Discovery of topological Weyl fermion lines and drumhead surface states in a room temperature magnet

Abstract: Topological matter is known to exhibit unconventional surface states and anomalous transport owing to unusual bulk electronic topology. In this study, we use photoemission spectroscopy and quantum transport to elucidate the topology of the room temperature magnet Co$_2$MnGa. We observe sharp bulk Weyl fermion line dispersions indicative of nontrivial topological invariants present in the magnetic phase. On the surface of the magnet, we observe electronic wave functions that take the form of drumheads, enabling us to directly visualize the crucial components of the bulk-boundary topological correspondence. By considering the Berry curvature field associated with the observed topological Weyl fermion lines, we quantitatively account for the giant anomalous Hall response observed in our samples. Our experimental results suggest a rich interplay of strongly correlated electrons and topology in this quantum magnet.

• The paper demonstrates the discovery of Weyl fermion lines and drumhead surface states in Co2MnGa using ARPES and first-principles calculations.
• The study reveals that these topological features underlie a giant anomalous Hall effect, with Hall conductivity reaching 1530 Ω⁻¹ cm⁻¹.
• The integration of quantum transport measurements with theoretical analysis provides new insights for advancing spintronics and quantum materials research.

Overview of "Discovery of topological Weyl fermion lines and drumhead surface states in a room temperature magnet"

The study presented in this paper investigates Co$_2$MnGa, a room temperature ferromagnet, revealing a rich interplay of topological electronic structures and magnetism. The researchers combine angle-resolved photoemission spectroscopy (ARPES), quantum transport measurements, and first-principles calculations to elucidate the presence of topological Weyl fermion lines and drumhead surface states, shedding light on their relationships with the anomalous Hall effect (AHE).

The primary focus is the Co$_2$MnGa system, free from spin-orbit coupling, which exhibits band degeneracies classified as Weyl lines within mirror planes of the crystal. These Weyl lines are two-fold degenerate band crossings that disperse over one dimension and are protected by crystalline symmetries. The strong correlation between magnetism and topology in Co$_2$MnGa is a significant finding as it informs the AHE, offering insights for spintronics and future quantum technology applications.

Key Findings and Methodology

• Weyl Line Characterization: Using ARPES, the researchers detected line nodes manifesting as well-delineated Weyl lines at energy points aligned with the crystal’s mirror symmetry planes. These Weyl lines contribute to the system’s complex topological nature and have been confirmed through ab initio calculations.
• Drumhead Surface States: The study also identifies drumhead surface states within Co$_2$MnGa through ARPES and theoretical predictions, which are crucial for confirming the bulk-boundary correspondence—a key characteristic of topological materials.
• Anomalous Hall Effect: The integration of ARPES data with quantum transport measurements revealed that the observed giant anomalous Hall effect in Co$_2$MnGa is deeply connected to the Berry curvature inherent in the Weyl line network. This intrinsic AHE component, observed even at 2 K with a Hall conductivity of 1530 $\Omega^{-1}$ cm$^{-1}$, positions Co$_2$MnGa as a distinctive ferromagnetic system demonstrating both large Berry curvature effects and Weyl topology in a single material.

Implications and Future Directions

The findings suggest that the strong intrinsic AHE and the topological structure of Co$_2$MnGa present promising opportunities for potential technological advancements, especially in the field of spintronics. The work paves the way for further examination of magnetic topological materials where Berry curvature and electronic topological structures interplay, potentially driving novel magnetic responses and functionalities.

The approach exemplified by this research—combining spectroscopy, transport measurements, and computational modelling—offers a robust framework for the exploration of other magnetic systems within the expansive landscape of 3D magnetic materials. With 1651 magnetic space groups described, the verification and characterization of additional magnetic Weyl semimetals could vastly enrich the understanding of topological matter in different magnetic contexts, opening new pathways for materials engineering and quantum computational applications.

In conclusion, this paper provides a comprehensive assessment of Co$_2$MnGa's topological features, emphasizing the role of magnetism in complex electronic behavior, while also setting the stage for extended investigations in topological and quantum materials research.