Mar 1 2019
According to a discovery by scientists from The University of Manchester in the United Kingdom, electrons in graphene act like a highly unique liquid.
(Image credit: The University of Manchester)
For the first time, electron fluid movement in graphene has been discovered to occur with two different viscosities, demonstrating that the classical Hall effect—a phenomenon familiar for over a century—is no more universal as it was considered to be.
In a study reported in Science this week, a team headed by Prof. Sir Andre Geim and Dr Denis Bandurin describes that the Hall effect can even be considerably modified in graphene. The effect was noticed at room temperature, which will have significant implications when using graphene to develop electronic devices.
Quite similar to molecules in liquids and gases, electrons in solids often tend to collide with one another; therefore, they can also behave like fluids. Electron fluids such as these perfect for finding new material behaviors where electron-electron interactions are specifically strong. The challenge is that a majority of the materials were hardly pristine enough to enable electrons to enter this abnormal viscous state. This is due to the fact that they include various impurities, which could scatter electrons before they can interact with one another and organize a viscous flow.
Graphene is best-suited for experiments since the two-dimensional carbon sheet is an extremely clean material consisting of only a few impurities and defects so that electron-electron interactions turn out to be the main source of scattering, resulting in a viscous electron flow.
In previous work, our group found that electron flow in graphene can have a viscosity which is 100 times higher than that of honey. In this first demonstration of electron hydrodynamics, we discovered unusual phenomena like negative resistance, electron whirlpools and superballistic flow.
Dr Denis Bandurin, The University of Manchester.
Much more abnormal effects arise upon applying a magnetic field to electrons in graphene when they are in the viscous state. Even earlier, theorists have thoroughly analyzed electro-magnetohydrodynamics due to its relevance for plasmas in neutron stars and in nuclear reactors. However, to date, there was no practical experimental system readily available to test those predictions.
As part of the experiments, the Manchester scientists developed graphene devices with several voltage probes positioned at different distances from the electrical current path. Some of them were located less than 1 μm away. Geim and team demonstrated that although the Hall effect is absolutely normal when evaluated at large macroscopic distances away from the current path, its magnitude quickly reduces upon being probed locally, using Hall contacts positioned close to the current injector.
“The behaviour is radically different from standard textbook physics,” stated Alexey Berdyugin, a PhD student who performed the experimental work. “We observe that if the voltage contacts are far from the current contacts, we measure the old, boring Hall effect. But, if we place the voltage probes near the current injection points—the area in which viscosity shows up most dramatically as whirlpools in electron flow—then we find that the Hall effect strongly diminishes.”
Changes in the electron flow caused by viscosity persist even at room temperature if graphene devices are smaller than one micron in size. Since this size has become routine these days as far as electronic devices are concerned, the viscous effects is important when making or studying graphene devices.
Alexey Berdyugin, PhD Student, The University of Manchester.