Structural phase transition induced by van Hove singularity in 5$d$ transition metal compound IrTe$_{2}$

Comprehensive studies of the electronic states of Ir 5$d$ and Te 5$p$ have been performed to elucidate the origin of the structural phase transition in IrTe$_{2}$ by combining angle-resolved photoemission spectroscopy and resonant inelastic X-ray scattering. While no considerable changes are observed in the configuration of the Ir 5$d$ electronic states across the transition, indicating that the Ir 5$d$ orbitals are not involved in the transition, we reveal a van Hove singularity at the Fermi level ($E_{\mathrm{F}})$ related to the Te $p_{\mathrm{x}}+p_{\mathrm{y}}$ orbitals, which is removed from $E_{\mathrm{F}}$ at low temperatures. The wavevector connecting the adjacent saddle points is consistent with the in-plane projection of the superstructure modulation wavevector. These results can be qualitatively understood with the Rice-Scott ``saddle-point'' mechanism, while effects of the lattice distortions need to be additionally involved.