A new material with a split personality -- part superconductor, part metal -- has been observed by a Princeton University-led research team.
The discovery may have implications for the development of next-generation electronics that could transform the way information is stored and processed.
The new material -- a crystal called a topological superconductor -- has two electronic identities at once. At very low temperatures, the interior of the crystal behaves like a normal superconductor, able to conduct electricity with zero resistance. At the same time, the surface is metallic, able to carry a current, albeit with some resistance.
This is in direct contrast to most existing materials that are classified as electronic states of matter, including metals, insulators and conventional superconductors, which are consistent in how they do, or don't, conduct electricity. For example, every single atom of every single copper wire is able to carry a current, which dissipates a bit as it travels. Similarly, all the molecules in normal superconductors conduct electricity without resistance when the material is placed at the appropriate temperature.
"The known states of electronic matter are insulators, metals, magnets, semiconductors and superconductors, and each of them has brought us new technology," said M. Zahid Hasan, an associate professor of physics at Princeton who led the research team. "Topological superconductors are superconducting everywhere but on the surface, where they are metallic; this leads to many possibilities for applications."
Hasan and his colleagues published their findings Nov. 1 in the journal Nature Physics.
According to Hasan, one of the most exciting potential uses for the material would be in energy-efficient quantum computers that would have the ability to identify errors in calculation as they occur and resist them during processing. The successful development of such machines is thought to hinge on catching and manipulating elusive particles called Majorana fermions, which were first predicted more than 70 years ago but never before observed, Hasan explained. The split electronic personality of the new superconductors with unusual surface properties, when placed in contact with a special kind of insulator, may enable scientists to coax the electrons whizzing about on the surface to become Majorana fermions, he added.
"These highly unusual superconductors are the most ideal nurseries to create and manipulate Majorana fermions, which could be used to do quantum computing in a fault-resistant way " said L. Andrew Wray, the first author of the paper, who received his doctoral degree from Princeton in 2010. "And because the particles would exist on a superconductor, it could be possible to manipulate them in low power-consumption devices that are not only 'green,' but also immune to the overheating problems that befall current silicon-based electronics."
The significant caveat is that any potential application could be several decades in development.
"Of course, it takes time to go from new physics to new technology -- usually 20 to 30 years, as was the case with semiconductors," Hasan said.
Initial find of insulators begins path to discovery
In 2007, a Hasan-led research team reported the discovery of three-dimensional topological insulators -- a strange breed of insulator with a metallic surface. While three-dimensional topological insulators may have potential for use in next-generation electronics, their properties alone are not ideal for use in quantum computers, Hasan said.
Quantum computers store and process information using the "quantum" behavior of subatomic particles -- phenomena that occur on the ultrasmall scale and are completely at odds with the world that can be seen by the naked eye, such as the ability of electrons to be in two different places at the same time. Quantum computers could one day enable the manipulation of data at speeds that far exceed today's conventional machines, which are rapidly approaching the fundamental limits of their computing capabilities.
However, efforts to create higher-performing quantum computers have been hampered by the notoriously fickle and unpredictable behavior of particles on the quantum scale.
For the past two years, Hasan and his collaborators have been tweaking the properties of a topological insulator called bismuth selenide to create a material with a metallic surface and a superconducting interior, which would have properties well suited to exploitation in the electronics of the future.
To make a superconductor with topological behavior, or unusual surface properties, Princeton chemistry professor Robert Cava and his research group invented a new kind of crystal by inserting atoms of copper into the atomic lattice structure of a semiconductor made out of the compound bismuth selenide. This process, called intercalation doping, is a method used to change the number of electrons in a material and tweak its electrical properties.
The scientists discovered that, with the right amount of doping, they were able to turn the crystal into a superconductor at very low temperatures -- below 4 degrees Kelvin, or around -452 degrees Fahrenheit. However, initial laboratory-based results suggested that the superconductor cannot retain topological properties indefinitely, though they do persist for months if the material is kept in a vacuum.
To assess the topological characteristics of the material, the researchers used a technique known as X-ray spectroscopy to bombard the crystal with X-rays and "pop" individual electrons out of the material. These electrons were then analyzed, providing a series of clues that allowed the team to determine the true nature of the crystal.