熱電材料可以實現熱能和電能的直接轉換，無須機械運動部件，這使得它在廢熱利用、低溫製冷等領域有廣泛的應用前景。在熱電效應中，反常能斯特效應無須外磁場驅動就能產生橫向電壓，可進一步減小器件的尺寸，同時電極可以同時貼在冷端可提升器件穩定性，近來受到了廣泛關注。

然而其熱電轉換效率卻是制約其實際應用的瓶頸因素。傳統的反常能斯特效應材料的發現要麼集中在特定材料體系比如哈斯勒金屬，要麼是偶然發現。如果從電子結構逆向設計的角度來尋找大的反常能斯特效應材料，可大大提高新候選體系的發現速度。

Fig. 2 Modulation of ANC for the four-band Dirac model with Zeeman term.

來自南方科技大學的劉奇航教授團隊，從理論上證明了在塞曼場下，一對狄拉克節點會呈現出隨化學勢變化奇函數分佈、具有雙峰特徵的反常霍爾電導率曲線，以及補償的載流子特性，從而獲得比傳統的，僅有一對外爾點的兩帶外爾半金屬模型有300%增強的反常能斯特熱電導率。結合上述能帶模型及第一性原理計算，該研究提供了兩個實際的候選材料：狄拉克半金屬Na3Bi和NaTeAu。在塞曼場下，它們在費米能級附近的反常能斯特熱電導率分別可達到0.4 Am^(-1) K^(-1) 和 1.3 Am^(-1) K^(-1)。其中NaTeAu的反常霍爾電導率曲線隨化學勢變化呈雙峰特徵，從而在費米能級附近有一個反常能斯特熱電導率的極大值，與塞曼場下的狄拉克能帶模型相符。

Fig. 3 Electronic structures and transport properties of Na3Bi under Zeeman field.

進一步地，該團隊通過把NaTeAu中50%的Na替換為Fe，獲得了具有塞曼劈裂的本徵鐵磁拓撲材料NaFeTe2Au2，它在費米能級附近展現出高達3.7 Am^(-1) K^(-1)的反常能斯特熱電導率。該研究還設計了一種全哈斯勒鐵磁材料Co2PdGe，同樣在費米能級附近有高達6.2 Am^(-1) K^(-1)的反常能斯特熱電導率。

總之，該研究提供了一種功能材料設計思路，首先設計特定的能帶結構然後篩選出符合這種電子結構的材料，這將提升新候選材料的發現速度。該文近期發表於npj Computational Materials9: 203 (2023)，英文標題與摘要如下，點選左下角「閱讀原文」可以自由獲取論文PDF。

Rational design of large anomalous Nernst effect in Dirac semimetals

Panshuo Wang, Zongxiang Hu, Xiaosong Wu & Qihang Liu

Anomalous Nernst effect generates a transverse voltage perpendicular to the temperature gradient. It has several advantages compared with the longitudinal thermoelectricity for energy conversion, such as decoupling of electronic and thermal transports, higher flexibility, and simpler lateral structure. However, a design principle beyond specific materials systems for obtaining a large anomalous Nernst conductivity (ANC) is still absent. In this work, we theoretically demonstrate that a pair of Dirac nodes under a Zeeman field manifests an odd-distributed, double-peak anomalous Hall conductivity curve with respect to the chemical potential and a compensated carrier feature, leading to an enhanced ANC compared with that of a simple Weyl semimetal with two Weyl nodes. Based on first-principles calculations, we then provide two Dirac semimetal candidates, i.e., Na3Bi and NaTeAu, and show that under a Zeeman field they exhibit a sizable ANC value of 0.4 Am^(-1) K^(-1) and 1.3 Am^(-1) K^(-1), respectively, near the Fermi level. Such approach is also applicable to ferromagnetic materials with intrinsic Zeeman splitting, as exemplified by a hypothetical alloy NaFeTe2Au2, exhibiting an ANC as high as 3.7 Am^(-1) K^(-1) at the Fermi level. Our work provides a design principle with a prototype band structure for enhanced ANC pinning at Fermi level, shedding light on the inverse design of other specific functional materials based on electronic structure.