SFB 1238 | July 21, 14:30

Topological skyrmion phases of matter

Ashley Cook

The search for novel topological phases of matter is on-going, with many recent efforts focusing on study of non-electronic and other analogues of the foundational electronic topological phases of matter of the ten-fold way. We return to study of the original electronic topological phases by introducing the topological skyrmion phases of matter. These are symmetry-protected topological phases defined by skyrmions, or topological spin textures, in momentum-space for Bloch Hamiltonians with more than two bands. These topological phases are protected by a generalized particle-hole symmetry C' present in some centrosymmetric superconductors. We first present a method for constructing various four-band toy models for skyrmion phases in two-dimensions, including models describing superconductors, and discuss some experimental signatures. We then introduce three-dimensional topological skyrmion phases and show that they display a bulk-boundary correspondence, trapping exotic localized "cross" zero-mode states at defects in the bulk and on the boundary of the system. Finally, we consider a more complex tight-binding model for spin-triplet superconductivity in transition metal oxides, which can realize topological skyrmion phases of matter in individual mirror subsectors, even for a parameter set previously used to model superconducting Sr2RuO4. Importantly, we find two types of topological phase transitions by which the skyrmion number can change. In a superconductor, the first kind occurs with a closing of the superconducting gap. The second kind, however, occurs when spin is not conserved and without the closing of direct energy gaps. As this type-II topological phase transition occurs without breaking the protecting symmetry of the topological phase in an effectively non-interacting system, it contradicts the "flat band" limit assumption of the ten-fold way classification scheme of topological phases of matter.


MPI PKS
Zoom ( URL https://uni-koeln.zoom.us/j/99781388489?pwd=U1FKR1ZRYzFTUUEwRzlodjlBbm1CZz09 )
Contact: S. Diehl