Magnetic field induced Weyl semimetal from Wannier-function-based tight-binding model

Abstract

Weyl semimetals (WSMs) have Weyl nodes where conduction and valence bands meet in the absence of inversion or time-reversal symmetry (TRS), or both. Interesting phenomena are expected in WSMs such as the chiral magnetic effect, anomalous Hall conductivity or Nernst effect, and unique quantum oscillations. The TRS-broken WSM phase can be driven from a topological Dirac semimetal (DSM) by magnetic field or magnetic dopants, considering that DSMs have degenerate Weyl nodes stabilized by rotational symmetry, i.e. Dirac nodes. Here we develop a Wannier-function-based tight-binding (WF-TB) model to investigate the formation of Weyl nodes and nodal rings induced by B field in the topological DSM Na$_3$Bi. The field is applied along the rotational axis. Remarkably, our study based on the WF-TB model shows that upon B field each Dirac node is split into four separate Weyl nodes along the rotational axis near the Fermi level; two nodes with single chiral charge (single Weyl nodes) and two with double chiral charge (double Weyl nodes). This result is in contrast to the common belief that each Dirac node splits into two Weyl nodes. In the effective models, the existence of double Weyl nodes ensures nonzero cubic terms in momentum. We further examine the evolution of Fermi arcs at a side surface as a function of chemical potential. The number of Fermi arcs is consistent with the corresponding Fermi surface Chern numbers, corroborating our finding of the double Weyl nodes. Furthermore, our study reveals the existence of nodal rings in the mirror plane below the Fermi level upon B field. These nodal rings persist with spin-orbit coupling, in contrast to typical nodal ring semimetals. Our WF-TB model can be used to compute interesting features arising from Berry curvature such as anomalous Hall and thermal conductivities, and our findings can be applied to other topological DSMs like Cd$_3$As$_2$.