Band structures of ( NbSe4 ) 3I and ( TaSe4 ) 3I : Reconciling transport, optics, and angle-resolved photoemission spectroscopy

Among the quasi-one-dimensional transition metal tetrachalcogenides ($M{\mathrm{Se}}₄$)${}_n\mathrm{I}$ ($M=\text{Nb}$,Ta), the $n=3$ compounds are the only ones not displaying charge density waves. Instead, they show structural transitions with puzzling transport behavior. They are semiconductors at the lowest temperatures, but their transport gaps are significantly smaller than those inferred from angle-resolved photoemission spectroscopy (ARPES) and optical conductivity. Recently, a metallic polytype of (${\mathrm{TaSe}}₄$)${}₃\mathrm{I}$ has been found with ferromagnetism and superconductivity coexisting at low temperature, in contrast to previous reports. In this work we present detailed $\mathit{ab}$ initio and tight-binding band-structure calculations for the different ($M{\mathrm{Se}}₄$)${}₃\mathrm{I}$ reported structures. We obtain good agreement with the observed transport gaps, and explain how ARPES and optics experiments effectively probe a gap between different bands due to an approximate translation symmetry, solving the controversy. Finally, we show how small extrinsic hole doping can tune the Fermi level through a Van Hove singularity in (${\mathrm{TaSe}}₄$)${}₃\mathrm{I}$ and discuss the implications for magnetism and superconductivity.