近日，中国科学院物理研究所的李世亮及其研究小组与香港大学的孟子杨以及日本原子能机构的Kenji Nakajima等人合作并取得一项新进展。经过不懈努力，他们获得笼目晶格反铁磁体中狄拉克自旋子的光谱证据。相关研究成果已于2024年5月9日在国际知名学术期刊《自然—物理学》上发表。
本文证明了笼目反铁磁体YCu3(OD)6Br2[Br0.33(OD)0.67]的自旋激发是圆锥形的，内部有一个自旋连续体，这与两个狄拉克自旋的卷积一致。从光谱测量得到的具有自旋速度的狄拉克自旋液体模型的预测与样品的低温比热是一致的。因此，这项研究为笼目晶格反铁磁体中狄拉克量子自旋液态的存在提供了确凿的光谱证据。然而，观测到的锥形自旋激发位置与最新的海森堡模型计算结果存在偏差，这表明狄拉克自旋子有一个意想不到的起源。
据悉，凝聚态系统中具有狄拉克色散的涌现准粒子可以用相对论电子的狄拉克方程来描述，类似于高能物理中的狄拉克粒子。例如，具有狄拉克色散的电子已经在石墨烯和拓扑绝缘体等电子系统中得到了深入的研究。然而，电荷并不是狄拉克费米子存在的先决条件，没有电荷自由度的狄拉克费米子的出现在理论上已经被预言在狄拉克量子自旋液体中会实现。这些准粒子的自旋为1/2，但它们是电中性的，因此被称为自旋子。
附：英文原文
Title: Spectral evidence for Dirac spinons in a kagome lattice antiferromagnet
Author: Zeng, Zhenyuan, Zhou, Chengkang, Zhou, Honglin, Han, Lankun, Chi, Runze, Li, Kuo, Kofu, Maiko, Nakajima, Kenji, Wei, Yuan, Zhang, Wenliang, Mazzone, Daniel G., Meng, Zi Yang, Li, Shiliang
Issue&Volume: 2024-05-09
Abstract: Emergent quasiparticles with a Dirac dispersion in condensed matter systems can be described by the Dirac equation for relativistic electrons, in analogy with Dirac particles in high-energy physics. For example, electrons with a Dirac dispersion have been intensively studied in electronic systems such as graphene and topological insulators. However, charge is not a prerequisite for Dirac fermions, and the emergence of Dirac fermions without a charge degree of freedom has been theoretically predicted to be realized in Dirac quantum spin liquids. These quasiparticles carry a spin of 1/2 but are charge-neutral and so are called spinons. Here we show that the spin excitations of a kagome antiferromagnet, YCu3(OD)6Br2[Br0.33(OD)0.67], are conical with a spin continuum inside, which is consistent with the convolution of two Dirac spinons. The predictions of a Dirac spin liquid model with a spinon velocity obtained from spectral measurements are in agreement with the low-temperature specific heat of the sample. Our results, thus, provide spectral evidence for a Dirac quantum spin liquid state emerging in this kagome lattice antiferromagnet. However, the locations of the conical spin excitations differ from those calculated by the nearest-neighbour Heisenberg model, suggesting the Dirac spinons have an unexpected origin.
DOI: 10.1038/s41567-024-02495-z
Source: https://www.nature.com/articles/s41567-024-02495-z