Enumeration of SpinSpace Groups: Toward a Complete Description of Symmetries of Magnetic Orders
DIPC Seminars
 Speaker

Chen Fang
Key Laboratory of Condensed Matter Theory and Computation, Institute of Physics, Chinese Academy of Sciences  When

2024/07/15
12:00  Place
 CFM Seminar Room
 Host
 Bogdan A. Bernevig
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Symmetries of threedimensional periodic scalar fields are described by 230 space groups (SGs). Symmetries of threedimensional periodic (pseudo)vector fields, however, are described by the spinspace groups (SSGs), which were initially used to describe the symmetries of magnetic orders. In SSGs, the real space and spin degrees of freedom are unlocked in the sense that an operation could have different spatial and spin rotations. SSGs give a complete symmetry description of magnetic structures and have natural applications in the band theory of itinerary electrons in magnetically ordered systems with weak spinorbit coupling. Altermagnetism, a concept raised recently that belongs to the symmetrycompensated collinear magnetic orders but has nonrelativistic spin splitting, is well described by SSGs. Because of the vast number and complicated group structures, SSGs have not yet been systematically enumerated. In this work, we exhaust SSGs based on the invariant subgroups of SGs, with spin operations constructed from three dimensional (3D) real representations of the quotient groups for the invariant subgroups. For collinear and coplanar magnetic orders, the spin operations can be reduced into lowerdimensional real representations. As the number of SSGs is infinite, we consider only SSGs that describe magnetic unit cells up to 12 times crystal unit cells. We obtain 157289 noncoplanar, 24 788 coplanarnoncollinear, and 1421 collinear SSGs. The enumerated SSGs are stored in an online database with a userfriendly interface. We develop an algorithm to identify SSGs for realistic materials and find SSGs for 1626 magnetic materials. We also discuss several potential applications of SSGs, including the representation theory, topological states protected by SSGs, structures of spin textures, and refinement of magnetic neutron diffraction patterns using SSGs. Our results serve as a solid starting point for further studies of symmetry and topology in magnetically ordered materials.