[Paper Review] Discovery of Van Hove Singularities: Electronic Fingerprints of 3Q Magnetic Order in a van der Waals Quantum Magnet

Hai-Lan Luo, Josue Rodriguez+12 more|arXiv (Cornell University)|Jan 16, 2026

2D Materials and Applications0 citations

TL;DR

An ARPES study of Co-intercalated TaS2 reveals van Hove singularities and an inverse Mexican hat dispersion, linking them to a 3Q magnetic order and its evolution with doping.

ABSTRACT

Magnetically intercalated transition metal dichalcogenides are emerging as a rich platform for exploring exotic quantum states in van der Waals magnets. Among them, CoxTaS2 has attracted intense interest following the recent discovery of a distinctive 3Q magnetic ground state and a pronounced topological Hall effect below a critical doping of x=1/3, both intimately tied to cobalt concentration. To date, direct signatures of this enigmatic 3Q magnetic order in the electronic structure remain elusive. Here we report a comprehensive doping dependent angle resolved photoemission spectroscopy study that unveils these long-sought fingerprints. Our data reveal an unexpected "inverse Mexican hat" dispersion along the K-M-K direction, accompanied by two van Hove singularities. These features are consistent with theoretical predictions for a 3Q magnetic order near three-quarters band filling on a cobalt triangular lattice. These results provide evidence of 3Q magnetic order in the electronic structure, establishing TMD van der Waals magnets as tunable materials to explore the interplay between magnetism and topology.

Motivation & Objective

• Investigate how cobalt intercalation alters the electronic structure of TaS2.
• Identify electronic fingerprints of the 3Q magnetic order in Co_xTaS2.
• Understand how doping and magnetic order affect Fermi surfaces and band dispersions.
• Explore the potential interplay between magnetism and topology in magnetically intercalated TMDs.

Proposed method

• Perform high-resolution ARPES on Co_xTaS2 across x = 0.29–0.36 at T = 7 K.
• Characterize Co- and TaS2-derived bands and their doping evolution using different surface terminations.
• Use in situ K deposition to tune the chemical potential and compare with Co-doping effects.
• Model the Co-derived bands with a tight-binding Hamiltonian on a √3×√3 Co triangular lattice including a 3Q exchange coupling J.
• Complement ARPES with DFT calculations (PBE, SOC, DFT+U) for Co_xTaS2 and TaS2 to assign orbital characters.
• Analyze band dispersions and Fermi surface reconstructions to identify signatures of 3Q order (inverse Mexican hat, VHSs).

Experimental results

Research questions

• RQ1 Do Co intercalants introduce Co-derived bands near the Fermi level in TaS2?
• RQ2 What are the electronic fingerprints of the 3Q magnetic order in Co_xTaS2 observed via ARPES?
• RQ3 How do Co doping and ionic gating shift TaS2-derived bands and Co-derived bands?
• RQ4 How does the 3Q order modify the Fermi surface and cause van Hove singularities?
• RQ5 Is there evidence for a transition from 3Q to helical magnetic order in the electronic structure?

Key findings

• Co intercalation injects electrons into TaS2 layers and creates Co-derived bands near EF.
• TaS2-derived α and β bands shift electron-doped with Co, with shifts ≈260–360 meV depending on direction.
• Emergence of shallow Co-derived γK and γM bands near EF, plus a dispersive ε band; γ bands largely originate from Co 3d orbitals.
• For 3Q order, an inverse Mexican hat-like dispersion appears along K-M-K, with two van Hove singularities away from M.
• Fermi surface shows triangular γ pockets that grow with doping, consistent with weakening 3Q order above a critical x_c.
• Spectral weight transfers from γ pockets to M point across x_c, signaling magnetic-order-driven reconstruction.