With this year’s Nobel Prize in Physics going to three physicists for their “theoretical discoveries of topological phase transitions and topological phases of matter,” it would appear that things are looking up for the nascent prospects of topological insulators.
Topological insulators (TIs) are materials that behave like conductors near their surfaces but act as insulators throughout the bulk of their interiors. While such materials had long been thought theoretically possible, only recently have research labs around the world begun producing materials with these properties. This has buoyed hopes that they could someday be used in technologies ranging from “spintronics” to quantum computers.
Now an international team of researchers from the National Institute of Standards and Technology (NIST), the University of California Los Angeles (UCLA), and the Beijing Institute of Technology in China have developed a way that makes it far easier to magnetize TIs, improving the odds that they’ll be applied to computing.
One of the main issues with TIs is that bringing out their unusual physical properties requires that they be kept at extremely low temperatures. Another potential showstopper has been magnetizing the material. There have been two basic ways of achieving this magnetization: Either you dope the TI with a small amount of magnetic material, or you create a layered structure using a magnetic material known as a ferromagnet. Both of these approaches have their downsides. The layered approach can create a magnetic field so powerful that it overpowers the TIs. And the doping method also disrupts the TI’s properties.
In research described in the journal Nature Materials, the researchers took the layering approach. But instead of using alternate layers of a ferromagnetic material, they used an antiferromagnetic (AFM) material. In a typical magnetic material, the atoms all have north poles pointing in the same direction. However, in AFM materials, the north poles of one layer point in one direction, and then in the opposite direction in the next layer. When multiple layers are stacked, the magnetism is canceled out.
So, what the researchers sought to achieve is exploiting the magnetism of the outermost layer of the AFM material. The research demonstrated that this single outer layer was enough to magnetize the TI without overwhelming its attractive properties.
Another benefit of using the AFM material is that it meant that the TI displayed the desired properties at slightly warmer temperatures (though 77 Kelvin still can’t be considered balmy). This allows the scientists to use liquid nitrogen to keep the material cold rather than liquid helium, so the logistics of studying the material are simpler.
“It makes them far easier to study,” said Alex Grutter of the NIST Center for Neutron Research, in a press release. “Not only can we explore TIs’ properties more easily, but we’re excited because to a physicist, finding one way to increase the operational temperature this dramatically suggests there might be other accessible ways to increase it again. Suddenly, room temperature TIs don’t look as far out of reach.”
With this incremental step, we shouldn’t expect to see spintronic devices and quantum computers that exploit TIs anytime soon, but at least we have been given a ray of hope to dream of such a day.
Dexter Johnson is a contributing editor at IEEE Spectrum, with a focus on nanotechnology.