Electrons Have Been Caught Disappearing and Reappearing Between Atomic Layers

Frank Ceballos/University of Kansas

Scientist have spotted a strange type of quantum movement occurring in electrons travelling between the atomic layers of a material.

Instead of travelling from the top to the bottom layer through the middle, the electrons were caught disappearing from the top layer and reappearing in the bottom letter a fraction of a second later - with no trace of them existing in between.

the researchers experimented with a van der Waals material that was made up of three 2D layers - similar to ultra-thin graphene - held together by van der Waals attractions.

The three layers the researchers tested were MoS2, WS2, and MoSe2 - all semiconducting materials, which means they can conduct electrons without resistance, and all of which respond to light with different colours.

That meant the researchers could use different coloured lasers to affect only electrons in one of the three layers, and not the others - and it allowed them to track where the electrons were travelling throughout the material.

To kick off the electrons, the team used an ultra-short laser pulse of just 100 femtoseconds (a femtosecond is one-quadrillionth of a second) to liberate some of the electrons in the top MoSe2 layer, so they could move freely.

"The colour of the laser pulse was chosen so that only electrons in the top layer can be liberated," said Zhao.

"We then used another laser pulse with the 'right' colour for the bottom MoS2 layer to detect the appearance of these electrons in that layer. The second pulse was purposely arranged to arrive at the sample after the first pulse by about 1 picosecond, by letting it travel a distance 0.3 mm longer than the first."

A picosecond is trillionth of a second, or around 1,000 femtoseconds.

The team found that the electrons moved from the top to the bottom layer of the material in that incredibly short time frame, taking just 1 picosecond to make the transition on average.

To figure out how they were getting there so quickly, they then used a third laser pulse with another colour to monitor the middle layer - and were surprised to find no electrons in there at all, defying regular physics.

"If electrons were things that followed 'common sense', like so-called classical particles, they'd be in the middle layer at some point during this one picosecond," said Zhao.

Thinking their result might have been a fluke, this bizarre quantum 'hopping' from the top to the bottom layer was further confirmed by simulations at the University of Nebraska-Lincoln.

The fact that it's shown up twice in two separate labs suggests that it's a real quantum phenomenon that lets electrons travel between atomic layers connected by van der Waals forces.

Further research is always needed to probe these new observed quantum phenomenon, but if verified, this is a good sign that van der Waals materials will be incredibly useful for electronics - and it's something that could be used to develop even more types of materials.

"This study showed electrons can transfer between these layers in a quantum fashion, just like in other conductors and semiconductors," said Zhao.

The research has been published in Nano Letters