A team of researchers has shown that sonic boom and Doppler-shifted sound waves can be generated in a graphene transistor, providing new insights into this world-famous material and its potential for use in nanoscale electronic technologies.
When a police car races towards you and passes by with its siren noise, you will hear a distinct change in the frequency of the siren noise. This is the Doppler effect.
When a jet plane’s speed exceeds the speed of sound (about 760 mph), the pressure it exerts on the air creates a shock wave that can be heard as a loud sonic boom or clap of thunder; This is the mach effect.
Scientists from Loughborough, Nottingham, Manchester, Lancaster and Kansas Universities have discovered that a quantum mechanical version of these phenomena occurs in an electronic transistor made from high-purity graphene.
Her new paper, Graphene’s Non-Equilibrium Fermions Reveal Doppler-Shifted Magnetophonon Resonances Accompanied by Mach Supersonic and Landau Velocity Effects, was published today in Nature Communications.
Graphene is often referred to as a “wonder material”. It is over 100 times stronger than steel and extremely light, over 100 times more conductive than silicon and of all known materials has the lowest specific electrical resistance at room temperature.
These properties make graphene well suited for a number of applications, including coatings to enhance touchscreens in phones and tablets and to speed up electronic circuits.
The research team used strong electric and magnetic fields to accelerate a stream of electrons in an atomically thin graphene monolayer made up of a hexagonal lattice of carbon atoms.
With a sufficiently high current density, which corresponds to about 100 billion amperes per square meter through the individual atomic layer of carbon, the electron current reaches a speed of 14 kilometers per second (approx. Emitting quantized bundles of sound energy, which are referred to as acoustic phonons.
This phonon emission is recognized as a resonant increase in the electrical resistance of the transistor; a supersonic boom is observed in graphs.
The researchers also observed a quantum mechanical analog of the Doppler effect at lower currents, when high-energy electrons jump between quantized cyclotron orbits and emit acoustic phonons with a Doppler-like shift up or down in their frequencies, depending on the direction of the sound relative to that of the waves frenzied electrons.
By cooling their graph transistor to the temperature of liquid helium, the team discovered a third phenomenon in which the electrons interact with one another through their electrical charge and make “phononless” jumps between quantized energy levels at a critical speed, the so-called Landau speed.
Dr. Mark Greenaway of Loughborough, one of the authors of the paper, said: “It is fantastic to see all these effects simultaneously in a graphene monolayer.
“It is the excellent electronic properties of graphene that enable us to study these out-of-equilibrium quantum processes in detail and understand how electrons in graphene that are accelerated by a strong electric field scatter and lose their energy.
“The Landau speed is a quantum property of superconductors and superfluid helium. It was therefore particularly exciting to discover a similar effect in the dissipative resonant magnetoresistance of graphene. “
The devices were manufactured at the National Graphene Institute at the University of Manchester.
Dr. Piranavan Kumaravadivel, Head of Device Design and Development Notes: “The size and high quality of our devices are key to observing these phenomena.
“Our devices are so large and pure that electrons interact almost exclusively with phonons and other electrons. We expect that these results will stimulate similar studies on non-equilibrium phenomena in other 2D materials.
“Our measurements also show that high-quality graphene layers can carry very high continuous current densities that come close to those in superconductors. Highly pure graphene transistors could find future applications in power electronics technologies on the nano scale. “
Greenaway MT, Kumaravadivel P, Wengraf J et al. Graphen’s non-equilibrium fermions exhibit Doppler-shifted magnetophonon resonances accompanied by Mach supersonic and Landau velocity effects. Nat. Commun 12, 6392 (2021). do:10.1038 / s41467-021-26663-4
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