Thursday, July 26, 2012

NJP accepted: Magnetic damping of a carbon nanotube NEMS resonator

There's very good news- our first Regensburg article on carbon nanotube nano-electromechanical systems, "Magnetic damping of a carbon nanotube NEMS resonator", was just accepted for publication by New Journal of Physics.
Let me give you a short introduction what we've been working on here. A very exciting discovery some time ago was that at low temperatures (T<0.1K) mechanical resonators made from single-wall carbon nanotubes show very large quality factors Q. That means, once vibrating they store energy for a long time, and the vibration decays only very slowly - a piano string with a similar Q would sound for over five minutes after hitting the key! 
Now this has all sorts of interesting side effects. It's so easy to keep the vibration going that it basically runs on its own once a current passes through the device and some prerequisites are given. The device switches between different stability regions, and the usually very predictable transport spectroscopy pattern of a carbon nanotube quantum dot gains strange shapes and sharp edges.
Amazingly, as soon as you apply a magnetic field, this effect is all gone again, and the transport spectrum becomes regular. The overall current does not change significantly, so our tunnel rates should not be influenced too much by the magnetic field. Which means, according to the theory, that our magnetic field has to tune the second available "knob", the quality factor Q of the mechanical vibration. And indeed if we now drive the system with a radio-frequency signal, we see that the resonance becomes broader in frequency in a high magnetic field - the quality factor decreases.
So what's the damping mechanism? Actually, that is pretty straightforward. In a magnetic field, the vibrating nanotube acts as an ac voltage source, generating a small voltage the same way as a macroscopic ac generator. In addition, high-frequency signals can be transmitted capacitively between, say, parallel cables. Consequently a small ac current flows across a parasitic circuit with a ~100kOhm resistance somewhere, which dissipates energy; the resulting upper limit for Q scales with 1/B2. We can compare this model with our observed Q(B), and see a very nice agreement. Effectively, we've built the world's smallest eddy current brake!

"Magnetic damping of a carbon nanotube NEMS resonator"
D. R. Schmid, P. L. Stiller, Ch. Strunk, and A. K. Hüttel
accepted for publication by New Journal of Physics; arXiv:1203.2319 (PDF)

1 comment:

  1. Congrats for article publication.great piece of article.It would be beneficial for me for the further studies as i am interested in nanotechnology and my area of interest is carbon nanotubes