Universal phase transition and band structures for spinless nodal-line and Weyl semimetals

We study a general phase transition between spinless topological nodal-line semimetal and Weyl semimetal phases. We classify topological nodal lines into two types based on their positions and shapes, and their phase transitions depend on their types. We show that a topological nodal-line semimetal becomes a Weyl semimetal by breaking time-reversal symmetry when the nodal lines enclose time-reversal-invariant momenta (type-A nodal lines). We also discuss an effect of crystallographic symmetries determining the band structure of the topological nodal-line semimetals. Thanks to protection by the symmetries, the topological nodal-line semimetals can transition into spinless Weyl semimetals or maintain the nodal lines in many crystals after inversion symmetry is broken.