Topological phase transitions of semimetal states in effective field theory models

Effective relativistic field theory models capable of realizing various gapless topological states are presented in this work. We study the topological phase transitions of the Weyl and nodal line semimetal states in effective field theories. When the one form field giving rise to the Weyl nodes lie perpendicular to the plane where the nodal ring lives, the nodal ring and Weyl nodes could coexist as the mirror symmetry responsible for the nodal ring is not broken. New phases including a three-node state and a triple degenerate state exist. The nodal ring is immediately destroyed when the one form field lies in the plane of the ring. However, we show that in an eight-component spinor model, Weyl nodes and nodal rings could still coexist even when the one form field is parallel to the plane, due to the expanded symmetry. Topological invariants are calculated which confirm the interesting nontrivial topology of the three-node state and the triple-degenerate node. This work presents potential topological phase transitions in multiphase topological systems, which may be experimentally detected.
Comment: 40 pages,15 figures, comments added, typos corrected. Final version to appear in EPJP