Cusped Electrical Conductivity in Spin-1 Chiral Fermion Systems Arising from Multifold Band Degeneracy

The energy-dependent electrical conductivity in spin-1 chiral fermion systems with disorder is studied using the self-consistent Born approximation. A distinct cusp-like feature appears at an energy different from the band-crossing point, arising from the multifold band-crossing structure formed by the Dirac and trivial bands. By decomposing the conductivity into band-resolved intraband and interband contributions, we clarify the microscopic origin of the cusp structure previously identified in spin-1 chiral fermion systems. These findings demonstrate the critical role of multifold band crossings and disorder-induced broadening of energy levels in determining the transport properties, offering theoretical insight into the unconventional conductivity behavior observed in topological semimetals hosting spin-1 chiral fermions.