Evolution of the electronic structure from electron-doped to hole-doped states in the two-dimensional Mott-Hubbard system La_1.17-xPb _xVS_3.17

The filling-controlled metal-insulator transition (MIT) in a two-dimensional Mott-Hubbard system La_1.17-xPb_xVS _3.17 has been studied by photoemission spectroscopy. With Pb substitution x, chemical potential μ abruptly jumps by ∼0.07 eV between x = 0.15 and 0.17, indicating that a charge gap is opened at x ≃0.16 in agreement with the Mott insulating state of the d² configuration. When holes or electrons are doped into the Mott insulator of x≃0.16, the gap is filled and the photoemission spectral weight at μ, ρ(μ), gradually increases in a similar way to the electronic specific-heat coefficient, although the spectral weight remains depressed around μ compared to that expected for a normal metal, showing a pseudogap behavior in the metallic samples. The observed behavior of ρ(μ)→0 for x→0.16 is contrasted with the usual picture that the electron effective mass of the Fermi-liquid system is enhanced towards the metal-insulator boundary. With increasing temperature, the gap or the pseudogap is rapidly filled up, and the spectra at T = 300 K appears to be almost those of a normal metal. Near the metal-insulator boundary, the spectra around μ are consistent with the formation of a Coulomb gap, suggesting the influence of long-range Coulomb interaction under the structural disorder intrinsic to this system.