Phase transitions in correlated-electron nanostructures

Many transition metal oxides undergo solid-state phase transitions as a result of strong electron-electron correlation effects. By studying nanostructures of such materials one can learn more about the nature of the mysterious correlated electronic phases, and also address general questions such as how phase transitions occur in small systems and in reduced dimensionality. A classic example is the dramatic metal-to-insulator transition shown by vanadium dioxide on cooling through 67°C. By making electrical devices from crystalline nanobeams of this material we have obtained a number of new results concerning this famous transition. We study the motion of a single metal-insulator domain wall electrically and optically, and find that the resistance of the wall is negligible. We find that the metallic phase can be supercooled but the insulating phase cannot be superheated. We find that the transition occurs via an intermediate insulating phase which is stabilized by c-axis tension. Most remarkably perhaps, the resistivity of the insulating phase turns out to be constant along the metal-insulator phase boundary, implying that the transition is controlled by the free carrier density, as expected for a Mott transition.