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Mo$_8$O$_{23}$ is a low-dimensional chemically robust transition metal oxidecoming from a prospective family of functional materials, MoO$_{3-x}$, rangingfrom a wide gap insulator $(x=0)$ to a metal $(x=1)$. The large number ofstoichometric compounds with intermediate $x$ have widely different properties.In Mo$_8$O$_{23}$, an unusual charge density wave transition has been suggestedto occur above room temperature, but its low temperature behaviour isparticularly enigmatic. We present a comprehensive experimental study of theelectronic structure associated with various ordering phenomena in thiscompound, complemented by theory. Density-functional theory (DFT) calculationsreveal a cross-over from a semi-metal with vanishing band overlap to narrow-gapsemiconductor behaviour with decreasing temperature. A buried Dirac crossing atthe zone boundary is confirmed by angle-resolved photoemission spectroscopy(ARPES). Tunnelling spectroscopy (STS) reveals a gradual gap openingcorresponding to a metal-to-insulator transition at 343 K in resistivity,consistent with CDW formation and DFT results, but with large non-thermalsmearing of the spectra implying strong carrier scattering. At lowtemperatures, the CDW picture is negated by the observation of a metallic Hallcontribution, a non-trivial gap structure in STS below $\sim 170$ K and ARPESspectra, that together represent evidence for the onset of the correlated stateat $70$ K and the rapid increase of gap size below $\sim 30$ K. The intricateinterplay between electronic correlations and the presence of multiple narrowbands near the Fermi level set the stage for metastability and suggestsuitability for memristor applications.

Unveiling the electronic transformations in the semi-metallic correlated-electron transitional oxide Mo$_8$O$_{23}$