A team of researchers from universities in Loughborough, Nottingham, Manchester, Lancaster and Kansas (US) has revealed that sonic boom and Doppler-shifted sound waves can be created in a graphene transistor.
When a police car speeds past you with its siren blaring, you hear a distinct change in the frequency of the siren’s noise. This is the Doppler effect. 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. The scientists discovered that a quantum mechanical version of these phenomena occurs in an electronic transistor made from high-purity graphene.
The researchers also observed a quantum mechanical analogue of the Doppler effect at lower currents when energetic electrons jump between quantized 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. 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 quantized energy levels at a critical speed, the so-called Landau velocity.
The devices were fabricated at the NGI in Manchester. Dr. Piranavan Kumaravadivel, who led device design and development, said: The large size and high quality of our devices are key for observing these phenomena. 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.... 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.
Loughborough University's Dr. Mark Greenway, one of the authors of the paper, commented: It is fantastic to observe of all these effects simultaneously in a graphene monolayer. It is due to graphene’s excellent electronic properties that we can 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. 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.