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Researchers at the University of Manchester’s School of Physics and Astronomy, in collaboration with the Massachusetts Institute of Technology (MIT), have recently published an article in Nature Physics on the flow of electron transport through graphene constrictions.
The research led by Nobel prize winner, Sir Andre Konstantin Geim, and MIT Physicist, Leonid Levitov, led to the discovery that the measured conductance in graphene constrictions exceeds the maximum possible conductance of free electrons1.
The work described here is based on previous research conducted by a team of MIT Researchers led by Levitov, who is also one of the Authors of the present paper.
In their paper published in Proceedings of the National Academy of Sciences (PNAS), Leonid Levitov’s team described a phenomenon called “superballistic flow”, which describes the flow of electrons passing through a narrow constriction within a piece of metal which can travel much faster than conventional electron speeds in metals2.
The collisions between the electrons can affect their flow in a variety of counterintuitive ways. It has been rather difficult to study the subject due to the simultaneous occurrence of different types of scattering mechanisms1.
Like the behavior exhibited by the gases escaping from a tiny hole, the electrons present in the quantum mechanical electron fluid that flows through the constriction of metals could travel at unusual speeds.
When you have a smaller number of gas molecules, they will be moving at random, and therefore hit the walls more often causing them to slow down but when you have higher number of gas molecules, most of the molecules will bump into each other more often than they will hit the walls1.
This collision process between the gas molecules in this case does not result in any loss of total energy, therefore, preventing overall slowing of their speed.
In fact, the higher the number of gas molecules, the faster their movement will be1. Based on this theory, Leonid Levitov’s team postulated that the electrons flowing through point-like constrictions in a metal can travel at speeds that exceed the fundamental speed limit, which is known as the Landauer’s ballistic limit.
This phenomenon of the flow of electrons with extraordinary speed and extremely low resistance when passing through a very narrow constriction in metals is called as “superballistic flow.” While superconductivity has zero resistance, it requires extremely low temperatures to achieve it, whereas the superballistic flow can occur at room temperature with no requirement of such extreme low temperatures.
In fact, the superballistic flow would increase with an increase in its temperature,1 therefore, it would be of a great advantage to utilize this phenomenon in electronic devices.
Sir Andre Konstantin Geim’s team in the current research studied the nature of electron flow through the constrictions of graphene.2 The results revealed that the conductivity in the graphene constrictions is higher than what is possible for free electrons.2
Interestingly, the conductivity was also found to increase with an increase in temperature of up to 150 K.2 This extraordinary conductivity in graphene constrictions is attributed to the collective movement of interacting electrons with preservation of the momentum loss and conservation of the total energy in the electron collisions that occur at the sample boundaries2.
Contrary to the popular notion that additional disorder will create extra electrical resistance, the current research puts forward a surprising discovery that electron scattering that occurs between the electrons flowing through the constrictions of graphene reduces resistance, thereby allowing them to travel at unusual speeds2.
Since this process can occur in metals at room temperature without requiring extreme low temperatures, this superballistic flow could be applied to variety of electronic applications in the future.
Image Credit:
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References:
- “Superballistic flow of viscous electron fluid through graphene constrictions” R. Kumar, D. Bandurin, et al. Nature Physics (2017). DOI: 10.1038/nphys4240.
- “Electrons go superballistic” – MIT News
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