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)

Monday, July 16, 2012

Gentoo in c't magazine

For the German speakers around here, today's edition of c't magazine (that's the company that people from UK/USA may know as "The H") includes an introductory article on Gentoo Linux: "Made to measure - Gentoo Linux: source code and rolling releases". So, go to your newsagent and grab it while it's hot! :)

Thursday, July 12, 2012

Lab::Measurement 3.00 released

I'm happy to be able to announce a first real release of the Lab::Measurement Perl package, providing a platform for measurement control with Perl.

Lab::Measurement is based on the packages Lab::VISA, Lab::Instrument, and Lab::Tools started by Daniel Schröer in 2005. Many people have contributed in the meantime, amongst others in roughly historical order Daniela Taubert, David Kalok, Florian Olbrich, and Alois Dirnaichner. The efforts of the last year have focussed on a general modularization, originally driven by a certain frustration with National Instruments NI-VISA support on Linux. Now, the hardware driver backend can be exchanged transparently, making measurements both with NI-VISA and with e.g. LinuxGPIB or the operating system serial port drivers on Linux and Windows possible.
Since VISA does not form a central part or even requirement anymore, the original use of Lab::VISA as name for the entire package became impractical, and we've decided to switch to Lab::Measurement instead. As version numbers of all components should still increase monotonously, our first release of the code rewrite then actually ended up as Lab::Measurement 3.00.

For downloads and documentation, including installation instructions for Linux and Windows and examples, visit the homepage of the package, Of course, if you're using Gentoo, the package is readily available in the main portage tree as dev-perl/Lab-Measurement.

Not all device drivers have been ported to the new internal architecture so far, but work is progressing swiftly. In the Regensburg nanophysics groups, we're using the new code already all the time in measurements at three different cryogenic setups. More drivers, bugfixes, and improvements are present in Git master. If you're willing to hack, I can only recommend that you give it a try. Contributors are always welcome; feel free to clone our git repository on Gitorious.