'Topological insulators' promising for spintronics, quantum computers

November 13, 2014  

Yang Xu

Purdue University doctoral student Yang Xu, lead author of a new research paper on "topological insulators," an emerging class of materials that could make possible "spintronic" devices and practical quantum computers far more powerful than today's technologies, is shown here inspecting devices made from topological insulators under a microscope before electrical measurements. (Purdue University photo / Ting-fung Chung)
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WEST LAFAYETTE, Ind. – Researchers have uncovered "smoking-gun" evidence to confirm the workings of an emerging class of materials that could make possible "spintronic" devices and practical quantum computers far more powerful than today's technologies.

The materials are called "topological insulators." Unlike ordinary materials that are either insulators or conductors, topological insulators are in some sense both at the same time - they are insulators inside but always conduct electricity via the surface. Specifically, the researchers have reported the clearest demonstration of such seemingly paradoxical conducting properties and observed the "half integer quantum Hall effect" on the surface of a topological insulator.

"This is unambiguous smoking-gun evidence to confirm theoretical predictions for the conduction of electrons in these materials," said Purdue University doctoral student Yang Xu, lead author of a paper appearing this week in the journal Nature Physics.

Yong P. Chen, a Purdue associate professor of physics and astronomy and electrical and computer engineering, led a team of researchers from Purdue, Princeton University and the University of Texas at Austin in studying the bismuth-based material.

"This experimental system provides an excellent platform to pursue a plethora of exotic physics and novel device applications predicted for topological insulators," Chen said.

For example, by further combining topological insulators with a superconductor, which conducts electricity with no resistance, researchers may be able to build a practical quantum computer. Such a technology would perform calculations using the laws of quantum mechanics, making for computers much faster than conventional computers at certain tasks such as database searches and code-breaking.

"One of the main problems with prototype quantum computers developed so far is that they are prone to errors," Chen said. "But if topologically protected, there is a mechanism to fundamentally suppress those errors, leading to a robust way to do quantum computing."

The topological insulators were synthesized at Purdue and fabricated into electrical devices at the Birck Nanotechnology Center in the university's Discovery Park.

The researchers for the first time demonstrated a three-dimensional material with an electrical resistance not dependent on the thickness of the material, a departure from conventional behavior. Whereas electrons usually have a mass, in the case of topological insulators the conducting electrons on the surface have no mass and are automatically "spin polarized," leading to the unique half-integer quantum Hall effect observed and also making the material promising for various potential applications.

Topological insulators could bring future computing platforms based on "spintronics." Conventional computers use the presence and absence of electric charges to represent ones and zeroes in a binary code needed to carry out computations. Spintronics, however, uses the "spin state" of electrons to represent ones and zeros.

"Compounds based on bismuth, antimony, telluride and selenide are the cleanest and most intrinsic topological insulators demonstrated so far, with no measurable amount of undesirable conduction inside the bulk that often spoils the topological conduction properties in earlier topological insulator materials," Chen said. 

The researchers also found evidence consistent with the conduction of electrons being "topologically protected," meaning its surface is guaranteed to be a robust conductor. Studying thin-slab-shaped samples cut from this material down to ever decreasing thickness while observing the conductance, the researchers found that the conductance - which occurs always and only at the surface - barely changes.

"For the thinnest samples, such topological conduction properties were even observed at room temperature, paving the way for practical applications," Xu said.

The paper was authored by Xu; Purdue research scientist Ireneusz Miotkowski, who created the high-quality materials; Princeton postdoctoral research associate Chang Liu; Purdue postdoctoral research associate Jifa Tian; UT Austin graduate student Hyoungdo Nam; Princeton graduate student Nasser Alidoust; Purdue graduate student Jiuning Hu; Chih-Kang Shih, Jane and Roland Blumberg Professor at UT Austin; M. Zahid Hasan, a Princeton professor of physics; and Chen.

In addition to the material growth and electrical measurements performed by the Purdue researchers, the Princeton and UT Austin groups contributed to this study by performing advanced characterizations that further confirmed important properties of the material as a topological insulator.

The research was funded by the Defense Advanced Research Projects Agency, which supports a Purdue-led program with participation from Princeton and other institutions aiming to develop energy efficient electronic devices based on topological insulators. The electrical measurements revealing the signature half-integer quantum Hall effect were performed at the National Science Foundation's National High Magnetic Field Laboratory. UT Austin's contribution to this study was supported the Welch Foundation and U.S. Army Research Office. 

Writer: Emil Venere, 765-494-4709, venere@purdue.edu 

Source: Yong P. Chen, 765-494-0947, yongchen@purdue.edu

Note to Journalists: The research paper is available from the Nature Press Office at press@nature.com, 212-726-9231


Observation of topological surface state quantum Hall effect in an intrinsic three-dimensional topological insulator

Yang Xu1,2, Ireneusz Miotkowski1, Chang Liu3,4, Jifa Tian1,2, Hyoungdo Nam5, Nasser Alidoust3,4, Jiuning Hu2,6, Chih-Kang Shih5, M. Zahid Hasan3, 4, Yong P. Chen1,2,6,*

 1Department of Physics and Astronomy, Purdue University

2Birck Nanotechnology Center, Purdue University

3Joseph Henry Laboratories, Department of Physics, Princeton University

4 Princeton Institute for Science and Technology of Materials, Princeton University

5 Department of Physics, University of Texas at Austin

6 School of Electrical and Computer Engineering, Purdue University

*Correspondence to: yongchen@purdue.edu 

A three-dimensional (3D) topological insulator (TI) is a quantum state of matter with a gapped insulating bulk yet a conducting surface hosting topologically-protected gapless surface states. One of the most distinct electronic transport signatures predicted for such topological surface states (TSS) is a well-defined half-integer quantum Hall effect (QHE) in a magnetic field, where the surface Hall conductivities become quantized in units of (1/2)e2/h (e being the electron charge, h the Planck constant) concomitant with vanishing resistance. Here, we observe well-developed QHE arising from TSS in an intrinsic TI of BiSbTeSe2. Our samples exhibit surface dominated conduction even close to room temperature, while the bulk conduction is negligible. At low temperatures and high magnetic fields perpendicular to the top and bottom surfaces, we observe well-developed integer quantized Hall plateaus, where the two parallel surfaces each contributing a half integer e2/h quantized Hall (QH) conductance, accompanied by vanishing longitudinal resistance. When the bottom surface is gated to match the top surface in carrier density, only odd integer QH plateaus are observed, representing a half-integer QHE of two degenerate Dirac gases. This system provides an excellent platform to pursue a plethora of exotic physics and novel device applications predicted for TIs, ranging from magnetic monopoles and Majorana particles to dissipationless electronics and fault-tolerant quantum computers.

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