Xu Su-Yang, Alidoust Nasser, Chang Guoqing, Lu Hong, Singh Bahadur, Belopolski Ilya, Sanchez Daniel S, Zhang Xiao, Bian Guang, Zheng Hao, Husanu Marious-Adrian, Bian Yi, Huang Shin-Ming, Hsu Chuang-Han, Chang Tay-Rong, Jeng Horng-Tay, Bansil Arun, Neupert Titus, Strocov Vladimir N, Lin Hsin, Jia Shuang, Hasan M Zahid
Department of Physics, Laboratory for Topological Quantum Matter and Spectroscopy (B7), Princeton University, Princeton, NJ 08544, USA.
Rigetti & Co Inc., 775 Heinz Avenue, Berkeley, CA 94710, USA.
Sci Adv. 2017 Jun 2;3(6):e1603266. doi: 10.1126/sciadv.1603266. eCollection 2017 Jun.
In quantum field theory, Weyl fermions are relativistic particles that travel at the speed of light and strictly obey the celebrated Lorentz symmetry. Their low-energy condensed matter analogs are Weyl semimetals, which are conductors whose electronic excitations mimic the Weyl fermion equation of motion. Although the traditional (type I) emergent Weyl fermions observed in TaAs still approximately respect Lorentz symmetry, recently, the so-called type II Weyl semimetal has been proposed, where the emergent Weyl quasiparticles break the Lorentz symmetry so strongly that they cannot be smoothly connected to Lorentz symmetric Weyl particles. Despite some evidence of nontrivial surface states, the direct observation of the type II bulk Weyl fermions remains elusive. We present the direct observation of the type II Weyl fermions in crystalline solid lanthanum aluminum germanide (LaAlGe) based on our photoemission data alone, without reliance on band structure calculations. Moreover, our systematic data agree with the theoretical calculations, providing further support on our experimental results.
在量子场论中，外尔费米子是相对论性粒子，它们以光速传播并严格遵守著名的洛伦兹对称性。其低能凝聚态类似物是外尔半金属，即电子激发模拟外尔费米子运动方程的导体。尽管在砷化钽中观测到的传统（I型）涌现外尔费米子仍大致遵守洛伦兹对称性，但最近有人提出了所谓的II型外尔半金属，其中涌现的外尔准粒子强烈破坏洛伦兹对称性，以至于无法与洛伦兹对称的外尔粒子平滑连接。尽管有一些关于非平凡表面态的证据，但II型体相外尔费米子的直接观测仍然难以实现。我们仅基于光电子能谱数据，在不依赖能带结构计算的情况下，给出了在晶体固体镧铝锗（LaAlGe）中II型外尔费米子的直接观测结果。此外，我们的系统数据与理论计算结果相符，为我们的实验结果提供了进一步的支持。
