Xu Bing, Fang Zhenyao, Sánchez-Martínez Miguel-Ángel, Venderbos Jorn W F, Ni Zhuoliang, Qiu Tian, Manna Kaustuv, Wang Kefeng, Paglione Johnpierre, Bernhard Christian, Felser Claudia, Mele Eugene J, Grushin Adolfo G, Rappe Andrew M, Wu Liang
Fribourg Center for Nanomaterials, Department of Physics, University of Fribourg, CH-1700 Fribourg, Switzerland.
Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104-6323.
Proc Natl Acad Sci U S A. 2020 Nov 3;117(44):27104-27110. doi: 10.1073/pnas.2010752117. Epub 2020 Oct 19.
We report the optical conductivity in high-quality crystals of the chiral topological semimetal CoSi, which hosts exotic quasiparticles known as multifold fermions. We find that the optical response is separated into several distinct regions as a function of frequency, each dominated by different types of quasiparticles. The low-frequency intraband response is captured by a narrow Drude peak from a high-mobility electron pocket of double Weyl quasiparticles, and the temperature dependence of the spectral weight is consistent with its Fermi velocity. By subtracting the low-frequency sharp Drude and phonon peaks at low temperatures, we reveal two intermediate quasilinear interband contributions separated by a kink at 0.2 eV. Using Wannier tight-binding models based on first-principle calculations, we link the optical conductivity above and below 0.2 eV to interband transitions near the double Weyl fermion and a threefold fermion, respectively. We analyze and determine the chemical potential relative to the energy of the threefold fermion, revealing the importance of transitions between a linearly dispersing band and a flat band. More strikingly, below 0.1 eV our data are best explained if spin-orbit coupling is included, suggesting that at these energies, the optical response is governed by transitions between a previously unobserved fourfold spin-3/2 node and a Weyl node. Our comprehensive combined experimental and theoretical study provides a way to resolve different types of multifold fermions in CoSi at different energy. More broadly, our results provide the necessary basis to interpret the burgeoning set of optical and transport experiments in chiral topological semimetals.
我们报道了手性拓扑半金属CoSi高质量晶体中的光导率，该晶体中存在被称为多重费米子的奇异准粒子。我们发现，光响应随频率可分为几个不同区域，每个区域由不同类型的准粒子主导。低频带内响应由双外尔准粒子的高迁移率电子口袋的窄德鲁德峰捕获，光谱权重的温度依赖性与其费米速度一致。通过减去低温下的低频尖锐德鲁德峰和声子峰，我们揭示了两个中间准线性带间贡献，它们在0.2 eV处由一个扭折分开。使用基于第一性原理计算的万尼尔紧束缚模型，我们将0.2 eV以上和以下的光导率分别与双外尔费米子和三重费米子附近的带间跃迁联系起来。我们分析并确定了相对于三重费米子能量的化学势，揭示了线性色散带和平带之间跃迁的重要性。更引人注目的是，如果考虑自旋轨道耦合，我们的数据在0.1 eV以下能得到最好的解释，这表明在这些能量下，光响应由一个先前未观察到的四重自旋 - 3/2节点和一个外尔节点之间的跃迁所支配。我们全面的实验与理论相结合的研究提供了一种方法，以分辨CoSi中不同能量下的不同类型多重费米子。更广泛地说，我们的结果为解释手性拓扑半金属中迅速涌现的一系列光学和输运实验提供了必要的基础。
