Type-II Dirac/Weyl semimetals are characterized by strongly tilted Dirac cones such that the Dirac/Weyl node emerges at the boundary of electron and hole pockets as a new state of quantum matter, distinct from the standard Dirac/Weyl points with a point-like Fermi surface which are referred to as type-I nodes. The type-II Dirac fermions were recently predicted by theory and have since been confirmed in experiments in the PtSe₂-class of transition metal dichalcogenides. However, the Dirac nodes observed in PtSe₂, PdTe₂, and PtTe₂ candidates are quite far away from the Fermi level, making the signature of topological fermions obscure as the physical properties are still dominated by the non-Dirac quasiparticles. Here, we report the synthesis of a new type-II Dirac semimetal NiTe₂ in which a pair of type-II Dirac nodes are located very close to the Fermi level. The quantum oscillations in this material reveal a nontrivial Berry’s phase associated with these Dirac fermions. Our first-principles calculations further unveil a topological Dirac cone in its surface states. Therefore, NiTe₂ may not only represent an improved system to formulate the theoretical understanding of the exotic consequences of type-II Dirac fermions, it also facilitates possible applications based on these topological carriers.

中文翻译：

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过渡金属二硫属元素化物NiTe _2中接近费米能级的拓扑II型狄拉克费米子

II型Dirac / Weyl半金属的特征是强烈倾斜的Dirac锥，使得Dirac / Weyl节点以新的量子态出现在电子和空穴腔的边界，这与标准Dirac / Weyl点不同，其点为-像费米曲面，被称为I型节点。II型狄拉克费米子最近已通过理论预测，并已在实验中在PtSe _2-类过渡金属_二卤化物中得到证实。但是，在PtSe ₂，PdTe ₂和PtTe _2中观察到Dirac节点候选物距离费米能级还很远，这使拓扑费米子的特征模糊不清，因为物理性质仍由非狄拉克准粒子占据主导地位。在这里，我们报告了一种新的II型Dirac半金属NiTe ₂的合成，其中一对II型Dirac节点非常靠近费米能级。这种材料中的量子振动揭示了与这些狄拉克费米子有关的非平凡的贝里相。我们的第一性原理计算进一步揭示了处于其表面状态的拓扑狄拉克锥。因此，NiTe ₂不仅可以代表一种改进的系统，以表达对II型狄拉克费米子的奇异后果的理论理解，还可以促进基于这些拓扑载体的可能的应用。