Nano Focus: Topological insulator Bi 2 Se 3 opens path to room-temperature spintronics
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pant also acts to protect the device, endowing it with superior environmental stability as compared with pristine graphene solar cells, which degrade over time. The factor of 4.5 increase in efficiency achieved though TFSA doping represents a significant improvement in performance, which could lead to these
cells acting as viable alternatives to expensive silicon diode cells and less stable organic cells. Alternatively, the doped graphene layer could itself be applied to a range of other substrates including flexible polymer semiconductors. Tobias Lockwood
Nano Focus Topological insulator Bi2Se3 opens path to room-temperature spintronics
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n the search for new materials with improved electrical conductivity, a team of researchers led by Tonica Valla of Brookhaven National Laboratory has found a potential candidate in the topological insulator Bi2Se3. Electrons on the metal surface of a topological insulator can flow with little resistance. Using angle-resolved photoemission spectroscopy (ARPES) at Brookhaven’s National Synchrotron Light Source and at the Advanced Light Source at Lawrence Berkeley National Laboratory (LBNL), the researchers discovered that the surface electrons of Bi2Se3 can flow at room temperature, making it an attractive candidate for practical applications like spintronics devices, plus farther-out ones like quantum computers. As reported in the May 4 issue of Physical Review Letters (10.1103/ PhysRevLett.108.187001), Valla, Alexei Fedorov of LBNL, Young Lee of the Massachusetts Institute of Technology, and their colleagues generated a direct graphic visualization of the sample’s electronic structure. The band structure of the surface states of a topological insulator like Bi2Se3 appear as two cones that meet at a point, called the Dirac point. There is no gap between the valence and conduction bands, only a smooth transition with increasing energy. This is similar to the band structure of graphene in which ARPES diagrams look like slices through the cones, an X centered on the Dirac point. Although graphene and topological
ARPES maps the electronic properties, including the band structure and Fermi surface, of the topological insulator bismuth selenide (left). Like graphene, the lower energy valence band of a topological insulator meets the higher energy conduction band at a point, the Dirac point, with no gap between the bands (center). Unlike graphene, however, the Fermi surface of a topological insulator does not usually pass through the Dirac point. For surface electrons, distinct spin states (red arrows) are associated with each different orientation in momentum space (right).
insulators have similar band structures, their other electronic characteristics are very different. The combinations of different speeds and orientations equivalent to a material’s highest particle energies (at zero degrees) make up its momentum space, mapped by the Fermi surface. While the Fermi surface of graphene lies between the conical bands at the Dirac point, this is not true of topological insulators. The
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