Structural phase transition associated with van Hove singularity in 5d transition metal compound IrTe₂

We have investigated the electronic states of IrTe₂ by using angle-resolved photoemission spectroscopy to elucidate the origin of the structural phase transition. Both the Ir 4f and Te 4d core level spectra exhibit dramatic splitting below the phase transition temperature T_s, suggesting that there exist two inequivalent Ir and Te sites with distinctly different electronic states in the distorted phase. The band related to the saddle points at the Fermi level (E_F) is strongly reconstructed, which removes the van Hove singularity from E_F below T_s. The wavevector connecting the adjacent saddle points is consistent with the in-plane superstructure modulation wavevector. These results indicate that the phase transition in IrTe₂ is intimately associated with the saddle points. As the van Hove singularity mainly originates from the Te p_x+p_y orbitals, the Te 5p electronic states play a dominant role in the structural phase transition.