Valleytronics refers to energy valleys that form in crystals and that can trap single electrons. These, according to the team at Princeton, have the potential to be used to store information.
In a new study, researchers observed that electrons in bismuth prefer to crowd into one valley rather than distributing equally into the six available valleys. According to the team, this behaviour creates a type of electricity called ferroelectricity, which involves the separation of positive and negative charges onto opposite sides of a material.
This study was made available online in May 2018 and published this month in Nature Physics.
The finding confirms a recent prediction that ferroelectricity arises naturally on the surface of bismuth when electrons collect in a single valley. These valleys are like pockets of low energy where electrons prefer to rest.
The researchers detected the electrons congregating in the valley using a technique called scanning tunnelling microscopy, which involves moving an extremely fine needle back and forth across the surface of the crystal. They did this at temperatures hovering close to absolute zero and under a very strong magnetic field, up to 300,000 times greater than Earth's magnetic field.
As a result of this research it might be possible to exploit electrons in future technologies. Crystals consist of highly ordered, repeating units of atoms, and with this order comes precise electronic behaviours. Silicon's electronic behaviours, for example, have driven modern advances in technology, but to extend its capabilities, researchers are now having to explore new materials. Valleytronics attempts to manipulate electrons to occupy certain energy pockets over others.
The existence of six valleys in bismuth raises the possibility of distributing information in six different states, where the presence or absence of an electron can be used to represent information. The finding that electrons prefer to cluster in a single valley is an example of "emergent behaviour" in that the electrons act together to allow new behaviours to emerge that wouldn't otherwise occur, according to Mallika Randeria, the first author on the study and a graduate student at Princeton working in the laboratory of Ali Yazdani, the Class of 1909 Professor of Physics.
"The idea that you can have behaviour that emerges because of interactions between electrons is something that is very fundamental in physics," Randeria said. Other examples of interaction-driven emergent behaviour include superconductivity and magnetism.
Image: Elliptical orbits of bismuth surface electrons in a large magnetic field. The orientation and interference patterns of the electronic states reveal that the electrons prefer to occupy a single valley. Image created using a theoretical model of the data