Things get even more interesting, however, when two such slices are put on top of each other with their crystal directions slightly twisted. This leads to an effect known from television: if someone is wearing a tie or dress made of a checked or striped fabric, strange patterns sometimes appear on the screen. These are also known as moiré patterns.
Something similar happens in Imamoğlu‘s materials. The twist between the two slices creates a kind of moiré-crystal lattice that amounts to a fictitious crystal with atoms that are farther apart than usual. Such a crystal has a much weaker influence on the motion of the electrons, meaning that the interactions between the electrons become more important by comparison.
“Thinking ‘more is better’, we additionally inserted a thin layer of an different material between the molybdenum diselenide slices”, says Yuya Shimazaki, leading postdoc in Imamoğlu‘s group. That slice of boron nitride ensures that, although the two twisted slices are very close to each other, electrons cannot tunnel back and forth between them. By applying an electric voltage to the material one can then control exactly how many electrons are present inside it. Finally, to find out how the electrons move inside this sandwich material, the researchers illuminated it with laser light, thus exciting the electrons.
“Our material allows us to study the electrons with optical means”, Imamoğlu explains. “That’s a big advantage over other 2D materials such as graphene.” From the light signals emitted by the excited electrons, many baffling properties of the electrons can be deduced. What most surprised the physicists was the behaviour of their material when it contained just as many electrons as there were lattice sites in the moiré patterns of the two slices.
In that case so-called Mott insulator states, in which exactly one electron occupies a lattice site, appeared in both slices. That state was rather peculiar as the Mott insulator states stabilized each other, such that even strong external electric fields could not move them and hence no current flowed. “That’s the first time such a behaviour was observed,” says Imamoğlu.
Ideal material for future investigations
The new material paves the way for a series of further exciting investigations. It is ideal for controlled experiments with strongly interacting electrons. The researchers can change the properties of the material and the strength of the interactions through the boron nitride layer and the angle between the molybdenum diselenide slices. This allows them to study complex physical processes that are hard to realize in other materials.