Title : Correlation between crystal structure, magnetic order, and topological states in Co3Sn2S2
Abstract:
Co3Sn2S2, a magnetic Weyl semimetal with a kagome lattice of cobalt ions, has triggered intense interest recently. In this talk, I will present crystal structure in both long-range and local scales, magnetism, and topological Weyl state of Co3Sn2S2 using elastic and inelastic neutron scattering techniques. Co3Sn2S2 was proposed to exhibit a coexistence of ferromagnetic (FM) order and antiferromagnetic (AFM) order below TC≈175 K, followed by a pure ferromagnetic (FM) order below TA≈135 K. The long-range ferromagnetic order along the c axis was confirmed from the half-polarized neutron technology below TC, which is correlated to the anomaly in the Co-Sn2 bond and lattice constant a. Inelastic neutron scattering revealed highly anisotropic magnon dispersions and linewidths below TC, and paramagnetic excitations above TC. Modeling the spin-wave spectra shows that the ground-state FM order is dominated by the unusual third neighbor “across-hexagon” Jd FM coupling with a weak frustrated next-nearest-neighbor bond. Our DFT calculations reveal that the symmetry-allowed two 120° antiferromagnetic orders support new Weyl states in TA<T<TC, with distinct numbers and locations of Weyl points as compared to those observed in T<TA1.
In the second part of the talk, I will go beyond long-range scale and move to the local scale in this system. A striking local symmetry breaking from rhombohedral R-3m to monoclinic Cm co-emerges with the onset of ferromagnetic order below TC using neutron total scattering technique. The mismatch of local and average crystallographic structures indicates that Co3Sn2S2 becomes an intrinsically lattice disordered system below TC. This provides new insight to the previously puzzling magnetic phase separation and spin glass like state in Co3Sn2S2. Furthermore, our DFT calculation indicates that the local symmetry breaking has the tendency to drive a local ferromagnetic moment reorientation by around 19o to the monoclinic a axis, and plays a detrimental role in the formation of the Weyl points associated with the ground-state FM order by breaking mirror symmetries and is expected to induce a broad topological surface band like feature. Our study provides the experimental realization of dominant third neighbor “cross-hexagon" magnetic coupling and a hidden local symmetry breaking in a Kagome magnet, and unveils an intimate interplay between crystal structure, magnetism, and Weyl states in Co3Sn2S22.
Audience Take-Away:
- The audience could use the methodology presented in this talk on their research on magnetic materials or other functional materials. The combination of the neutron diffraction, neutron total scattering, neutron spectroscopy and DFT calculation is proved to be powerful tools to explore the interplay between crystal structure, magnetic order and topological states.
- This research shows the importance to study the crystal structure in not only long-range scale but also the local scale for understanding the unusual magnetic and electronic ground states of other topological materials.
- Our work indicates that the previously puzzling magnetic properties were due to an incomplete understanding of the crystalline structure of Co3Sn2S2. It would motivate further experimental studies to discover and control local structure for realizing new structure-property relationships and engineering desired physical properties more broadly in topological semimetals and kagome magnets.
