Hidden symmetry-breaking in a kagome Ising ferromagnet

TL;DR

This paper uncovers evidence of time-reversal symmetry breaking in the kagome metal TbV$_6$Sn$_6$, caused by subtle magnetic interactions between V kagome layers and Tb local moments, revealed through neutron scattering.

Contribution

It demonstrates the coexistence of symmetry-breaking magnetic order with Ising ferromagnetism without structural changes, highlighting a novel interplay between kagome and rare-earth magnetism.

Findings

Inequivalent Tb site excitations indicate symmetry breaking.

No structural distortions detected despite magnetic splitting.

Spectroscopy reveals subtle broken symmetries in the material.

Abstract

Kagome metals can host unconventional electronic phenomena that emerge from their frustrated lattice geometry and associated band topology. Correlated electronic orders, such as charge-density waves and superconductivity, are observed to intertwine with subtle time-reversal symmetry breaking whose microscopic origin is not currently understood. Here, we provide evidence for such time-reversal symmetry breaking in the kagome metal TbV$_6$Sn$_6$ arising from staggered magnetic moments within the kagome layers. TbV$_6$Sn$_6$ consists of metallic V kagome layers separated by Tb triangular layers that host Ising ferromagnetic order. Deep in the ferromagnetic state, the Tb Ising doublet ground state should display a single, dispersionless spin-flip excitation. Instead, inelastic neutron scattering reveals two sharp excitations associated with inequivalent Tb sites, demonstrating that a symmetry-broken phase coexists with Ising ferromagnetism. No additional structural or magnetic phase transitions are detected, and first-principles calculations rule out lattice distortions as the origin of the splitting. We attribute this effect to time-reversal symmetry breaking encoded by small V moments that couple to the Tb sublattice and leave a measurable spectral fingerprint. Our results establish rare-earth local moment spectroscopy as a sensitive probe of subtle broken symmetries and highlight an unexpected interplay between kagome magnetism and rare-earth local moment magnetism.