Type-II Weyl semimetals
Type-II Weyl semimetals
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DOI：
10.1038/nature15768
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发表时间：
2015-11-26
期刊：
影响因子：
64.8
通讯作者：
Bernevig, B. Andrei
中科院分区：
文献类型：
--
作者：
Soluyanov, Alexey A.;Gresch, Dominik;Bernevig, B. Andrei
费米子--基本粒子如电子--被分为狄拉克、马约拉纳和外尔三种。马约拉纳费米子和外尔费米子直到最近发现了拓扑超导体和半金属等凝聚态系统才被实验观测到，在这些系统中，它们是以低能激发的形式出现的（1-6）。在这里，我们提出了一种以前被忽视的外尔费米子类型的存在，出现在电子和空穴口袋之间的边界在一个新的物质阶段。这个粒子被外尔（7）遗漏了，因为它打破了高能物理中严格的洛伦兹对称性。然而，在凝聚态物理学中不存在洛伦兹不变性，通过推广狄拉克方程，我们发现了新的外尔费米子类型。特别是，而外尔半金属材料托管外尔费米子以前被认为有标准的外尔点与点状费米表面（我们称之为I型），我们发现了II型外尔点，这仍然是一个受保护的交叉，但出现在接触的电子和空穴口袋II型外尔半金属。我们预测，WTe 2是一个拓扑半金属托管新粒子作为低能量激发围绕这样的II型外尔点的例子。WTe 2中II型外尔点的存在意味着它的许多物理性质与具有点状费米表面的标准外尔半金属非常不同。
Fermions-elementary particles such as electrons-are classified as Dirac, Majorana or Weyl. Majorana and Weyl fermions had not been observed experimentally until the recent discovery of condensed matter systems such as topological superconductors and semimetals, in which they arise as low-energy excitations(1-6). Here we propose the existence of a previously overlooked type of Weyl fermion that emerges at the boundary between electron and hole pockets in a new phase of matter. This particle was missed by Weyl(7) because it breaks the stringent Lorentz symmetry in high-energy physics. Lorentz invariance, however, is not present in condensed matter physics, and by generalizing the Dirac equation, we find the new type of Weyl fermion. In particular, whereas Weyl semimetals-materials hosting Weyl fermions-were previously thought to have standard Weyl points with a point-like Fermi surface (which we refer to as type-I), we discover a type-II Weyl point, which is still a protected crossing, but appears at the contact of electron and hole pockets in type-II Weyl semimetals. We predict that WTe2 is an example of a topological semimetal hosting the new particle as a low-energy excitation around such a type-II Weyl point. The existence of type-II Weyl points in WTe2 means that many of its physical properties are very different to those of standard Weyl semimetals with point-like Fermi surfaces.