Triply Degenerate Nodal Point Semimetals

Searching for the analogues of elementary particles in solids has become a hot topic since the discovery of Dirac semimetal (DSM) and Weyl semimetal (WSM). In DSM and WSM, the low energy excitation around the Dirac and Weyl nodes can be described by massless Dirac and Weyl equation, respectively. However, the space group symmetries in solids can protect energy nodes with degeneracy rather than four and two in DSM and WSM, respectively. In this talk, I will introduce our proposals of another nodal point semimetal with triply degenerate nodal points (TDNPs) around the Fermi level. The triple points are the accidental degenerate points of band crossing among a nondegenerate band and a double degenerate bands due to band inversion. The three component fermions are quasiparticles excited from TDNPs. They are intermediate fermions between Dirac and Weyl fermions and have no counterpart in high-energy field theory. The TDNP semimetal can have different longidinal magnetoresistance from DSM and WSM. Some recent experimental observations of TDNPs and magnetoresistance have been also discussed.