University of Waterloo
200 University Avenue West
Waterloo, Ontario, Canada N2L 3G1
Phone: (519) 888-4567 ext 32215
Fax: (519) 746-8115
JC Séamus Davis
Cornell University / Brookhaven Nat. Lab. / St Andrew’s University
The discovery of the surface states of topological insulators (TI) was a key advance in condensed matter physics. This is because, if an energy gap can be opened in their Dirac-like spectrum, these states should have a long list of highly exotic new properties including the quantum anomalous Hall effect (QAHE), axionic electrodynamics, and even electric-field induced magnetic monopoles. However, after almost a decade of research into TI materials, these phenomena have proven extremely elusive.
To explore this mystery, we recently introduced the Dirac-mass ‘gapmap’ technique to visualize the atomic-scale spatial conformations of Dirac mass . It reveals radically new perspectives on the physics of ferromagnetic topological insulators. By simultaneously visualizing the mass gap Δ(r) and the ferromagnetic dopant atoms, we discover intense nanoscale disorder in the Dirac-mass and demonstrate that this is directly related to fluctuations in the magnetic-dopant atom density n(r). The fact that TI QAHE phenomenology has only been detected at millikelvin temperatures, can now be understood. The key realization from these discoveries is that, in TIs dominated by atomic-scale magnetic-dopant atom phenomena, the primary chiral edge states at the sample boundary can hybridize through the percolating internal edges at the Δ(r) domain walls so that the QAHE (and all other exotic TI surface state phenomena) will be suppressed.