ARPES Line Shapes in FL and non-FL Quasi-Low-Dimensional Inorganic Metals

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Quasi-low-dimensional (quasi-low-D) inorganic materials are not only ideally suited for angle resolved photoemission spectroscopy (ARPES) but also they offer a rich ground for studying key concepts for the emerging paradigm of non-Fermi liquid (non-FL) physics. In this article, we discuss the ARPES technique applied to three quasi-low-D inorganic metals: a paradigm Fermi liquid (FL) material TiTe₂, a well-known quasi-1D charge density wave (CDW) material K₀.₃MoO₃ and a quasi-1D non-CDW material Li₀.₉Mo₆O₁₇. With TiTe₂, we establish that a many body theoretical interpretation of the ARPES line shape is possible. We also address the fundamental question of how to accurately determine the kF value from ARPES. Both K₀.₃MoO₃ and Li₀.₉Mo₆O₁₇ show quasi-1D electronic structures with non-FL line shapes. A CDW gap opening is observed for K₀.₃MoO₃, whereas no gap is observed for Li₀.₉Mo₆O₁₇. We show, however, that the standard CDW theory, even with strong fluctuations, is not sufficient to describe the non-FL line shapes of K₀.₃MoO₃. We argue that a Luttinger liquid (LL) model is relevant for both bronzes, but also point out difficulties encountered in comparing data with theory. We interpret this situation to mean that a more complete and realistic theory is necessary to understand these data.