JIJITANG2014-07-02 4:26 PM


As smartphones get smarter and computers compute faster, researchers are looking for ways to speed up the processing of information. Now, scientists have made a step forward with a new class of materials that could be used in future technologies.

Researchers have discovered a quantum effect that lets electrons—the negative-charge-carrying particles that make today’s electronic devices possible—dash through the insides of these materials with very little resistance.

The discovery is the latest chapter in the story of a curious material known as a “topological insulator,” in which electrons whiz along the surface without penetrating the interior. The newest research indicates that these electrons also can flow through the interior of some of these materials.

“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,” says M. Zahid Hasan, a professor of physics at Princeton University, who led the discovery.

The first paper in Nature Communications, published May 7, demonstrates that fast electrons can flow in the interior of crystals made from cadmium and arsenic, or cadmium arsenide. The second paper, published May 12, explores fast electrons in a material made from the elements bismuth and selenium.


In most materials, including copper and other metals that conduct electricity, electrons navigate an obstacle course of microscopic outcroppings, ledges, and other imperfections that obstruct the tiny particles and send them scattering in the wrong directions. This causes resistance and the conversion of electrical current into heat, which is why electronic appliances become warm during use.

In topological insulators and the new class of materials that the researchers studied, the unique properties of the atoms combine to create quantum effects that coax electrons into acting similar to a light wave instead of like individual particles.

These waves can weave around and dodge—and even move through—barriers that would typically stop most electrons. These properties were theoretically proposed by Charles Kane and a team at the University of Pennsylvania from 2005 to 2007 and first observed experimentally in solid materials by the Hasan group in 2007 and 2008.

In 2011, the Hasan group detected this fast electron-flow in the interior of a material made from combining several elements—bismuth, thallium, sulfur, and selenium.

In the new study in cadmium arsenide, the electrons have an average velocity that is 10,000 times more than that of the previous bismuth-based materials identified by the group.

“This is a big deal,” Hasan says. “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 most promising application for these materials may be for a proposed “topological quantum computer” based on novel electronics that would use a property of electrons known as “spin” to do calculations and transmit information.


The quantum behavior in this new class of materials has led them to be called “topological Dirac semi-metals” in reference to English quantum physicist and 1933 Nobel Prize-winner Paul Dirac, who noted that electrons could behave like light.

Semi-metals that are “topological” are ones that retain their spatial electronic properties—and their speedy electrons—even when deformed by certain types of stretching and twisting.

The speeds achieved by these electrons have led to comparisons to another novel electronic material, graphene. The new class of materials has the potential to be superior to graphene in some aspects, Hasan says, because graphene is a single layer of atoms in which electrons can flow only in two dimensions. Cadmium arsenide permits electrons to flow in three dimensions.

The new study redefines what it means to be a topological material, according to Su-Yang Xu, a graduate student in Hasan’s lab and co-first author of the May 7 paper with postdoctoral research associate Madhab Neupane of Princeton and Raman Sankar of National Taiwan University.

“The term topological insulator is now quite famous, and the yet term ‘insulator’ means that there are no electrons flowing in the bulk of the material,” Xu says. “Our study shows that electrons are flowing in the bulk of the material, so clearly cadmium arsenide is not an insulator, but it is still topological in nature, so this is a totally new type of quantum matter,” he says.


The team made the discovery using a technique called angle-resolved photoemission spectroscopy. The researchers shined a very powerful X-ray beam—using a particle accelerator at the Advanced Light Source at Lawrence Berkeley National Laboratory—onto the surface of the material then monitored the electrons as they were knocked out of the interior.

“When the electron comes out, we measure its energy and velocity, and what we found is that electrons coming out of the cadmium arsenide had measurements that were similar to what is seen in particles that are massless,” Neupane says.

In the second paper, Neupane and coauthors present a model for controlling the spin direction of the electron particles in a different material, bismuth selenide.

“The Princeton group showed in exquisite details that electrons in certain solids obey the three-dimensional massless Dirac equation,” says Patrick Lee, a professor of physics at MIT who was not involved in the work.

“While predicted by theoretical calculations, this behavior has never been seen before in real materials until this past year. This work adds greatly to the ongoing excitement of how topology can impact electronic states in real materials.”

Additional researchers from Princeton, National Tsing Hua University in Taiwan, National University of Singapore, Northeastern University, and National Taiwan University contributed to the May 7 paper.

The May 12 paper includes coauthors from Princeton, Penn State, Helmholtz Centre Berlin for Materials and Energy, MAX-lab in Sweden, National Tsing Hua University in Taiwan, Academia Sinica in Taiwan, National University in Singapore, and Northeastern University.

The US Department of Energy’s Office of Basic Energy Sciences provided primary support for both studies.






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