The noncentrosymmetric transition-metal monopnictides NbP, TaP, NbAs, and TaAs are a family of Weyl semimetals in which pairs of protected linear crossings of spin-resolved bands occur. These so-called Weyl nodes are characterized by integer topological charges of opposite sign associated with singular points of Berry curvature in momentum space. In such a system, anomalous magnetoelectric responses are predicted, which should only occur if the crossing points are close to the Fermi level and enclosed by Fermi surface pockets penetrated by an integer flux of Berry curvature, dubbed Weyl pockets. TaAs is shown to possess Weyl pockets, whereas TaP and NbP have trivial pockets enclosing zero net flux of Berry curvature. Herein, via measurements of the magnetic torque, resistivity, and magnetization, a comprehensive quantum-oscillation study of NbAs is presented, the last member of this family where the precise shape and nature of the Fermi surface pockets is still unknown. Seven distinct frequency branches, three of which have not been observed before, are detected. A comparison with density functional theory calculations suggests that the two largest pockets are topologically trivial, whereas the low frequencies might stem from tiny Weyl pockets. The enclosed Weyl nodes are within a few meV of the Fermi energy.

中文翻译：

---

NbAs 中接近费米能的外尔节点

非中心对称过渡金属单核苷酸 NbP、TaP、NbAs 和 TaAs 是 Weyl 半金属家族，其中发生了成对的自旋分辨带的受保护线性交叉。这些所谓的外尔节点的特征是与动量空间中贝里曲率的奇异点相关的相反符号的整数拓扑电荷。在这样的系统中，预测了异常磁电响应，只有当交叉点接近费米能级并且被称为外尔袋的整数通量贝里曲率穿透的费米表面袋包围时，才会发生异常磁电响应。TaAs 被证明具有 Weyl 口袋，而 TaP 和 NbP 具有微不足道的口袋，其中包含零的 Berry 曲率净通量。在此，通过测量磁转矩、电阻率和磁化强度，介绍了 NbAs 的全面量子振荡研究，这是该家族的最后一个成员，其中费米表面口袋的精确形状和性质仍然未知。检测到七个不同的频率分支，其中三个以前没有被观察到。与密度泛函理论计算的比较表明，两个最大的口袋在拓扑上是微不足道的，而低频可能源于微小的外尔口袋。封闭的外尔节点在费米能量的几兆电子伏范围内。与密度泛函理论计算的比较表明，两个最大的口袋在拓扑上是微不足道的，而低频可能源于微小的外尔口袋。封闭的外尔节点在费米能量的几兆电子伏范围内。与密度泛函理论计算的比较表明，两个最大的口袋在拓扑上是微不足道的，而低频可能源于微小的外尔口袋。封闭的外尔节点在费米能量的几兆电子伏范围内。