Exotic Topological Insulators Electrify Physics
What's Hot in January/February 2011
By Dr. Simon Mitton
The physics Top Ten is fired up this period by the palpable excitement generated by the entry of a paper (#6) devoted to topological insulators. These are exotic materials that are insulators in the bulk, but their remarkable surface conditions lead to new phases, including unhindered unidirectional electron flow at the surface. This type of material allows electrons to travel at room temperature without loss of energy. Papers describing developments in this emerging field point to many remarkable possibilities for applications in condensed matter physics and nanotechnology, as well as even faster computer microchips.
Wilkinson Microwave Anisotropy Probe (see papers #1 and
#2) has journeyed into deep space on a voyage to explore some of the
deepest mysteries of the cosmos. Scientists can determine much about the
content, shape, history, and the ultimate fate of the Universe by
constructing a full-sky picture of the oldest light in the
Universe.
Credit: NASA /
WMAP Science Team.
To understand this new field, we should begin with graphene, a 2D form of carbon that is electronically insulating. Once theorists understood how to apply band theory to graphene they used the language of topology to explain the physics. Graphene is a 2D topological insulator known as a quantum spin Hall insulator. It was predicted in 2005 to exist in this phase state and was observed two years later in quantum well structures in compounds made from elements much heavier than C.
Theorists, who in fact have been taking the lead in this new research field, then asked if the novel insulators could have a 3D configuration, and indeed that seemed possible. But carbon is a light element with a small band gap, so a quest was launched to see if the physics took place in heavy materials with strong spin-orbit interactions, a required property of topological insulators. Soon, three groups of theorists independently discovered a generalization in 3D of the insulator state, and that was created in the laboratory using a semiconducting bismuth antimony alloy.
It turned out, however, that BiSb alloys have a complicated surface structure and a low band gap energy, properties that made it difficult to think of useful applications in spintronics and electronics. The disappointments led investigators, who were guided by theorists, to experiment with Bi2Se3, Bi2Te3, and Sb2Te3, by making detailed investigations of their surface structures. An important advantage of these materials is that they are pure compounds (i.e., stoichiometric) rather than alloys, and so can be made to a high degree of purity.
In Hot Paper #6 Yulin Chen, Zhi-Xun Shen and their colleagues in Stanford and Beijing describe the behavior of surface electrons in Bi2Se3. By using X-rays to examine samples they found clear signs of the free flow of electrons at the surface. This demonstrates that Bi2Se3 is an ideal candidate for the simplest kind of topological insulator. In fact the set-up showed that Bi2Se3 performed at higher temperatures than the theorists had expected. That’s important because it will simplify the development of future application.
"The physics Top Ten is fired up this period by the palpable excitement generated by the entry of a paper (#6, [in the tab below]) devoted to topological insulators."
As the paper points out, pure crystals of Bi2Se3 are bulk insulators, but real crystals have imperfections that may allow leakage of current. The experimenters fixed that by doping the crystals with 0.67% Sn. The history of condensed matter physics suggests that for applications, the production of high-quality materials needs to be perfected. In this case the ability to tune the energy levels in the bulk material by doping is a promising step forward.
The high citation rate for this paper is witness to the emergence of topological insulators as a new aspect of condensed matter physics. Physicists have known for some time that the electric field and magnetic field are coupled inside some insulators. What is new here is that in a topological insulator, this coupling is quantized, and that gives rise to many possibilities for tuning by doping with small amounts of magnetic material. Their unusual surfaces offer all kinds of possibilities in spintronics, and researchers have barely scratched the surface. Also, the new topological insulators provide a development platform for new semiconductors, and hence commercial applications. There is optimism in this field, and it should develop in exciting new directions.
Dr. Simon Mitton is a science writer and Fellow of St. Edmund’s College, University of Cambridge, U.K.
What's Hot in Physics | |||
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Rank | Paper |
Cites This Period Jul-Aug 10 |
Rank Last Period May-Jun 10 |
1 | E. Komatsu, et al., "Five-year Wilkinson Microwave Anisotropy Probe observations: Cosmological interpretation," Astrophys. J. Suppl. Ser., 180(2): 330-76, February 2009. [14 institutions worldwide] *406EI | 143 | 1 |
2 | J. Dunkley, et al., "Five-year Wilkinson Microwave Anisotropy Probe observations: Likelihoods and parameters from the WMAP data," Astrophys. J. Suppl. Ser., 180(2): 306-29, February 2009. [14 U.S. and Canadian institutions] *406EI | 48 | 2 |
3 | S.H. Park, et al., "Bulk heterojunction solar cells with internal quantum efficiency approaching 100%," Nature Photonics, 3(5): 297-302, May 2009. [U. Calif., Santa Barbara; Gwangju Inst. Sci. & Tech., S. Korea; U. Laval, Quebec City, Canada] *447UY | 47 | 3 |
4 | K.N. Abazajian, et al., "The Seventh Data Release of the Sloan Digital Sky Survey," Astrophys. J. Suppl. Ser., 182(2): 543-58, June 2009. [110 institutions worldwide] *448UE | 43 | † |
5 | O. Adriani, et al., "An anomalous positron abundance in cosmic rays with energies 1.5-100 GeV," Nature, 458(7238): 607-9, 2 April 2009. [17 institutions worldwide] *427RK | 38 | 4 |
6 | Y.L. Chen, et al., "Experimental realization of a three-dimensional topological insulator, Bi2Te3", Science, 325(5937): 178-81, 10 July 2009. [Stanford U., CA; Lawrence Berkeley Natl. Lab., CA; Chinese Acad. Sci., Beijing] *468FK | 36 | † |
7 | F.-C. Hsu, et al., "Superconductivity in the PbO-type structure alpha-FeSe," PNAS, 105(38): 14262-4, 23 September 2008. [Acad. Sinica, Taipei, Taiwan; Natl. Tsing Hua U., Hsinchu, Taiwan; Duke U., Durham, NC] *353TY | 35 | 5 |
8 | H.Y. Chen, et al., "Polymer solar cells with enhanced open-circuit voltage and efficiency," Nature Photonics, 3(11): 649-53, November 2009. [Solarmer Energy, Inc., El Monte, CA; U. Calif., Los Angeles; U. Chicago, IL] *526PG | 30 | † |
9 | M. Kowalski, et al., "Improved cosmological constraints from new, old, and combined supernova data sets," Astrophys. J., 686(2): 749-78, 20 October 2008. [41 institutions worldwide] *364YB | 29 | 6 |
10 | K.K. Ni, et al., "A high phase-space-density gas of polar molecules," Science, 322(5899): 231-5. 10 October 2008. [U. Colorado, Boulder; Temple U., Phila., PA; NIST and U. Maryland, Gaithersburg] *358FK | 29 | 9 |
SOURCE: Thomson Reuters Hot Papers Database. Read the Legend. |