Topological states of matter are typically found in systems where electron-electron interactions are weak. The electrons flow independently, their wave functions wind around each other in patterns protected by symmetry, and the topology is a property of the single-particle band structure. Strong interactions between electrons — the kind found in heavy-fermion materials where electrons behave as though they are hundreds of times more massive than free electrons — were expected to destroy topological order. When particles interact strongly, the single-particle picture breaks down, and with it the framework in which topology is defined.
Published in Nature Physics, Qimiao Si at Rice University and Silke Paschen at the Vienna University of Technology demonstrated the opposite. In a heavy-fermion material, strong electron interactions do not destroy topology — they create it. The topological state exists because of the interactions, not despite them.
The structural insight is about the relationship between destruction and creation at the same site. The strong interactions that collapse the single-particle band structure — the framework in which topology was originally defined — simultaneously build a new framework in which a different topology emerges. The old topology requires non-interacting electrons. The new topology requires strongly interacting electrons. The same parameter (interaction strength) destroys one and creates the other.
This is a phase transition in the definition of the phenomenon. Topology was defined in terms that made it incompatible with strong interactions. The experimental finding required redefining topology in terms that accommodate interactions. The obstacle was not physical but conceptual: the phenomenon existed in the material before it existed in the theory. The interactions were always there. The topology was always there. The incompatibility was in the description.