Vanadium dioxide begins to transform itself from an insulator to a conductor when heated above 341 degrees Kelvin (about 154 degrees Fahrenheit or 68 degrees Celsius).
For decades scientists have puzzled over how this transformation to a fully metallic stateknown as Mott metal-insulator transitionoccurs. Balatsky, a Los Alamos condensed-matter theorist, believed, like many other scientists, that the transition begins when metallic puddles begin forming at sites of impurities or imperfections within the lattice. The puddles grow until they touch, and at that point the material becomes conductive, or superconductive.
We had evidence to believe that metallic puddles were forming in an inhomogenous manner within the material at the transition phase, but we had no way of proving it, said Balatsky. If you had the right glasses that could see something extremely small, you could see this process occurring.
The right glasses came in the form of a microscopic viewing technique known as near-field scanning optical microscopy, which has been used to inspect viruses and nano-transistors. The microscope sees infrared light reflecting off of a surface only 20-billionths of a meter (or 20 nanometers) wide. A single virus is less than 20 nanometers wide, while a typical human hair is about 100,000 nanometers wide.
Using this nanoscale viewer, the UCSD-LANL-Max Planck-ETRI team was able to watch metallic puddles form within vanadium oxide at the exact temperature where the Mott transition was expected to occur. These infrared nanoscope images have revealed for the first time a new type of metal phase existing only during the transition of the material from its insulating state to its conducting state.
The new findings will help researchers worldwide better describe and understand underlying physical laws of how charges propagate through correlated materials.
|Contact: James E. Rickman|
DOE/Los Alamos National Laboratory