印度科学研究所Anindya Das团队研究了魔角扭曲双层石墨烯中涌现局域矩的热电探针。2025年3月31日出版的《自然—物理学》杂志发表了这项成果。
最近对魔角扭曲双层石墨烯的实验表明，平坦带的形成表明，电子相关效应可能在这种材料中占主导地位。然而，缺少显示强相关性的明显特征的全球输运测量，例如由平坦带引起的局部矩。
研究组通过它们对从热电测量中提取的熵的影响来证明出现的局部矩的存在。除了狄拉克点处的热功率的符号变化和平坦带的完全填充外，还观察到四分之一填充带附近的符号变化，这些变化不随温度从5 K到60 K的变化而变化。这与课题组在研究相关性较弱的扭曲双层石墨烯器件时看到的温度依赖性交叉点形成鲜明对比。
此外，研究组发现施加磁场会降低热功率，这与在部分自旋极化下在层状氧化物中观察到的自旋熵抑制一致。无论是稳健的交叉点还是磁场的抑制，都不能仅从带费米子的贡献来解释；相反，该数据表明，主要贡献来自强相关平带的出射局域矩的熵。
附：英文原文
Title: Thermopower probes of emergent local moments in magic-angle twisted bilayer graphene
Author: Ghosh, Ayan, Chakraborty, Souvik, Dutta, Ranit, Agarwala, Adhip, Watanabe, K., Taniguchi, T., Banerjee, Sumilan, Trivedi, Nandini, Mukerjee, Subroto, Das, Anindya
Issue&Volume: 2025-03-31
Abstract: Recent experiments on magic-angle twisted bilayer graphene have shown the formation of flat bands, suggesting that electronic correlation effects are likely to dominate in this material. However, a global transport measurement showing distinct signatures of strong correlations—such as local moments arising from the flat bands—is missing. Here we demonstrate the presence of emergent local moments through their impact on entropy extracted from thermopower measurements. In addition to sign changes in the thermopower at the Dirac point and full filling of the flat bands, we observe sign changes near the quarter-filled bands that do not vary with temperature from 5K to 60K. This is in contrast to temperature-dependent crossing points seen in our study on twisted bilayer graphene devices with weaker correlations. Furthermore, we find that applying a magnetic field reduces the thermopower, consistent with spin entropy suppression observed in layered oxides under partial spin polarization. Neither the robust crossing points nor the suppression by a magnetic field can be explained solely from the contributions of band fermions; instead, our data suggest a dominant contribution coming from the entropy of the emergent localized moments of a strongly correlated flat band.
DOI: 10.1038/s41567-025-02849-1
Source: https://www.nature.com/articles/s41567-025-02849-1