Electrons in solids need not behave exactly like electrons in free space. In particular, they may have an 'effective mass' which is different from the real electron mass, and describes the way the electron in the solid responds to a force. Recently, materials have been discovered in which the electron mass is exactly zero. In such materials the electron moves at a constant speed regardless of its energy, akin to particles of light which travel at constant speed. These materials are called 'Dirac materials' because the electron motion is described by the same Dirac equation which describes electrons moving close to the speed of light. Dirac materials include graphene (a single atom thick plane of carbon atoms) and the metallic surface of bismuth selenide. This project will study how defects in graphene and bismuth selenide can be used to change the electronic properties of the materials, such as inducing magnetism or a transition from conducting to insulating behavior.

Technical Abstract

This project will study the surface modification and resulting modification of electronic transport properties of two Dirac electronic materials systems, bilayer graphene and strong topological insulator Bi2Se3. Bilayer graphene (BLG) will be prepared by mechanical exfoliation, and irradiated with He+ ions in UHV to induce vacancies. Differences in cross section for atom removal are expected to result in breaking the top-bottom and A-B symmetries in defect concentration. Further symmetry-breaking could be provided by gating. Broken sublattice symmetry of defects is expected to lead to ferromagnetism in BLG via the RKKY effect, and this effort will search for evidence of ferromagnetism in defective BLG in anisotropic magnetoresistance as well as in spin-injection experiments. Thin films of Bi2Se3 will be prepared by molecular beam epitaxy on insulating substrates, and electronic transport measurements will be performed in situ on the films without breaking vacuum. The central issues that will be studies are interface and bulk doping in as-prepared films, doping upon annealing or exposure to ambient, and scattering by magnetic and non-magnetic impurities.

National Science Foundation (NSF)
Division of Materials Research (DMR)
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Guebre X. Tessema
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University of Maryland College Park
College Park
United States
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