Magnetic atoms that create exotic surface property also sow the seeds of its destruction
The discovery of "topologically protected" electrical conductivity on the surface of some materials whose bulk interior acts as an insulator was among the most sensational advances in the last decade of condensed matter physics—with predictions of numerous unusual electronic states and new potential applications. But many of these predicted phenomena have yet to be observed.
Now, a new atomic-scale study of the surface properties of one of these ferromagnetic topological insulators reveals that these materials may not be what they had seemed.
The research—conducted at the U.S. Department of Energy's Brookhaven National Laboratory and published in the Early Edition of the Proceedings of the National Academy of Sciences—revealed extreme disorder in a fundamental property of the surface electrons known as the "Dirac mass."
Like the mass imparted to fundamental particles by their interactions with the recently confirmed Higgs field, 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.
"What we have discovered is that the Dirac mass is extremely disordered at the nanoscale, which was completely unanticipated," said J.C. Séamus Davis, a senior physicist at Brookhaven Lab and a professor at Cornell University and St. Andrew's University in Scotland, who led the research. "The analogous situation in elementary particles would be if the Higgs field was random throughout space so that the electron mass (and the mass of a car or a person) was randomly different at every location. It would be an extremely chaotic universe!"
In the ferromagnetic topological insulators, Davis said, the chaos eventually destroys the exotic surface state.
"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," Davis said. "This new understanding will likely result in revisions of the basic research directions in this field."
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.
This research was funded by the DOE Office of Science.
Brookhaven National Laboratory is supported by the Office of Science of the U.S. Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.
One of ten national laboratories overseen and primarily funded by the Office of Science of the U.S. Department of Energy (DOE), Brookhaven National Laboratory conducts research in the physical, biomedical, and environmental sciences, as well as in energy technologies and national security. Brookhaven Lab also builds and operates major scientific facilities available to university, industry and government researchers. Brookhaven is operated and managed for DOE's Office of Science by Brookhaven Science Associates, a limited-liability company founded by the Research Foundation for the State University of New York on behalf of Stony Brook University, the largest academic user of Laboratory facilities, and Battelle, a nonprofit applied science and technology organization.
Karen McNulty Walsh
Public Affairs Specialist
Karen McNulty Walsh | newswise
Breakthrough with a chain of gold atoms
17.02.2017 | Universität Konstanz
New functional principle to generate the „third harmonic“
16.02.2017 | Laser Zentrum Hannover e.V.
In the field of nanoscience, an international team of physicists with participants from Konstanz has achieved a breakthrough in understanding heat transport
Cells need to repair damaged DNA in our genes to prevent the development of cancer and other diseases. Our cells therefore activate and send “repair-proteins”...
The Fraunhofer IWS Dresden and Technische Universität Dresden inaugurated their jointly operated Center for Additive Manufacturing Dresden (AMCD) with a festive ceremony on February 7, 2017. Scientists from various disciplines perform research on materials, additive manufacturing processes and innovative technologies, which build up components in a layer by layer process. This technology opens up new horizons for component design and combinations of functions. For example during fabrication, electrical conductors and sensors are already able to be additively manufactured into components. They provide information about stress conditions of a product during operation.
The 3D-printing technology, or additive manufacturing as it is often called, has long made the step out of scientific research laboratories into industrial...
Nature does amazing things with limited design materials. Grass, for example, can support its own weight, resist strong wind loads, and recover after being...
Nanometer-scale magnetic perforated grids could create new possibilities for computing. Together with international colleagues, scientists from the Helmholtz Zentrum Dresden-Rossendorf (HZDR) have shown how a cobalt grid can be reliably programmed at room temperature. In addition they discovered that for every hole ("antidot") three magnetic states can be configured. The results have been published in the journal "Scientific Reports".
Physicist Dr. Rantej Bali from the HZDR, together with scientists from Singapore and Australia, designed a special grid structure in a thin layer of cobalt in...
13.02.2017 | Event News
10.02.2017 | Event News
09.02.2017 | Event News
17.02.2017 | Medical Engineering
17.02.2017 | Medical Engineering
17.02.2017 | Health and Medicine