Room 10.01, Bldg. 30.23 CS and Zoom
University of Groningen
The dimensionality and symmetry of the order parameter determine the topology of defects in ordered states of matter. The diversity and complexity of topological defects grow with the number of dimensions of the order parameter space. A prominent example is the A-phase of superfluid 3He . The realization of a three-dimensional order in a solid allows studying the wealth of states predicted for the superfluid 3He. The Fe-langasite, Ba3TaFe3Si24O14, hosts a non-collinear antiferromagnetic order described by an SO(3) matrix . This unique material is both chiral and magnetically frustrated. The langasite spin lattice is built of triangles formed by antiferromagnetically coupled Fe3+-ions (S = 5/2). It shows a non-collinear 120o spin ordering at the scale of one unit cell, a spiral with a period of 7 lattice constants, and spin superstructures at the 100 nm scale. Lifshitz invariants allowed by the lack of inversion symmetry give rise to novel modulated magnetic phases and stabilize particle-like topological defects  previously discussed in very different physical contexts, e.g., nuclear physics and superfluid 3He.
 G. Volovik, Symmetry in Superfluid 3He. (Elsevier, 1990).
 M. Ramakrishnan et al., npj Quantum Materials 4, 60 (2019).
 E. Barts and M. Mostovoy, npj Quantum Mater. 6, 104 (2021).