Scientists explore a hydrodynamic semiconductor where electrons flow like water

Scientists explore a hydrodynamic semiconductor where electrons flow like water

23 July 2022

(Nanowerk News) You don’t normally want to mix electricity and water, but electricity behaving like water has the potential to improve electronic devices. Recent work from the groups of engineer James Hone at Columbia and theoretical physicist Shaffique Adam at the National University of Singapore and Yale-NUS builds new understanding of this unusual hydrodynamic behavior that changes some old assumptions about the physics of metals.

The study was published in the journal The progress of science (“Dissipation-activated hydrodynamic conductivity in a tunable bandgap semiconductor”).

In the work, the team studied the behavior of a new semiconductor in which negatively charged electrons and positively charged “holes” simultaneously carry current. They found that this current can be described by just two “hydrodynamic” equations: one that describes how the electrons and holes slide against each other, and a second that describes how all the charges move together through the material’s atomic network. electrons can flow like water around obstacles in a semiconductor In a new semiconductor, electrons can flow like water around obstacles. This hydrodynamic behavior could provide more efficient devices. (Image: Rina Goh, National University of Singapore)

“Simple formulas usually mean simple physics,” said Hone, who was surprised when Adams’ postdoc, Derek Ho, built the new model, which challenges assumptions many physicists learn about metals early in their training. “We were all taught that in a normal metal, all you really need to know is how an electron bounces off different kinds of imperfections,” Hone said. “In this system, the basic models we learned about in our first courses simply do not apply.”

In metal wires carrying an electric current, there are many moving electrons that largely ignore each other, like riders on a crowded subway. As the electrons move, they inevitably encounter either physical defects in the material carrying them or vibrations that cause them to scatter. The current slows down and energy is lost. But in materials that have smaller numbers of electrons, those electrons actually interact strongly with each other and will flow together, like water through a pipe. They still run into the same flaws, but their behavior is completely different: instead of thinking about individual electrons scattering randomly, you now have to treat the entire set of electrons (and holes) together, Hone said.

To experimentally test their simple new model of hydrodynamic conductivity, the team studied bilayers the graph – a material made from two atomically thin slices of carbon. Hone’s graduate student Cheng Tan measured electrical conductivity from room temperature down to near absolute zero when he varied the density of electrons and holes.

Tan and Ho found an excellent match between the model and their results. “It is striking that experimental data agree so much better with hydrodynamic theory than old ‘standard’ theory of conductivity,” Ho said.

The model worked when the material was tuned in a way that allows the conductivity to be turned on and off, and the hydrodynamic behavior was prominent even at room temperature. “It is truly remarkable that bilayer graphene has been studied for over 15 years, but until now we have not properly understood its conductivity at room temperature,” said Hone, who is also the Wang Fong-Jen Professor and chair of the Department of Mechanical Engineering. at Columbia Engineering.

Conductivity with low resistance at room temperature can have very practical applications. Existing superconducting materials, which conduct electricity without resistance, must be kept incredibly cold. Materials capable of hydrodynamic flow could help researchers build more efficient electronic devices – known as viscous electronics – that do not require such intense and expensive cooling.

At a more fundamental level, the team verified that the sliding motion between electrons and holes is not specific to graphene, said Adam, an associate professor from the Department of Materials Science and Engineering at the National University of Singapore and the Division of Science at Yale-NUS College. Because this relative motion is universal, researchers should be able to find it in other materials—especially as improvement in manufacturing techniques continues to yield purer and purer samples, which the Hone Lab has focused on developing over the past decade. In the future, researchers may also design specific geometries to further improve the performance of devices built to take advantage of this unique water-like collective behavior.

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