Self-destructive effects of magnetically-doped ferromagnetic topological insulators
Magnetic atoms that create exotic surface property also sow the seeds of its destruction
Proceedings of the National Academy of Sciences
Precision studies
Under Davis' guidance, Brookhaven Lab postdoctoral fellows Inhee Lee and Chung Koo Kim studied nearly perfect ferromagnetic topological insulator crystals grown by Brookhaven physicist Genda Gu. They used a spectroscopic imaging, scanning tunneling microscope (SI-STM) designed and built by Davis at Brookhaven to scan the surface of these crystals atom-by-atom. This tool has the precision to simultaneously reveal the positions of the magnetic dopant atoms and the resulting Dirac mass.
Prior to this work, scientists had assumed that these magnetic dopant atoms were not detrimental to the topological surface states. But no one had directly studied how the spatial arrangements of the magnetic dopant atoms at the atomic scale influenced the Dirac-mass because there were no reliable techniques to do so, until now.
The new atom-by-atom SI-STM data revealed not only the intense nanoscale disorder in the Dirac mass, but also showed that this disorder is directly related to fluctuations in the density of the magnetic dopant atoms on different parts of the crystal surface. In the paper, the scientists also provide the first direct evidence for the actual mechanism of how surface ferromagnetism arises in a topological insulator, and determine directly the strength of the surface-state magnetic-dopant interactions.
"The Dirac-mass 'gapmap' technique introduced here reveals radically new perspectives on the physics of ferromagnetic topological insulators," Davis said.
"The key realization from these discoveries--aside from a clear and direct picture of what is going on at the atomic scale - is that, in ferromagnetic topological insulators dominated by this magnetic-dopant atom phenomena, many of the exotic and potentially valuable phenomena expected for these materials are actually being quantum mechanically short circuited by the random variations of Dirac mass," he said.
Of course, there may still be a way to achieve all the exotic physics expected of ferromagnetic topological insulators- if scientists can develop ways to control the dopant-induced Dirac-mass gap disorder. Hence the idea of a whole new research direction for this field.
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