Feromagnets short circuited by quantum phenomena

Researchers have found that feromagnetic topological insulators are ‘short circuited’ by quantum phenomena.

Discovered in the last decade, feromagnetic topological insulators conduct electricity on their surface but act as an insulator on the inside. While these materials have many practical applications, many predicted phenomena have yet to be observed.

A new atomic-scale study of the surface properties of one of these ferromagnetic topological insulators has revealed that this is because their surface conductivity is undermined by quantum phenomena. The research revealed extreme disorder in a fundamental property of the surface electrons known as the “Dirac mass” that ‘short circuited’ the surface conductivity.

Dirac mass results from surface particles’ interactions with magnetic fields. These fields are created by the presence of magnetic atoms substituted into the material’s crystal lattice to convert it into a ferromagnetic topological insulator.

The research team studied nearly perfect ferromagnetic topological insulator crystals. They used a spectroscopic imaging, scanning tunneling microscope (SI-STM) to scan the surface of these crystals atom-by-atom with 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.

J.C. Séamus Davis, a senior physicist at Brookhaven Lab and a professor at Cornell University and St. Andrews University, said:

“What we have discovered is that the Dirac mass is extremely disordered at the nanoscale, which was completely unanticipated.”

“Our findings explain why many of the electronic phenomena expected to be present in ferromagnetic topological insulators are in fact suppressed by the very atoms that generate this state, and offer insight into the true atomic-scale mechanism by which the observed properties arise. This new understanding will likely result in revisions of the basic research directions in this field.”

“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.”

The research team also included Brookhaven Lab postdoctoral fellows Inhee Lee and Chung Koo Kim and Brookhaven physicist Genda Gu.

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