High-resolution angle-resolved photoemission study of η-Mo₄O₁₁

We have performed high-resolution angle-resolved photoemission spectroscopy (ARPES) on a quasi two-dimensional (2D) conductor η-Mo₄O₁₁. The band dispersions near E_F and the Fermi-surface topology determined by ARPES show a qualitatively good agreement with the band-structure calculation, although the observed width of Mo 4d bands forming the Fermi surface is two-to-three times larger in the calculation. We have clearly identified two nesting vectors in the ARPES-derived Fermi surface, which show a good agreement with the CDW wave vectors obtained from x-ray diffraction experiments. The present ARPES results indicate that the CDW instability in η-Mo₄O₁₁ is driven by "the hidden 1D Fermi-surface nesting" due to the quasi 1D chains in the crystal.