Evolution of Weyl nodes in Ni-doped thallium niobate pyrochlore [Tl.sub.2-x][Ni.sub.x]N[b.sub.2][O.sub.7]

The magnetic Weyl semimetal (WSM) is important for fundamental physics and potential applications due to its spontaneous magnetism, robust band topology, and enhanced Berry curvature. It possesses many unique quantum effects, including a large intrinsic anomalous Hall effect, Fermi arcs, and chiral anomaly. In this work, using ab initio calculations, we propose that Ni-doped pyrochlore [Tl.sub.2]N[b.sub.2][O.sub.7] is a magnetic WSM caused by the exchange field splitting on bands around its quadratic band crossing point. The exchange field tuned by Ni 3d on- site Coulomb interaction parameter U drives the evolution of Weyl nodes and the resulting topological phase transition. As Weyl nodes can exist at generic points in the Brillouin zone and are hard to identify exactly, their creation and annihilation, i.e., the change in their number, chirality, and distribution, have been consistently confirmed with a combined theoretical approach, which employs parity criterion, symmetry indicator analysis, and the Wilson loop of the Wannier center. We find that Weyl nodes remain in a large range of U and are close to the Fermi level, which makes the experimental observation very possible. We think that this method and our proposal of magnetic WSM will be useful in finding more WSMs and add to the understanding of the topological phase transition. magnetic Weyl semimetal, pyrochlore, topological phase transition, Chern number, anomalous Hall effect PACS number(s): 73.20.At, 73.43.Nq, 73.20.-r