Solids are generally divided into metals, which conduct electricity, and insulators, which do not. Some oxides straddle this boundary, however: a material's structure and properties suggest it should be a metal, but sometimes it behaves as an insulator. Researchers at the University of California, Santa Barbara are digging into the mechanisms of this transformation and are aiming to harness it for use in novel electronic devices.

In the atomic structure of metals, some electrons are non-localized, meaning they are not associated with any particular atom. These electrons, which have enough energy to move around within the crystal lattice of the metal, are called conduction electrons. The movement of these electrons in a particular direction constitutes an electric current; it is the presence of conduction electrons in metals which makes them good conductors of electricity (see video here).

In 1937, Sir Nevill Mott explained that some solids we would expect to act as metals, since they have high concentrations of conduction electrons, actually behave as insulators instead. This class of materials is now referred to as Mott insulators. Today, everyone agrees that this phenomenon is caused by interactions between the conduction electrons. The nanoscale mechanisms of the transformation from conductor to insulator have remained a topic of debate, however.

It makes sense that the number and intensity of interactions between conduction electrons, along with the effects we observe from these interactions, should increase as the number and density of the electrons increases. Using quantum-mechanical computer simulations, the Santa Barbara group has been studying what happens at the level of atoms and electrons if we start with a conventional insulator without conduction electrons, and then gradually increase the number of electrons. For low concentrations of conduction electrons, the material behaves as a metal and conducts electricity, as expected. The electrons spread out evenly in the material, allowing for electrical conduction (see Fig. 1). At a critical electron density, however, the material suddenly turns back into an insulator. Our simulations reveal that this transformation is linked to particular distortions in the atomic structure of the material, which cause electrons to "lock up" and localize on particular atoms, thereby turning the material into an insulator (see Fig. 2 and video below).

These investigations are not just theoretical: the necessary electron densities can actually be achieved in thin layers of strontium titanate (SrTiO₃) squeezed between other oxides.  The interfaces between layers contain large concentrations of electrons, which researchers hope to harness for novel electronic devices. Experimental evidence reveals that if these electrons are forced into an ultrathin SrTiO₃ layer, the transition to an insulator is triggered (see Fig. 3). As we continue to learn more about the specific mechanisms that trigger this change, we can develop methods to control the process, incorporating Mott insulators as switches to build smaller electrical components and memory devices.