New femtosecond technique enables insulator classification

Experts have been heavily discussing why exactly electrically insulating materials insulate as they do. Based on different mechanisms, a classification scheme for insulators has been in use since the 1960s - a theoretical one. However, it has been yet impossible to distinctly classify all insulators due to a lack of suitable experimental approaches. A team of physicists from Kiel University (Germany) and the University of Colorado in Boulder (USA) has now developed a new method to distinguish different insulators unambiguously.

No notebook, no mobile phone, no digital camera would be functional without electrical insulators. The hunt for precise knowledge about state-of-the-art or future insulators is just as rapid as the development of new, better electronic devices. For this reason, insulator research is currently one of the hottest topics in solid-state science.

Following the common scientific procedures in physics, such insulators are first described using universal equations and simulated by computer models. The theoretical results about materials then need to be verified by experiments in the lab. It is this experimental verification that failed for a number of insulators in the past.

For many years expert discussions have been without any final answer about the insulator class. This study now provides a completely new experimental approach to classifying the insulating behaviour of materials objectively.

The science team made use of a special effect: some electrical conductors turn into insulators when strongly cooled down. At the same time, their electric state changes, and when the materials warm up again, their electronic properties also change. The scientists now use the speed of this change to distinguish different classes of insulators.

Inconceivably, small time scales are applied in this method. For the classifications they use a laser beam camera to produce a film from individual images taken within femtoseconds. To illustrate how amazing the technology is to achieve this it can be compared to video used in cinema and HD TV where 30 images per second is at the high end of frame rates. By comparison, if you took one picture every femtosecond for a period of one second, you would end up with 1.000.000.000.000.000 single pictures.

From the resulting film it can be seen how some materials display electronic changes that take about one to 50 femtoseconds, while others display changes between 100 to 200 femtoseconds. In this way the scientists can distinguish one insulator class from another.

Titanium diselenide (TiSe₂) is one of the most discussed insulator materials, which has now been precisely classified. On top of the precise classification of TiSe₂, the scientists gave the first experimental evidence for a new class of insulators, the so-called excitonic insulators.

This new classification method using the femtosecond camera technique was the first systematic application to a scientific question.