Origin of a shallow electron pocket: $\beta$-band in Co$_{1/3}$TaS$_2$ studied by angle-resolved photoemission spectroscopy

TL;DR

This study reveals that a shallow electron pocket in Co$_{1/3}$TaS$_2$ originates from strong electron correlations on Co sites, which are essential for understanding its electronic structure beyond standard DFT methods.

Contribution

The paper demonstrates that the $eta$-feature in Co$_{1/3}$TaS$_2$ is correlation-driven and not surface-related, using advanced theoretical modeling beyond standard DFT+U.

Findings

The $eta$ pocket is reproduced by cluster perturbation theory (CPT).

Reduced Co content diminishes the $eta$ feature.

Strong correlations and long-range order are necessary for the $eta$ feature.

Abstract

We investigate the electronic structure and Fermi surface of Co$_{1/3}$TaS$_2$ using angle-resolved photoemission spectroscopy (ARPES) combined with theoretical modeling beyond standard density functional theory (DFT+U). A shallow electron pocket, the so-called $\beta$ feature, is observed at the Fermi level near the corner of the superlattice Brillouin zone, representing the first experimental observation of this feature in an intercalated TaS$_2$ compound. Similar pockets have been reported in $X_{1/3}$NbS$_2$ ($X$ = Co, Cr, Ni), where their surface versus bulk origin remains actively debated. Because conventional DFT+U does not capture this feature, we employ cluster perturbation theory (CPT) to incorporate an explicit treatment of strong electron correlations ($U$) on the Co sites. CPT successfully reproduces the $\beta$ feature, demonstrating its origin from correlation-driven bulk states rather than surface effects. To further substantiate this conclusion, we studied a reduced Co-content sample, Co$_{0.22}$TaS$_2$, where the reduced charge transfer modifies the Co-derived states near the Fermi level. Its electronic structure remains largely similar to that of pristine 2H-TaS$_2$, showing only a minor overall energy shift and lacking the $\beta$ feature, consistent with disrupted long-range Co ordering and modified orbital character near the Fermi level. We demonstrate that the $\beta$ feature arises from strong local correlations on the Co sites and requires long-range crystallographic order among intercalated Co atoms to maintain coherence. These results highlight the importance of strong electronic correlations in magnetically intercalated transition-metal dichalcogenides and provide a microscopic understanding of features not captured by conventional DFT+U.