Band structures of (NbSe$_4$)$_3$I and (TaSe$_4$)$_3$I: Reconciling transport, optics and ARPES

TL;DR

This study uses ab-initio and tight binding calculations to reconcile discrepancies between transport, ARPES, and optical data in quasi-one-dimensional transition metal tetrachalcogenides, revealing how experimental probes relate to the band structure.

Contribution

The paper provides a detailed theoretical analysis of (NbSe$_4$)$_3$I and (TaSe$_4$)$_3$I structures, explaining experimental discrepancies and exploring doping effects on electronic properties.

Findings

Good agreement with observed transport gaps.

ARPEs and optics probe gaps between different bands.

Small hole doping can tune the Fermi level near a Van Hove singularity.

Abstract

Among the quasi one-dimensional transition metal tetrachalcogenides (MSe$_4$)$_n$I (M=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 ARPES and optical conductivity. Recently, a metallic polytype of (TaSe$_4$)$_3$I has been found with ferromagnetism and superconductivity coexisting at low temperature, in contrast to previous reports. In this work we present detailed ab-initio and tight binding band structure calculations for the different (MSe$_4$)$_n$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 (TaSe$_4$)$_3$I and discuss the implications for magnetism and superconductivity.