Mysterious Mott transition could make better electronics

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By eeNews Europe

The Mott transition, a not well-understood phenomenon, is known to occur in transition metal chalcogenides and transition metal oxides. Generally thought of as a being thermally driven, the insulator-metal transition can also be modified by electrical current, pressure and doping and by quantum confinement within atomic-scale thin layers

The latest discovery was made by an international team of researchers that included the MESA+ Insitute at the University of Twente in the Netherlands and the U.S. Department of Energy’s Argonne National Laboratory.

It connects the classical and quantum mechanical views of the transition together with shedding light on non-equilibrium physics, which is itself poorly understood and yet governs most real-world systems. Much of physics is based on the primary assumption that a system has settled to a steady state and is in equilibrium. The finding may also represent a step towards developing a more efficient form of electronics based on the Mott transition devices, according to the researchers.

Although the researchers had set up the experiment to study pinned or mobile magnetic vortices that are part of the signature of superconductivity they recognized that system behaved like a Mott transition driven by electronic current rather than by temperature.

Whether or not the Mott transition is a classical or quantum phenomenon is not well understood and researchers have not until now directly observed a Mott transition, in which a phase transition from an insulating to a metallic state is induced by driving an electrical current through the system, according to the Argonne National Laboratory.

At the University of Twente, researchers built a system containing 90,000 superconducting niobium nanometer-scale islands on top of a gold film. In this configuration, magnetic vortices find their minimum energy configuration by settling into energy dimples in an arrangement like an egg crate – and make the material act as a Mott insulator, since the vortices won’t move if the applied electric current is small.

When they applied a large enough electric current, however, the scientists saw a dynamic Mott transition as the system flipped to become a conducting metal; the properties of the material had changed as the current pushed it out of equilibrium.

Next; implications

“We can controllably induce a phase transition between a state of locked vortices to itinerant vortices by applying an electric current to the system,” said Hans Hilgenkamp, head of the University of Twente research group, in a statement issued by the Argonne National Laboratory. “Studying these phase transitions in our artificial systems is interesting in its own right, but may also provide further insight in the electronic transitions in real materials.”

Valerii Vinokur, an Argonne Distinguished Fellow and corresponding author on the study, said: “Furthermore, this system will be key to building a general understanding of out-of-equilibrium physics, which would be a major breakthrough in physics.”

Mott systems could offer an alternative to the silicon transistor. Since they can be flipped between conducting and insulating states with small changes in voltage, they may be able to encode 1s and 0s at smaller scales and with superior performance to silicon transistors.

The results were printed in the study “Critical behavior at a dynamic vortex insulator-to-metal transition,” published in Science. Other co-authors are associated with the Siberian Branch of Russian Academy of Science, the Rome International Center for Materials Science Superstripes, Novosibirsk State University, the Moscow Institute of Physics and Technology and Queen Mary University of London.

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