The research, led by Nitin Samarth, Professor and Downsbrough Head of Physics at Penn State and David D. Awschalom, Liew Family Professor and deputy director in the Institute of Molecular Engineering at the University of Chicago, was published on October 9, 2015 inScience Advances, the new online journal of the American Association for the Advancement of Science, where it was featured on the journal's front page.
The new technique is more flexible than advanced nanofabrication facilities based on chemical processing because it allows for rewritable "optical fabrication" of the topological insulators. "This observation came as a complete surprise", David D. Awschalom stated. "It's one of those rare moments in experimental science where a seemingly random event - turning on the room lights - generated unexpected effects with potentially important impacts in science and technology."
The electrons in topological insulators have unique quantum properties that many scientists believe will be useful for developing spin-based electronics and quantum computers. However, making even the simplest experimental circuits with these materials has proved difficult because traditional semiconductor engineering techniques tend to destroy their fragile quantum properties. Even a brief exposure to air can reduce their quality.
InScience Advances, the researchers report the discovery of an optical effect that allows them to "tune" the energy of electrons in these materials using light, and without ever having to touch the material itself. They have used this effect to draw and erase one of the central components of a transistor - the p-n junction - in a topological insulator for the first time.
Like many advances in science, the path to this discovery had an unexpected twist. "To be honest, we were trying to study something completely different", stated Andrew Yeats, a graduate student in David D. Awschalom's laboratory and the paper's lead author. "There was a slow drift in our measurements that we traced to a particular type of fluorescent lights in our lab. At first we were glad to be rid of it, and then it struck us - our room lights were doing something that people work very hard to do in these materials."
The researchers went back to Bulley & Andrews, the contractor that renovated the lab space for more information about the lights. "I've never had a client so obsessed with the overhead lighting", stated Frank Floss, superintendent for Bulley & Andrews Construction. "I could have never imagined how important it would turn out to be."
The researchers found that the surface of strontium titanate, the substrate material on which they had grown their samples, becomes electrically polarized when exposed to ultraviolet light, and their room lights happened to emit it at just the right wavelength. The electric field from the polarized strontium titanate was leaking into the topological insulator layer, changing its electronic properties.
David D. Awschalom and his colleagues found that, by intentionally focusing beams of light on their samples, they could draw electronic structures that persisted long after the light was removed. "It's like having a sort of quantum Etch-a-Sketch in our lab", he stated. They also found that bright red light counteracted the effect of the ultraviolet light, allowing them to both write and erase. "Instead of spending weeks in the clean room and potentially contaminating our materials", David D. Awschalom said, "now we can sketch and measure devices for our experiments in real time. When we're done, we just erase it and make something else. We can do this in less than a second."
At Penn State, Nitin Samarth stated: "One exciting aspect of this work is that it's noninvasive. Since the electrical polarization occurs in an adjacent material, and the effect persists in the dark, the topological insulator remains relatively undisturbed." Nitin Samarth added: "With these fragile quantum materials, sometimes you have to use a light touch."
To test whether the new technique might interfere with the unique properties of topological insulators, the team measured their samples in high magnetic fields. They found promising signatures of an effect called weak anti-localization, which arises from quantum interference between the different simultaneous paths that electrons can take through a material when they behave as waves.
To better understand the physics behind the effect, the researchers conducted a number of control measurements which showed that the optical effect is not unique to topological insulators, but that it can act on other materials grown on strontium titanate, as well.
"In a way, the most exciting aspect of this work is that it should be applicable to a wide range of nanoscale materials such as complex oxides, graphene, and transition metal dichalcogenides", stated David D. Awschalom. "It's not just that it's faster and easier. This effect could allow electrical tuning of materials in a wide range of optical, magnetic, and spectroscopic experiments where electrical contacts are extremely difficult or simply impossible."
The research was supported by the U.S. Office of Naval Research, Air Force Office of Scientific Research, and Army Research Office.
A. L. Yeats, Y. Pan, A. Richardella, P. J. Mintun, N. Samarth, D. D. Awschalom are the authors of "Persistent Optical Gating of a Topological Insulator". The paper appears inScience Advances1, e1500640 (2015). DOI: 10.1126/sciadv.1500640.