Topological semimetals (TSMs) are characterized by bulk band crossings in their electronic structures, which are expected to give rise to gapless electronic excitations and topological features that underlie exotic physical properties. The most famous examples are Dirac and Weyl semimetals, in which the corresponding low-energy fermionic excitations, i.e., the Dirac and Weyl fermions, are direct analogs of elementary particles in quantum field theory. The last decade has witnessed an explosion of research activities in the field of TSMs thanks to precise theoretical predictions, well-controlled material synthesis, and advanced characterization techniques including angle-resolved photoemission spectroscopy, scanning tunneling microscopy, magnetotransport measurements, optical spectroscopy, etc. Here recent progress in three-dimensional TSMs is reviewed with an emphasis on their characteristic bulk electronic structures, including dimensionality (such as zero-dimensional nodal points, one-dimensional nodal lines, and two-dimensional nodal surfaces), degeneracy (twofold, threefold, fourfold, sixfold, or eightfold) of the band crossing, the slope (type I and type II) and order (linear, quadratic, or cubic) of the band dispersion near the crossing, the characteristic topological invariants (such as monopole charges), and the crystallographic symmetries that stabilize the band crossings. The distinct signatures of the various topological semimetal phases, such as the nontrivial surface states (including Fermi arcs of Dirac and Weyl semimetals) and the unique transport and optical responses (such as chiral anomaly-induced negative magnetoresistance in Dirac and Weyl semimetals), are also reviewed.

中文翻译：

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三维拓扑半金属的实验观点

拓扑半金属（TSM）的特征在于其电子结构中存在体带交叉，这有望引起无间隙的电子激发和构成奇异物理特性的拓扑特征。最著名的例子是狄拉克（Dirac）和魏尔（Weyl）半金属，其中相应的低能铁离子激发，即狄拉克（Dirac）和魏尔（Weyl）费米子，是量子场论中基本粒子的直接类似物。过去十年见证了TSM领域研究活动的爆炸式增长，这要归功于精确的理论预测，可控的材料合成以及先进的表征技术，包括角分辨光发射光谱，扫描隧道显微镜，磁传输测量，光学光谱等。在这里，我们回顾了三维TSM的最新进展，重点介绍了其独特的体电子结构，包括尺寸（例如零维节点，一维节点线和二维节点表面），退化（两倍，三倍）。 ，四倍，六倍或八倍），交叉附近的频带色散的斜率（I型和II型）和阶次（线性，二次或三次），特征拓扑不变性（例如单极电荷） ，以及使能带交叉稳定的晶体学对称性。各种拓扑半金属相的独特标志，