Valleys are maxima and minima of electron energies in a crystalline solid. A new field of physics, valleytronics, exploits the electron's ‘valley degree of freedom’ for data storage and logic applications. A method to control electrons in different valleys could yield super-efficient computer chips.
The team used a ferromagnetic compound to pull the valleys apart and keep them at different energy levels. This is said to lead to an increase in the separation of valley energies by a factor of 10 more than the one obtained by applying an external magnetic field.
"Normally there are two valleys in these atomically thin semiconductors with the same energy. These are called 'degenerate energy levels' and limit our ability to control individual valleys. An external magnetic field can be used to break this degeneracy,” explained Professor Hao Zeng.
“Our new approach makes the valleys more accessible and easier to control, and this could allow valleys to be useful for future information storage and processing."
When one valley is occupied by electrons, the switch is ‘on’. When the other valley is occupied, the switch is ‘off’. The work shows that the valleys can be positioned in such a way that a device can be turned ‘on’ and ‘off’ with a tiny amount of electricity.
The researchers created a 2D heterostructure, with a film of magnetic europium sulphide on the bottom and a single layer of the semiconductor material dichalcogenide tungsten diselenide on top. The magnetic field of the bottom layer forced the energy separation of the valleys in the WSe2.
"As long as we have the magnetic material there, the valleys will stay apart," Prof Zeng said. "This makes it valuable for non-volatile memory applications."