Hidden Link, Creation, and Annihilation of Nodal Lines with Z2 Monopole Charges in PT-Symmetric Systems

Nodal lines are formed when the conduction and valence bands are inverted if spin-orbit coupling is negligible in systems with time-reversal T and inversion P symmetries. Those nodal lines have two interesting properties distinguished from nodal points. First, they can have various shapes such as links and knots. Second, they can carry two topological charges: one is the well-known \pi Berry phase, and the other is the Z2 monopole charge. In this talk, we show that the Z2 monopole charge originates from the exotic hidden link. A nodal line carrying the Z2 monopole charge is always linked to the nodal line formed between the two topmost occupied bands. Then we show how this links of Z2 monopoles can be pair-created by a process we call double band inversion. Finally, we show that pair-creation and pair-annihilation of nodal lines with Z2 monopole charge can mediate a topological phase transition between a normal insulator and a three-dimensional Stiefel-Whitney insulator (3D SWI).