Electronic structure of monolayer-CrTe$_2$: an antiferromagnetic 2D van der Waals material

Magnetic van der Waals materials are an important building block to realize spintronic functionalities in heterostructures of two-dimensional (2D) materials. Yet, establishing their magnetic and electronic properties and the interrelationship between the magnetic ground state and electronic structure is often challenging because only a limited number of techniques can probe magnetism and electronic structure on length scales of tens to hundreds of nanometers. Chromium chalcogenides are a class of 2D magnetic materials for which a rich interplay between structure and magnetism has been predicted. Here, we combine angle-resolved photoemission and quasi-particle interference imaging to establish the electronic structure of a monolayer of CrTe$_2$ on graphite. From a comparison of model calculations with spectroscopic mapping using angle-resolved photoemission spectroscopy and scanning tunnelling microscopy we establish the magnetic ground state and the low energy electronic structure. We demonstrate that the band structure of monolayer CrTe$_2$ is captured well by density functional theory (DFT) in a DFT+U framework when a Coulomb repulsion of $U=2.5\mathrm{eV}$ is accounted for.

💡 Research Summary

This paper presents a comprehensive study of the electronic structure and magnetic ground state of monolayer CrTe₂, a two‑dimensional van‑der‑Waals (vdW) material that exhibits antiferromagnetic order. The authors combine molecular‑beam epitaxy (MBE) growth, scanning tunnelling microscopy (STM), angle‑resolved photoemission spectroscopy (ARPES), quasi‑particle interference (QPI) imaging, and density‑functional theory with a Hubbard‑U correction (DFT+U) to obtain a unified picture of the material’s properties.

Growth and Structural Characterisation
Monolayer CrTe₂ films were grown on highly oriented pyrolytic graphite (HOPG) using an optimized MBE protocol that employs Ge as a sacrificial element to suppress competing phases such as Cr₂Te₃. In‑situ reflection high‑energy electron diffraction (RHEED) and ex‑situ STM reveal a lattice constant of 3.55 ± 0.15 Å and an apparent step height of ~1 nm, consistent with a single‑layer thickness. High‑resolution STM images display two distinct atomic modulations: a zig‑zag pattern and a perpendicular stripe pattern, each forming domains of roughly 20 nm. The zig‑zag modulation breaks the six‑fold symmetry of the underlying lattice and has previously been identified as a zz‑AFM (zig‑zag antiferromagnetic) order by spin‑polarised STM.

Electronic Structure from ARPES
ARPES measurements performed at the Diamond Light Source (I05 beamline) between 40 K and 90 K reveal a metallic Fermi surface centred at Γ. The constant‑energy map shows a sharp inner hexagon surrounded by triangular features and a broader outer hexagon, together with high‑intensity pockets near the K points. These features cannot be reproduced by non‑magnetic (NM) or ferromagnetic (FM) DFT calculations, which either lack central intensity (NM) or concentrate all bands at Γ (FM).

Theoretical Modelling (DFT+U)
To interpret the data, the authors carried out DFT calculations with the plane‑wave codes Quantum ESPRESSO and VASP, adding a Hubbard‑U term on the Cr 3d orbitals. Calculations were performed for three magnetic configurations: NM, FM, and zz‑AFM. For all cases a Hubbard U ranging from 0 to 6 eV was explored. The zz‑AFM configuration enlarges the primitive cell, folding the Brillouin zone and generating additional band replicas. By unfolding the calculated bands to the original Brillouin zone and symmetrising over the three possible domain orientations (rotations by 60° and 120°), the authors obtain a spectral function that matches the ARPES Fermi surface remarkably well when U ≈ 2.5 eV. This value pushes the Cr‑derived bands away from the Fermi level, leaving Te‑p states as the dominant contributors near EF, which explains why the experimental bands require only a modest rigid energy shift (≈ +50 meV) and no further many‑body renormalisation. In contrast, U = 0 eV produces an unobserved Van Hove singularity at Γ and retains Cr‑d weight at EF, underscoring the necessity of including electronic correlations.

QPI Imaging and Real‑Space Confirmation
To further validate the magnetic ground state, the authors performed spectroscopic mapping with STM and extracted QPI patterns via Fourier transformation. The QPI maps display sharp peaks forming a rectangular symmetry, consistent with scattering vectors expected for the zz‑AFM Brillouin zone. Continuum local density of states (CLDOS) calculations for the zz‑AFM unit cell (U = 2.5 eV, negative scattering potential) reproduce the experimental QPI features, whereas analogous calculations for NM and FM cells generate qualitatively different patterns. This real‑space technique allows the authors to isolate a single magnetic domain (as seen in the atomic‑resolution STM image) and directly link its electronic interference signatures to the antiferromagnetic band structure.

Key Insights and Implications

1. Combined Momentum‑ and Real‑Space Probes – By integrating ARPES (momentum space) with QPI (real space), the study overcomes the limitations of each technique alone, especially the domain averaging inherent in ARPES on micron‑scale beams.
2. Magnetic Ground State Identification – The zig‑zag antiferromagnetic order is unequivocally identified as the ground state for the experimentally measured lattice constant, confirming theoretical predictions that the magnetic order is highly sensitive to strain.
3. Role of Correlations – A moderate Hubbard‑U (≈ 2.5 eV) on Cr 3d orbitals is essential to reproduce the experimental spectra, indicating that electron‑electron interactions, though not strong enough to produce Mott physics, significantly reshape the low‑energy band structure.
4. Dominance of Te‑p States – Near the Fermi level the electronic states are primarily Te‑derived, which explains why the DFT band dispersions require little renormalisation beyond the U‑shift. This insight is valuable for designing heterostructures where the magnetic layer is coupled to other 2D conductors or semiconductors.
5. Spintronic Relevance – The ability to grow large‑area monolayer CrTe₂, to determine its antiferromagnetic order, and to map its metallic bands provides a solid platform for antiferromagnetic spin‑orbit torque devices, magnonic waveguides, and proximity‑induced phenomena in vdW heterostructures.

In summary, the paper delivers a thorough experimental‑theoretical characterization of monolayer CrTe₂, establishing zig‑zag antiferromagnetism as the intrinsic magnetic ground state and demonstrating that DFT+U with U ≈ 2.5 eV accurately captures its low‑energy electronic structure. The methodology—combining ARPES, QPI, and advanced DFT—sets a benchmark for future investigations of correlated 2D magnetic materials and paves the way for their integration into next‑generation spintronic technologies.