The evolution of Weyl nodes in Ni doped thallium niobate pyrochlore Tl$_{2-x}$Ni$_x$Nb$_2$O$_7$

TL;DR

This paper uses ab initio calculations to demonstrate how Ni doping induces a magnetic Weyl semimetal phase in Tl$_2$Nb$_2$O$_7$, revealing the evolution of Weyl nodes and topological phase transitions driven by exchange field tuning.

Contribution

It introduces a theoretical approach to identify and analyze the evolution of Weyl nodes in Ni-doped Tl$_2$Nb$_2$O$_7$, advancing understanding of magnetic WSMs and their topological phase transitions.

Findings

Weyl nodes are caused by exchange field splitting around quadratic band crossing.

Weyl nodes evolve with the Coulomb interaction parameter U, leading to topological phase transitions.

Weyl nodes are near the Fermi level, facilitating experimental detection.

Abstract

Magnetic Weyl semimetal (WSM) is of great importance both 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 large intrinsic anomalous Hall effect, Fermi arcs, and chiral anomaly. In this work, using ab initio calculations, we propose that Ni doped pyrochlore Tl$_2$Nb$_2$O$_7$ is a magnetic WSM caused by 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. Since Weyl nodes can exist at generic point in Brillouin zone and are hard to be exactly identified, the creation and annihilation of them, 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 Wannier center. We find that Weyl nodes remain in a quite large range of $U$ and are close to Fermi level, which makes the experimental observation very possible. We think this method and our proposal of magnetic WSM will be useful in finding more WSMs and adding our understanding on the topological phase transition.