# Insulating ground state and 2-k magnetic structure of candidate Weyl Hydrogen atom K$_2$Mn$_3$(AsO$_4$)$_3$

## TL;DR

This study investigates K2Mn3(AsO4)3, a candidate Weyl semi-metal, revealing complex magnetic structures that break the symmetry needed for Weyl nodes and showing it is actually an insulator, not a semi-metal.

## Contribution

The paper provides comprehensive experimental and theoretical analysis showing that K2Mn3(AsO4)3 is an insulator with complex magnetic order, challenging previous predictions of Weyl semi-metal behavior.

## Key findings

- Magnetic transitions at 8 K and 4 K with complex magnetic structures.
- Crystals are optically transparent with a large band-gap.
- Density functional theory predicts an insulating ground state.

## Abstract

The ideal Weyl 'Hydrogen-atom' semi-metal exhibits only a single pair of Weyl nodes and no other trivial states at the Fermi energy. Such a material would be a panacea in the study of Weyl quasi particles allowing direct unambiguous observation of their topological properties. The alluaudite-like K$_2$Mn$_3$(AsO$_4$)$_3$ compound was recently proposed as such a material. Here we use comprehensive experimental work and first principle calculations to assess this prediction. We find K$_2$Mn$_3$(AsO$_4$)$_3$ crystallizes in the $C2/c$ symmetry with a quasi-1D Mn sublattice, growing as small needle-like crystals. Bulk properties measurements reveal magnetic transitions at $\approx$ 8 and $\approx$ 4 K which neutron scattering experiments show correspond to two distinct magnetic orders, first a partially ordered ferrimagnetic $\mathbf{k_1}$= (0, 0, 0) structure at 8 K and a second transition of $\mathbf{k_2}$= (1, 0, 0) at 4 K to a fully ordered state. Below the second transition, both ordering vectors are necessary to describe the complex magnetic structure with modulated spin magnitudes. Both of the best-fit magnetic structures in this work are found to break the symmetry necessary for the generation of the Weyl nodes, though one of the magnetic structures allowed by $\mathbf{k_1}$ does preserve this symmetry. However, the crystals are optically transparent and ellipsometry measurements reveal a large band-gap, undermining expectations of semi-metallic behavior. Density functional theory calculations predict an insulating antiferromagnetic ground state, in contrast to previous reports, and suggest potential frustration on the magnetic sublattice. Given the wide tunability of the alluaudite structure we consider ways to push the system closer to semi-metallic state.