Flexing an Electron Gas
Flexoelectricity is a polarization that occurs in insulators when they are bent or stretched in a nonuniform way. The effect is usually small, but recent work has explored using it in certain photovoltaic materials. Now researchers have shown that flexing a metal oxide structure—consisting of a lanthanum aluminate (LAO) film above a strontium titanate (STO) substrate—can alter the resistance of a 2D electron gas trapped between the layers. This flexoelectric effect might be useful in future energy-harvesting devices or mechanical sensors.
Thanks to their compact nature and ultralow resistance, 2D electron gases are attractive for electronic device applications. The electron gas in LAO/STO structures is especially interesting, as it exhibits superconductivity and ferromagnetism (see 25 July 2011 Viewpoint). Previous work has shown that the electronic properties of LAO/STO are sensitive to various inputs, such as light and spin-polarized currents. Flexoelectricity offers a new “handle” for manipulating the LAO/STO system.
Ji-yan Dai from The Hong Kong Polytechnic University and colleagues bent an LAO/STO structure while monitoring the resistance through the electron gas. They discovered that the resistance in the gas increased by as much as 3% when the structure’s edges were bent down, forming a “hill” shape. Flexing the sample in a “valley” shape decreased the resistance by as much as 0.3%. The team showed that these resistance changes were caused by flex-induced polarization in the thin film of LAO. The electric field from this polarization altered the resistance in the electron gas by driving electrons into or out of the LAO/STO interface, depending on the flex orientation.
This research is published in Physical Review Letters.
Michael Schirber is a Corresponding Editor for Physics based in Lyon, France.