Nodal-line Quantum Effects Enable Anomalous and Nonlinear Hall Effects in NbMnP

The anomalous Hall effect, a phenomenon observed in magnetic materials, is gaining renewed attention thanks to research into complex magnetic structures. Ibuki Terada of Osaka Metropolitan University, alongside Vu Thi Ngoc Huyen from Tohoku University and Yuki Yanagi of Toyama Prefectural University, and et al., have investigated this effect in the noncollinear antiferromagnetic metal NbMnP, a material with a unique mixture of magnetic components. Their work reveals a surprisingly large anomalous Hall effect, suggesting a connection between the material’s magnetic order and the potential for nonlinear Hall effects. Using first-principles calculations, the team demonstrate that enhanced Berry curvature and Berry-connection-polarization dipole, originating from spin-orbit coupling and nodal lines, are key to understanding the Hall response in NbMnP. This research positions NbMnP as a crucial model system for exploring transport phenomena driven by nodal lines within parity-mixed antiferromagnets, potentially paving the way for novel spintronic devices.

This research positions NbMnP as a crucial model system for exploring transport phenomena driven by nodal lines within parity-mixed antiferromagnets, potentially paving the way for novel spintronic devices.

Berry Curvature Mapping in NbMnP

The study of the anomalous Hall effect in NbMnP began with a detailed theoretical investigation of its intrinsic properties, employing first-principles calculations combined with the Wannier interpolation method. Researchers modelled the electronic band structure of NbMnP to understand how its unique magnetic order, a mixture of even-parity B3g and odd-parity B2u components, influences Hall effect phenomena. This computational work accurately described the quantum geometry of its Bloch bands, establishing a foundation for understanding the material’s behaviour.

Scientists harnessed these calculations to map the Berry curvature and Berry-connection-polarization dipole within NbMnP, revealing that these geometric quantities are strongly enhanced on a specific mirror plane. They discovered that these enhancements originate from spin-orbit coupling inducing gap openings along nodal lines within the material’s electronic structure. This innovative approach pinpointed the precise mechanisms driving the anomalous Hall response, demonstrating a direct link between the material’s band structure and its observed transport properties.

This work pioneered a method for analysing parity-mixed antiferromagnets, materials possessing both even and odd-parity magnetic components. By examining the interplay between these components, the study demonstrated how the co-existence of B3g and B2u magnetic order enables the anomalous Hall effect, despite the absence of net magnetisation. Furthermore, the team’s calculations provided crucial insight into the nonlinear Hall effect, predicting its emergence due to the breaking of inversion symmetry by the odd-parity B2u component.

NbMnP Anomalous Hall Effect Driven by Berry Curvature

Scientists have achieved a significant breakthrough in understanding the anomalous Hall effect (AHE) within the noncollinear antiferromagnetic metal NbMnP. The research team investigated the intrinsic anomalous and nonlinear Hall effects of NbMnP, employing first-principles calculations and the Wannier interpolation method to explore the quantum geometry of Bloch bands. Experiments revealed that the intrinsic Hall response of NbMnP is predominantly governed by strongly enhanced Berry curvature and a Berry-connection-polarization dipole located on a specific mirror plane.

These enhanced geometric quantities originate from spin-orbit-coupling-induced gap openings along nodal lines within the material’s band structure. Measurements confirm that the interplay between these nodal lines and spin-orbit coupling is crucial in driving the observed AHE. Detailed analysis shows the magnetic structure of NbMnP, experimentally determined through neutron powder diffraction, consists of four manganese atoms forming two antiparallel pairs with magnetic moments of approximately 1.2 μB.

The coexistence of the B3g and B2u components within the antiferromagnetic state is expected to give rise to both AHE and nonlinear Hall effects, with the B3g component permitting anomalous Hall conductivity. The observation of a substantial AHE in NbMnP supports the presence of the B3g component, while the nonlinear Hall effect provides evidence for the B2u component. This breakthrough delivers a deeper understanding of transport phenomena originating from nodal-lines in parity-mixed antiferromagnets.

NbMnP’s Berry Curvature Drives Hall Effects Noncollinear antiferromagnetic

This work details a theoretical investigation into the anomalous and nonlinear Hall effects observed in the noncollinear antiferromagnetic metal NbMnP. Through first-principles calculations and the Wannier interpolation method, researchers demonstrated that the material’s intrinsic Hall response is strongly influenced by enhanced Berry curvature and Berry-connection-polarization dipole, particularly on a specific mirror plane.

These geometric quantities are fundamentally linked to spin-orbit coupling induced gap openings along nodal lines within the material’s band structure. The findings reveal that the anomalous Hall effect arises from symmetry breaking caused by the even-parity magnetic component, achieving an anomalous Hall conductivity of -366 S/cm. Simultaneously, the nonlinear Hall effect is driven by the odd-parity magnetic component, with nonlinear Hall conductivities reaching the order of a few mS/V.

NbMnP therefore exhibits both Berry-curvature-driven and Berry-connection-polarization-driven Hall effects due to the coexistence of these magnetic components. The authors acknowledge that their analysis focused on intrinsic contributions, leaving the investigation of dissipative Hall responses for future work. NbMnP is presented as a valuable model system for understanding transport phenomena in parity-mixed antiferromagnets with nodal lines. The study establishes a clear link between the material’s unique magnetic structure, its band geometry, and the resulting Hall effects, offering a foundation for the development of novel spintronic devices.