Most electronics devices are structures that contain interfaces between relatively simple materials such as silicon, some sort of metal and silicon oxide, which simply acts as a insulator. However, some researchers are keen to use more complex oxides — with properties such as superconductivity, ferromagnetism and ferroelectricity — which could result in new and more efficient types of devices.

This is like putting two slices of bread on top of each other and finding that a slice of ham appears in the middle

It used to be very difficult to make tiny devices using complex oxides, but thanks to recent progress in experimental techniques, scientists can now create atomically abrupt interfaces between these materials by growing them on top of each other in sandwich-like structures.

Ultrathin superconductor

Now, Andrea Caviglia of the University of Geneva and colleagues at the University of Paris-Sud 11 and the University of Augsburg, have shown that potentially useful electronic states can be found at the interface between two complex insulating oxides: lanthanum aluminate, LaAlO3, and strontium titanate, SrTiO3 (Nature 456 624). And that these states are very sensitive to external disturbances, such as electric fields.

Their work builds on experiments done last year when the team discovered that a thin superconducting layer forms between the two insulators when atomically thin layers of the materials are grown on top of each other (Science 317 1196). "This is like putting two slices of bread on top of each other and finding that a slice of ham appears in the middle," quip the researchers.

The team has applied the same principle used in the CMOS field effect transistor to modulate the transport properties of this superconducting layer. In a CMOS field-effect transistor, an external voltage is employed to change a semiconducting channel's resistance to an electrical current. "We report that we can switch the interface from a superconducting state to an insulating state just by applying an electric field to it," said the scientists. "In other words, you can drive the system from being a perfect conductor (which offers no resistance to electrical current) to being an insulator (which has a very high resistance to electrical current) just by applying a voltage."

Faster and more efficient

A transistor made with a superconductor would carry current without any dissipation and should therefore be very efficient. Thanks to the lack of electrical resistance, the device would run faster than its semiconducting counterparts while using up much less power. Scientists have been trying to achieve such a result with high-temperature superconductors (which belong to same family as LaAlO3 and SrTiO3 because they have very similar crystal structures) for the last 20 years.

Although there are no immediate commercial applications for this research because of the very low device operation temperatures involved, the electric field effect could be applied to other materials, such as ferromagnets. "We can now imagine switching a magnetic system on and off just by applying a voltage, something that would have immediate applications in electronic devices," said the team.

The researchers next want to apply their technique in the field of nanoelectronics. "We will realize nanoscale devices in which superconductivity can be dynamically defined using local electric fields," they said.