Weyl metallic state induced by helical magnetic order

Weyl semimetals (WSMs), characterized by Weyl nodes—points in momentum space where singly degenerate bands cross—have attracted significant attention due to their exotic transport properties. Realizing WSMs requires breaking either inversion symmetry (P) or time-reversal symmetry (T). While ferromagnetic WSMs are straightforward, their stray fields limit miniaturization. Antiferromagnetic (AFM) WSMs are more promising, but most AFM structures preserve P × T symmetry, preventing Dirac point splitting into Weyl nodes. Non-collinear AFM structures offer an alternative, exemplified by Mn3X (X = Sn, Ge) with chiral AFM structure supporting Weyl nodes but exhibiting experimentally challenging band broadening. Centrosymmetric Eu compounds with non-collinear Eu spin configurations, such as EuCo2P2, EuNi2As2, EuZnGe, and EuCuSb, show promise due to large exchange coupling between localized Eu 4f states and conduction electrons, but DFT calculations on these complex structures are challenging. This study focuses on EuCuAs, a simpler system with commensurate non-collinear AFM order, to investigate Weyl node generation.

The study builds upon prior work demonstrating the potential for magnetic control of topological electronic band features. Previous research highlighted the interest in magnetic Weyl semimetals for their exotic charge and spin transport properties, with a focus primarily on ferromagnetic or ferrimagnetic compounds. However, the limitations of these materials due to stray magnetic fields motivated the search for alternative systems, particularly antiferromagnetic ones with non-collinear spin arrangements. The challenges in computational modeling of complex incommensurate magnetic structures in similar Eu compounds are also acknowledged, emphasizing the significance of studying EuCuAs as a simpler model system.

The research employed a multifaceted approach combining various experimental and computational techniques. Bulk properties were characterized using temperature-dependent susceptibility and field-dependent magnetization measurements. Magnetotransport measurements provided insights into the relationship between charge transport and Eu magnetism. The magnetic structure was investigated using powder neutron diffraction (PND), single-crystal neutron diffraction (ND), and resonant elastic x-ray magnetic scattering (REXS). These techniques provided information on the magnetic propagation vector and spin arrangement. Spherical neutron polarimetry (SNP) was utilized to further refine the magnetic structure analysis, overcoming challenges related to neutron absorption by Eu. Density Functional Theory (DFT) calculations were performed to determine the electronic band structure, considering different magnetic configurations. Finally, Angle-resolved photoemission spectroscopy (ARPES) measurements were used to validate the DFT calculations.

Below a Néel temperature (TN) of 14.5 K, EuCuAs exhibits a planar helical spin arrangement with a magnetic propagation vector of (0, 0, τ), where τ ≈ 0.5. The magnetic susceptibility shows anisotropy, with higher susceptibility along the c-axis than perpendicular to it, indicating that Eu moments primarily lie in the ab plane. Magnetization measurements reveal a metamagnetic transition at around 0.3 T, corresponding to a reorientation of Eu moments within the ab plane. The resistivity shows a peak at TN, suppressed by an applied magnetic field, suggesting scattering from Eu spin fluctuations. Neutron and x-ray diffraction studies confirm the helical spin structure, although the exact value of τ shows some sample dependence. The analysis indicates that the magnetic structure is a planar helix with a period of approximately four Eu layers. In a magnetic field applied perpendicular to the c-axis, a transition to a canted double-period AFM structure is observed, followed by a gradual canting of Eu moments along the field direction toward a fully polarized state. DFT calculations, validated by ARPES measurements (with a 0.4 eV energy shift to account for hole doping), reveal the presence of two quadratic Weyl points with Chern numbers ±2 at the A point in the Brillouin zone, induced by the helical magnetic order. A spin Hamiltonian model is developed to describe the magnetic behavior, accounting for the competing exchange interactions and anisotropy.

The findings demonstrate that a fully compensated helical magnetic order in EuCuAs can induce a Weyl metallic state. This is significant because it expands the range of systems where magnetic order can influence electronic band topology, offering an alternative to ferromagnetic systems with their limitations on device miniaturization. The observed metamagnetic transition and the associated change in the electronic band structure highlight the strong coupling between magnetism and charge transport. The agreement between DFT calculations and ARPES measurements validates the computational model and confirms the existence of Weyl points. The developed spin Hamiltonian provides a framework for understanding the magnetic interactions and their role in stabilizing the helical structure.

This study conclusively demonstrates the induction of Weyl nodes in EuCuAs by helical magnetic order. This discovery expands the landscape of magnetic topological materials, offering a pathway towards smaller-scale devices due to the absence of stray fields in the compensated antiferromagnetic structure. Future research could focus on identifying similar systems with simpler band structures to better isolate the effects of Weyl fermions and investigate the potential applications of this phenomenon.

The sample dependence of the precise value of the propagation vector τ suggests potential sensitivity to slight variations in stoichiometry or other material parameters. The presence of multiple bands near the Fermi level in EuCuAs complicates the analysis of the Weyl fermion response. The spin Hamiltonian model, while providing a good description of the magnetic properties, is an approximation and could be further refined by incorporating additional interaction terms.