Interplay between local symmetry breaking, magnetic order and Weyl states in Co₃Sn₂S₂

Co₃Sn₂S₂, a magnetic Weyl semimetal with a kagome lattice of cobalt ions, has triggered intense interest recently. Co₃Sn₂S₂ was proposed to exhibit a coexistence of ferromagnetic (FM) order and antiferromagnetic (AFM) order below T_C≈ 175 K, followed by a pure ferromagnetic (FM) order below T_A≈ 135 K. The ferromagnetic order along the c axis was confirmed from our half-polarized neutron technology below T_C. Using neutron total scattering, we found a striking local symmetry breaking from rhombohedral R-3m to monoclinic Cm co-emerges with the onset of ferromagnetic order below T_C. The mismatch of local and average crystallographic structures indicates that Co₃Sn₂S₂ becomes an intrinsically lattice disordered system below T_C. This provides new insight to the previously puzzling magnetic phase separation and spin glass like state in Co₃Sn₂S₂. Our DFT calculations reveal that both the symmetry-allowed 120° antiferromagnetic orders support new Weyl states in T_A<T<T_C, with distinct numbers and locations of Weyl points. Upon cooling below T_A, inelastic neutron scattering revealed highly anisotropic spin-wave dispersions and linewidths. Modeling the spin-wave spectra shows that the ground-state FM order is dominated by the FM third-neighbor “across-hexagon” J_d coupling with a weak frustrated next-nearest-neighbor bond. Furthermore, our DFT calculation indicates that the local symmetry breaking plays a detrimental role in the formation of the Weyl points associated with the FM order by breaking mirror symmetries and is expected to induce a broad topological surface band like feature. Our study unveils an intimate interplay between local symmetry breaking, magnetic order and Weyl states in Co₃Sn₂S₂.