Resistance is low: New quantum effect

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  • Published: Jun 1, 2014
  • Author: David Bradley
  • Channels: X-ray Spectrometry
thumbnail image: Resistance is low: New quantum effect

Massless electrons

Electrons can flow rapidly due to a novel quantum effect in topological Dirac semi-metals. Credit: Image courtesy of M. Zahid Hasan and Suyang Xu

In experiments using X-ray angle-resolved photoemission spectroscopy, US researchers have discovered a new quantum effect that enables electrons to move through the interior of certain materials with very little resistance.

Technologists are forever seeking new ways to boost the performance of computer chips. One thing over which they have little control is the speed with which electrons move. However, scientists at Princeton University have now taken a step forward in developing a new class of materials that could be used in future technologies. They have discovered a new quantum effect that allows electrons to move rapidly through so-called topological insulator materials with very little resistance.

In conventional conductors, such as copper and other metals, electrons can travel through the material but must traverse microscopic obstacles such as edges and imperfections in the metal. These scatter the electrons somewhat, the root cause of electrical resistance and the reason why a large fraction of the electric current is lost as heat energy and why electronic components become warm in use.

Conducting insulators

Topological insulators show time-reversal symmetry protected edge states, which meant that electrons could scoot along the surface of such materials meeting very little resistance. The interior of such materials was seemingly inaccessible to a current.

Topological insulators were first predicted in 1986 to occur in quantum wells (very thin layers) of mercury telluride sandwiched between cadmium telluride. While band inversion in Hg(Cd)Te was reported that year by Pankratov and collaborators, topological insulators were not observed completely until 2007. The new work reveals that the negatively charged particles can also find their way into the interior where they behave more like light waves than electrons whizzing through and bypassing obstacles without loss of energy as heat. Of course, in so doing, these electrons break the definition of a topological insulator as a material on the surface of which a current might flow without accessing the insides. And, so they are an entirely new class of material, displaying a new quantum effect.

"With this discovery, instead of facing the challenge of how to use only the electrons on the surface of a material, now you can just cut the material open and you have light-like electrons flowing in three dimensions inside the materials," explains Princeton physicist Zahid Hasan. Hasan worked with colleagues Su-Yang Xu and Madhab Neupane at Princeton, and others in Germany, Singapore, Sweden and Taiwan. They have published sequential papers in Nature Communications detailing the work. The first paper demonstrates that fast electrons can flow in the interior of crystals made from cadmium and arsenic, or cadmium arsenide. The second paper details their exploration of fast electrons in a bismuth-selenium material.

Exotic quantum effects

Charles Kane and a team at the University of Pennsylvania working during the period 2005 to 2007 predicted theoretically the existence of a new class of topological insulator in which the materials' structure would conspire to create a quantum effect that would allow electrons to move like light waves rather than particles and so skirt around obstacles. The Hasan group made the first observations of this phenomenon in 2007 and 2008. In 2011, they detected fast electron flow in a bismuth, thallium, sulfur, selenium compound. But it is only now that they have essentially removed the electronic speed limit.

Their work with cadmium arsenide shows that the electrons have an average velocity that is 10,000 times higher than that of the previous bismuth-based materials identified by the group. "This is a big deal," Hasan explains. "It means the electrons can flow quite easily in the material and many more exotic quantum effects can now be studied. That just wasn't possible in the past." The studies were carried out with angle-resolved photoemission spectroscopy using the powerful X-ray beam of the Advanced Light Source at Lawrence Berkeley National Laboratory.

The most promising application for these materials may be for a proposed "topological quantum computer" based on novel electronics that would exploit electron known spin to do calculations and transfer information. Of course, it was Paul Dirac in the first half of the twentieth century, who suggested that electrons might have a wave-like character and so behave as if they were massless. The speeds observed by Hasan and colleagues are comparable to those observed in the graphite monolayer material graphene, discovered in 1962 by chemist Hanns-Peter Boehm, although first posited by nineteenth century chemist Benjamin Collins Brodie. The new class of materials will outstrip graphene in many aspects, Hasan suggests, avoiding the two-dimensionality of that supposed wonder material. Cadmium arsenide permits electrons to flow in three dimensions.

Related Links

Nature Commun, 2014, 5, #3786: "Observation of a three-dimensional topological Dirac semimetal phase in high-mobility Cd3As2"

Nature Commun, 2014, 5, #3841: "Observation of quantum-tunnelling-modulated spin texture in ultrathin topological insulator Bi2Se3 films"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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