'Flipping' graphene from conductor to insulator
28 May 2015
Spiralling pulses of laser light might be used to change the nature of graphene, turning it from a metal into an insulator.
A new study predicts that researchers could use spiraling pulses of laser light to change the nature of graphene, turning it from a metal into an insulator and giving it other peculiar properties that might be used to encode information.
The results, published in the journal, Nature Communications, pave the way for experiments that create and control new states of matter with this specialized form of light, with potential applications in computing and other areas.
“It’s as if we’re taking a piece of clay and turning it into gold, and when the laser pulse goes away the gold goes back to clay,” says Thomas Devereaux, a professor at the US Department of Energy’s (DoE's) SLAC National Accelerator Laboratory and director of the Stanford Institute for Materials and Energy Sciences (SIMES), a joint SLAC/Stanford venture.
“But in this case, our simulations show that we could theoretically change the electronic properties of the graphene, flipping it back and forth from a metallic state, where electrons flow freely, to an insulating state," says Devereaux. "In digital terms this is like flipping between zero and one, on and off, yes and no; it can be used to encode information in a computer memory, for instance. What makes this cool and interesting is that you could make electronic switches with light instead of electrons.”
Devereaux led the study with Michael Sentef, who began the work as a post-doctoral researcher at SLAC and is now at the Max Planck Institute for the Structure and Dynamics of Matter in Germany.
Graphene is a superb conductor of heat and electricity and one of the strongest materials known. But despite many attempts, scientists have not found a way to turn it into a semiconductor – the material at the heart of microelectronics.
An earlier study demonstrated that it might be possible to take a step in that direction by hitting a material with circularly polarized light – light that spirals either clockwise or counterclockwise as it travels, a quality that can also be described as right- or left-handedness. This would create a 'band gap' - a range of energies that electrons cannot occupy, which is one of the hallmarks of a semiconductor.
In the SIMES study, theorists used the DoE’s National Energy Research Scientific Computing Center at Lawrence Berkeley National Laboratory to perform large-scale simulations of an experiment in which graphene is hit with circularly polarized pulses of a few femtoseconds in duration.
“Previous studies were based on analytical calculations and on idealized situations,” says Martin Claassen, a Stanford graduate student in Devereaux’s group who made key contributions to the study. “This one tried to simulate what happens in as close to real experimental conditions as you can get, right down to the shape of the laser pulses. Doing such a simulation can tell you what types of experiments are feasible and identify regions where you might find the most interesting changes in those experiments.”
The simulations show that the handedness of the laser light would interact with a slight handedness in the graphene, which is not entirely uniform. This interaction leads to interesting and unexpected properties, says SLAC staff scientist and study co-author Brian Moritz.
Not only does it produce a band gap, but it also induces a quantum state in which the graphene has a so-called 'Chern number' of either one or zero, which results from a phenomenon known as Berry curvature and offers another on/off state that scientists might be able to exploit.
Going beyond graphene
While this study does not immediately open ways to make electronic devices, it does give researchers fundamental insights that advance the science in that direction. The results are also relevant to materials called dichalcogenides, which are also two-dimensional sheets of atoms arranged in a honeycomb structure.
Dichalcogenides are the focus of intense research at SIMES and around the world because of their potential for creating 'valleytronic' devices. In valleytronics, electrons move through a two-dimensional semiconductor as a wave with two energy valleys whose characteristics can be used to encode information.
Possible applications include light detectors, low-energy computer logic and data storage chips and quantum computing. In addition to the work on graphene, members of the research team have also been simulating experiments involving the interaction of light with dichalcogenides.
“Ultimately we’re trying to understand how interaction with light can alter a material’s character and properties to create something that’s both new and interesting from a technological point of view,” Moritz adds.
In addition to SLAC, Stanford, SIMES and the Max Planck Institute for the Structure and Dynamics of Matter, other members of the research team were from Berkeley Lab, the University of Tokyo and Georgetown University.
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