Metal-semiconductor transition and Luttinger-liquid behavior in quasi-one-dimensional BaVS₃ studied by photoemission spectroscopy

The quasi-one-dimensional conductor BaVS₃ exhibits successive phase transitions at ∼240 K (linear chains to zigzag chains), ∼70 K (metallic to semiconducting), and ∼35 K (paramagnetic to antiferromagnetic). We have made high-resolution ultraviolet-photoemission-spectroscopy (UPS), x-ray-photoemission-spectroscopy, electrical-resistivity, and magnetic-susceptibility measurements on BaVS₃. UPS spectra near the Fermi level of the metallic phase exhibit a power-law dependence on the electron binding energy, indicating that conduction electrons in BaVS₃ behave as a Luttinger liquid. The power-law exponent is large (<∼1), indicating that electron-electron interaction is long ranged and possibly that electron-phonon interaction is also important. The spectra exhibit gradual changes with temperature. In particular, a semiconducting gap starts to open well above the metal-to-semiconductor transition temperature and fully develops below it. We propose that the gradual orthorhombic distortion of the Jahn-Teller type below ∼240 K lowers one of the d levels, d_xy, and that below ∼70 K electrons are fully transferred to the d_xy band. The d_xy band then becomes half-filled, resulting in the opening of a Mott-Hubbard gap.