The Hall effect becomes viscous in graphene

Researchers at the University of Manchester in the UK have discovered that the Hall effect, a phenomenon well known for over a century, is no longer as universal as it is thought so.

In the research paper published in Science This week, the group led by professors Sir Andre Geim and Denis Bandurin discovered that the Hall effect could even have a significant impact if the electrons interacted strongly with each other, giving rise to a viscous flow. The new phenomenon is important at room temperature, which can have important consequences for the manufacture of electronic or optoelectronic devices.

Just like molecules in gases and liquids, electrons in solids often collide with each other and can therefore also behave like viscous fluids. Such electronic fluids are ideal for finding new material behaviors in which electron-electron interactions are particularly strong. The problem is that most materials are rarely pure enough to allow electrons to enter the viscous regime. Indeed, they contain many impurities that electrons can disperse before having time to interact and organize a viscous flow.

Graphene can be very useful here: the carbon sheet is a very clean material that contains only a few defects, impurities and phonons (temperature-induced crystal lattice vibrations), so that the electron-electron interactions become the main source diffusion, which leads to a viscous electron flow.

"In previous work, our group had found that the flow of electrons in graphene could have a viscosity of up to 0.1 m²s^-1, which is 100 times higher than that of honey, "said Dr. Bandurin" During this first demonstration of the hydrodynamics of electrons, we discovered very unusual phenomena such as negative resistance, swirls of 39, electrons and superballistic flows "

Even more unusual effects occur when a magnetic field is applied to graphene electrons when they are in a viscous state. Theorists have already extensively studied electro-magnetohydrodynamics because of its importance for plasma in nuclear reactors and neutron stars, as well as for fluid mechanics in general. But no practical experimental system to test these predictions (such as a significant negative magnetoresistance and abnormal Hall resistivity) was available until now.

In their latest experiments, Manchester researchers made graphene devices with numerous voltage probes placed at different distances from the electrical current path. Some of them were within one micron of each other. Geim and his colleagues have shown that if the Hall effect is perfectly normal if it is measured at great distances from the current path, its magnitude decreases rapidly if it is probed locally, at the same time. help from contacts close to the current injector.

"The behavior is radically different from the classical physics of textbooks," says Alexey Berdyugin, Ph.D. student who led the experimental work. "We observe that if the voltage contacts are far away from the current contacts, we measure the old boring Hall effect instead of this new" viscous Hall effect. "But, if we place the voltage probes near the dots of Injection of current, the area in which the viscosity appears in the most spectacular way as swirls in the electron flow – then we see that the Hall effect decreases.

"Qualitative changes in electron flux caused by viscosity persist even at room temperature if graphene devices are smaller than one micron," says Berdyugin. As this size has become a routine for electronic devices, the viscous effects are important when manufacturing or studying graphene devices. "

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More information:
"Hall viscosity measurement of graphene electronic fluid" Science (2019). science.sciencemag.org/lookup/… 1126 / science.aau0685