Symmetry-breaking pathway towards the unpinned broken helix

Abstract

One of the prime material candidates to host the axion insulator state is EuIn${}_2$As${}_2$. First-principles calculations predicted the emergence of this exotic topological phase based on the assumption of a collinear antiferromagnetic structure. However, neutron scattering measurements revealed a more intricate magnetic ground state, characterized by two coexisting magnetic wavevectors, reached by successive thermal phase transitions. The proposed high and low temperature phases were a spin helix and a state with interpenetrating helical and antiferromagnetic order, termed a broken helix, respectively. Despite its complexity, the broken helix still protects the axion state because the product of time-reversal and a rotational symmetry is preserved. Here we identify the magnetic structure associated with these two phases using a multimodal approach that combines symmetry-sensitive optical probes, scattering, and group theoretical analysis. We find that the higher temperature phase hosts a nodal structure rather than a helix, characterized by a variation of the magnetic moment amplitude from layer to layer, with the moment vanishing entirely in every third Eu layer. The lower temperature structure is similar to the broken helix, with one important difference: the relative orientation of the magnetic structure and the lattice is not fixed, resulting in an `unpinned broken helix'. As a result of the breaking of rotational symmetry, the axion phase is not generically protected. Nevertheless, we show that it can be restored if the magnetic structure is tuned with externally-applied uniaxial strain. Finally, we present a spin Hamiltonian that identifies the spin interactions needed to account for the complex magnetic order in EuIn${}_2$As${}_2$. Our work highlights the importance of the multimodal approach in determining the symmetry of complex order-parameters.

32 pages, 21 figures

References in corpus (16)

• Higher-order Topology of Axion Insulator EuIn${}_2$As${}_2$
• Three-state nematicity and magneto-optical Kerr effect in the charge density waves in kagome superconductors
• Spirit: Multifunctional Framework for Atomistic Spin Simulations
• Spontaneous topological Hall effect induced by non-coplanar antiferromagnetic order in intercalated van der Waals materials
• A Single Pair of Weyl Fermions in Half-metallic EuCd2As2 Semimetal
• Noncoplanar multiple-$Q$ spin textures by itinerant frustration: Effects of single-ion anisotropy and bond-dependent anisotropy
• Tetrahedral triple-Q magnetic ordering and large spontaneous Hall conductivity in the metallic triangular antiferromagnet Co1/3TaS2
• Ab-initio overestimation of the topological region in Eu-based compounds
• Field-induced Topological Hall effect in antiferromagnetic axion insulator candidate EuIn${}_2$As${}_2$
• Signature of band inversion in the antiferromagnetic phase of axion insulator candidate EuIn2As2
• Dipolar Magnetic Interactions and A-type Antiferromagnetic Order in the Zintl Phase Insulator EuZn2P2
• EuCd${}_2$As${}_2$: a magnetic semiconductor
• Spin-carrier coupling induced ferromagnetism and giant resistivity peak in EuCd${}_2$P${}_2$
• Elastocaloric signatures of symmetric and antisymmetric strain-tuning of quadrupolar and magnetic phases in DyB2C2
• Strain-tuned quantum criticality in electronic Potts-nematic systems
• Influence of magnetism, strain and pressure on the band topology of EuCd${}_2$As${}_2$

Cited by in corpus (6)

• Incommensurate broken helix induced by nonstoichiometry in the axion insulator candidate EuIn${}_2$As${}_2$
• Doping-tunable Fermi surface with persistent topological Hall effect in axion candidate EuIn${}_2$As${}_2$
• Nature of field-induced transitions and hysteretic magnetoresistance in non-collinear antiferromagnet EuIn2As2
• Incommensurate magnetic order in an axion insulator candidate EuIn${}_2$As${}_2$ investigated by NMR measurement
• Non-relativistic linear Edelstein effect in helical EuIn2As2
• Linear magneto-conductivity as a DC probe of time-reversal symmetry breaking