Electron-phonon instability in graphene revealed by global and local noise probes

phys.org | 4/4/2019 | Staff
shardonay (Posted by) Level 3
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Understanding nonequilibrium phenomena to effectively control it is an outstanding challenge in science and engineering. In a recent study, Trond. I. Andersen and colleagues at the departments of physics, chemistry, materials science and engineering in the USA, Japan and Canada used electricity to drive ultraclean graphene devices out-of-equilibrium and observe the manifested instability as enhanced current fluctuations and suppressed conductivity at microwave frequencies.

Using the experimental setup, they found that direct current at high drift velocities generated a large increase in the noise at gigahertz frequencies and the noise grew exponentially in the direction of the current. Andersen and co-workers credited the observed emission mechanism, to the amplification of acoustic phonons by the Cerenkov effect (a characteristic blue glow resulting from charged particles passing through an insulator at a speed greater than the speed of light in that medium) and have now published the results on Science.

Scientists - Nonequilibrium - Fluctuations - Field - Sensors

The scientists spatially mapped the nonequilibrium current fluctuations using nanoscale magnetic field sensors to reveal that they grew exponentially along the direction of carrier flow. Andersen et al. credited the observed dependence of the phenomenon on density and temperature, to electron-phonon Cerenkov instability at supersonic drift velocities. Supersonic drift velocities occurred when the population of certain phonons increased with time due to forced Cerenkov emission, when the drift velocity of electron conduction was greater than the velocity of sound (VD>VS) in the medium. The experimental results can offer the opportunity to generate tunable terahertz frequencies and construct active phononic devices on two-dimensional materials.

Nonequilibrium phenomena driven in electronic and optical systems display rich dynamics, which can be harnessed for applications as Gunn diodes and lasers. Two-dimensional materials such as graphene, are an increasingly popular new platform to explore such phenomena. For instance, modern ultraclean graphene devices demonstrate high mobilities and can be driven to high electronic...
(Excerpt) Read more at: phys.org
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