Nov 04, 2021 |
Graphene research sounds out new possibilities for electronic technologies
(Nanowerk News) A team of researchers have revealed that sonic boom and Doppler-shifted sound waves can be created in a graphene transistor, giving new insights into this world-famous material and its potential for use in nanoscale electronic technologies.
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When a police car speeds towards you and passes by with its siren blaring, you can hear a distinct change in the frequency of the siren’s noise. This is the Doppler effect.
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When a jet aircraft’s speed exceeds the speed of sound (about 760 mph),the pressure it exerts upon the air produces a shock wave which can be heard as a loud supersonic boom or thunderclap; this is the Mach effect.
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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.
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Their new publication has been published in Nature Communications ("Graphene’s non-equilibrium fermions reveal Doppler-shifted magnetophonon resonances accompanied by Mach supersonic and Landau velocity effects").
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Graphene is frequently referred to as a “wonder” material. It is over 100 times stronger than steel while being extremely light, over 100 times more conductive than silicon and has the lowest electrical resistivity at room temperature of all known materials.
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These properties make graphene well suited for a range of applications, including coatings to improve touch screens in phones and tablets and to enhance the speed of electronic circuits.
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The research team used strong electric and magnetic fields to accelerate a stream of electrons in anatomically-thin graphene monolayer composed of a hexagonal lattice of carbon atoms.
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At a sufficiently high current density, equivalent to around 100 billion amps per square meter passing through the single atomic layer of carbon,the electron stream reaches a speed of14 kilometers per second (around 30,000 mph)and starts to shake the carbon atoms, thus emitting quantised bundles of sound energy called acoustic phonons.
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This phonon emission is detected as a resonant increase in the electrical resistance of the transistor - a supersonic boom is observed in graphene.
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The researchers also observed a quantum mechanical analogue of the Doppler effect at lower currents when energetic electrons jump between quantised cyclotron orbits and emit acoustic phonons with a Doppler-like up-shift or down-shift of their frequencies, depending on the direction of the sound waves relative to that of the speeding electrons.
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By cooling their graphene transistor to liquid helium temperature, the team detected a third phenomenon in which the electrons interact with each other through their electrical charge and make “phononless” jumps between quantised energy levels at a critical speed, the so-called Landau velocity.
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Loughborough's Dr Mark Greenway, one of the authors of the paper, said: “It is fantastic to observe all of these effects simultaneously in a graphene monolayer.
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“It is due to graphene’s excellent electronic properties that allow us to investigate these out-of-equilibrium quantum processes in detail and understand how electrons in graphene, accelerated by a strong electric field, scatter and lose their energy.
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“The Landau velocity is a quantum property of superconductors and superfluid helium. So it was particularly exciting to detect a similar effect in the dissipative resonant magnetoresistance of graphene.”
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The devices were fabricated at the National Graphene Institute, University of Manchester.
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Dr Piranavan Kumaravadivel, who led device design and development notes, “the large size and high quality of our devices are key for observing these phenomena.
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“Our devices are sufficiently large and pure that electrons interact almost exclusively with phonons and other electrons. We expect that these results will inspire similar studies of non-equilibrium phenomena in other 2D materials.
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“Our measurements also demonstrate that high-quality graphene layers can carry very high continuous current densities which approach those achievable in superconductors. High purity graphene transistors could find future applications in nanoscale power electronic technologies.”
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