicted to have a pronounced pseudo-gap across the Fermi level and experimentally proved to show the drastic change in the electronic transport properties and the large enhancement of the thermoelectric power on the deviation of the composition from its stoichiometry. The purpose of this research is to experimentally clarify the relation between their electronic structures and thermoelectric properties of the Heusler-type Fe_2VAl-related alloys with use of the normal and inverse photoelectron spectroscopy, and thus to obtain a new guideline for developing thermoelectric materials.In this study, the electronic structures have been investigated for the stoichiometric Fe_2VAl, off-stoichiometric (Fe_<2/3>V_<1/3>)_<100-y>Al_y and Fe_<2-x>V_<1+x>Al, and substituted Fe_2VAl_<1-z>Si_z. It is experimentally shown for the first time that the existence of the pseudo-gap and the relative position of the Fermi level in the pseudo-gap are strongly related to their thermoelectric properties. The change in the electronic structure near the Fermi level and thermoelectric properties with the composition is explained qualitatively by a rigid band model for (Fe_<2/3>V_<1/3>)_<100-y>Al_y and Fe_2VAl_<1-z>Si_z but not for Fe_<2-x>V_<1+x>Al. This arises from the large modification of the electronic structure near the Fermi level due to the transition metal d states in contrast to the sp valence states of Al and Si. These mid-gap d states are responsible for the unusual transport properties at low temperatures. Thus it is important to control the d states near the Fermi level for the further development of these Heusler-type alloys as thermoelectric materials. We suggest that the substitution of the forth element for the Fermi level positioning and the microscopic structural control for the low thermal conductivity are effective for the development.