Ma J-Z, Nie S M, Yi C J, Jandke J, Shang T, Yao M Y, Naamneh M, Yan L Q, Sun Y, Chikina A, Strocov V N, Medarde M, Song M, Xiong Y-M, Xu G, Wulfhekel W, Mesot J, Reticcioli M, Franchini C, Mudry C, Müller M, Shi Y G, Qian T, Ding H, Shi M
Swiss Light Source, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland.
Institute of Condensed Matter Physics, École Polytechnique Fédérale de Lausanne, CH-10 15 Lausanne, Switzerland.
Sci Adv. 2019 Jul 12;5(7):eaaw4718. doi: 10.1126/sciadv.aaw4718. eCollection 2019 Jul.
Weyl fermions as emergent quasiparticles can arise in Weyl semimetals (WSMs) in which the energy bands are nondegenerate, resulting from inversion or time-reversal symmetry breaking. Nevertheless, experimental evidence for magnetically induced WSMs is scarce. Here, using photoemission spectroscopy, we observe that the degeneracy of Bloch bands is already lifted in the paramagnetic phase of EuCdAs. We attribute this effect to the itinerant electrons experiencing quasi-static and quasi-long-range ferromagnetic fluctuations. Moreover, the spin-nondegenerate band structure harbors a pair of ideal Weyl nodes near the Fermi level. Hence, we show that long-range magnetic order and the spontaneous breaking of time-reversal symmetry are not essential requirements for WSM states in centrosymmetric systems and that WSM states can emerge in a wider range of condensed matter systems than previously thought.
作为涌现准粒子的外尔费米子可以出现在外尔半金属（WSMs）中，其中能带是非简并的，这是由空间反演或时间反演对称性破缺导致的。然而，磁诱导WSMs的实验证据很少。在这里，我们使用光电子能谱观察到，在EuCdAs的顺磁相中，布洛赫能带的简并已经解除。我们将这种效应归因于巡游电子经历准静态和准长程铁磁涨落。此外，自旋非简并能带结构在费米能级附近有一对理想的外尔点。因此，我们表明，长程磁有序和时间反演对称性的自发破缺对于中心对称系统中的WSM态不是必要条件，并且WSM态可以出现在比以前认为的更广泛的凝聚态系统中。
