Vanadium dioxide (VO2) has long been a puzzle to condensed-matter physicists — above 68 °C it acts as a conductor, but below that temperature it is an insulator. Some say that collective motion of the electrons is mainly responsible for this transition, whereas others suggest that interactions between the electrons and the vibrations in the material's crystal lattice contribute to the odd behaviour.
David Cobden and his colleagues at the University of Washington in Seattle approached the mystery using nanobeams of VO2, which are devoid of the defects and variations that obscure the results of experiments on larger samples. (Pictured, a 40-micrometre-long nanobeam imaged during warming shows dark metallic domains appearing and widening.) Their findings suggest that electron–electron interactions are behind the transition.
The same approach may prove useful in studying other materials. As for VO2, it may find uses in electronics or mirrors that can be switched on and off at will.