Sunday, July 27, 2014

Perl in Gentoo: dev-lang/perl, virtuals, and perl-core packages

We've got the stabilization of Perl 5.18 upcoming, so what better chance is there to explain a bit how the Perl-related ebuilds in Gentoo work...

First of all, there is dev-lang/perl. This contains the Perl core distribution, installing the binaries and all the Perl modules that are bundled with Perl itself.

Then, there is the perl-core category. It contains independent ebuilds for Perl modules that are also present in the core Perl distribution. Most Perl modules that are bundled with Perl are also in addition released as independent tarballs. If any of these packages is installed from perl-core, its files are placed such that the perl-core download overrides the bundled copy. This means you can also update part of the bundled Perl modules, e.g. in case of a bug, without updating Perl itself.

Next, there are a lot of virtuals "virtual/perl-..." in the virtual category of the portage tree. What are these good for? Well, imagine you want to depend on a specific version of a module that is usually bundled with Perl. For example, you need Module::CoreList at at least version 3.  This can either be provided by a new enough Perl (for example, now hardmasked Perl 5.20 contains Module::CoreList 3.10), or by a separate package from perl-core (where we have Module::CoreList 5.021001 as perl-core/Module-CoreList-5.21.1).
To make sure that everything works, you should never directly depend on a perl-core package, but always on the corresponding virtual (here virtual/perl-Module-CoreList; any perl-core package must have a corresponding virtual). Then both ways to fulfil the dependency are automatically taken into account. (Future repoman versions will warn if you directly depend on perl-core. Also you should never have anything perl-core in your world file!)

Last, we have lots of lots of modules in the dev-perl category. Most of them are from CPAN, and the only thing they have in common is that they have no copy inside core Perl.

I hope this clarifies things a bit. More Perl posts coming...

Friday, June 27, 2014

Emmy Noether grant extended

Today we've received the good news that our Emmy Noether project on the electronic and nano-electromechanical properties of carbon nanotubes has been given a positive intermediate evaluation from the referees. This means funding for an additional period will be granted. Cheers!

Tuesday, June 10, 2014

Please test =app-admin/perl-cleaner-2.14

We've made a few small updates to perl-cleaner that should get you around subslot issues much better in the future.
If you are planning to do any major Perl update on your Gentoo box in the near future, please as a first step update to =app-admin/perl-cleaner-2.14, which is currently in ~arch but in my opinion a good stabilization candidate. This will hopefully give you a much better upgrade of your Perl modules.
Of course any feedback is appreciated, and if you encounter problems, please file bugs! If nothing unexpected happens, =app-admin/perl-cleaner-2.14 will go stable in a month.

Sunday, May 25, 2014

PRB Rapid Comm. accepted: Sub-gap spectroscopy of thermally excited quasiparticles in a Nb contacted carbon nanotube quantum dot

Excellent news- our manuscript "Sub-gap spectroscopy of thermally excited quasiparticles in a Nb contacted carbon nanotube quantum dot" was just accepted for publication by Physical Review B as a Rapid Communication.
Once again we visit the topic of a carbon nanotube quantum dot with superconducting contacts, and again we use niobium for these contacts. Only, this time the connection between the nanotube and the superconductor is pretty bad, i.e., very low electronic tunnel rates. In the end this means that the superconductor does not influence the localized electronic system very much. However, in the metallic contacts we still have a superconductor, meaning electrons pair up into Cooper pairs, and for free quasiparticles carrying only one electron charge an energy gap evolves (the so-called BCS density of states).
Since we're using niobium, we can see superconducting effects over a fairly large temperature range. If we increase the temperature enough, thermal quasiparticles are excited over this energy gap. This precisely is what we observe in our experiment, as additional discrete lines in the transport spectrum. A detailed theoretical analysis of single electron tunneling, in a close cooperation with the research group Prof. Dr. M. Grifoni, confirms our results very well, especially also the temperature dependence of the features visible in the measurements.
In addition there is an interesting bonus to be had here. The thermally activated processes lead to a distinct double-peak of the conductance at zero bias, and the relative height of the two maxima is controlled by the degeneracy of the quantum dot ground states involved in tunneling. This means that looking at the thermally activated current provides additional information to identify the carbon nanotube level spectrum, even if it is not immediately clear from the usual "Coulomb diamond spectroscopy".

"Sub-gap spectroscopy of thermally excited quasiparticles in a Nb contacted carbon nanotube quantum dot"
M. Gaass, S. Pfaller, T. Geiger, A. Donarini, M. Grifoni, A. K. Hüttel, and Ch. Strunk
Phys. Rev. B 89, 241405(R) (2014), arXiv:1403.4456 (PDF)

Thursday, February 20, 2014

Lab::Measurement moves to Github

We've decided to migrate the primary Git repository of our code from Gitorious to Github. The new project page of Lab::Measurement is
and you can anonymously clone the code e.g. from the following URL:
This means we'll also be able to use the Github issue tracker in the future. The copy on Gitorious will eventually be removed.

Monday, January 27, 2014

Lab::Measurement 3.31 released

Since we came up with a few good ideas at the last moment, we're skipping the announcement of Lab::Measurement 3.30 and hereby directly announce a new and shiny Lab::Measurement 3.31!
So why a version number increase again? First of all, the structure of the XPRESS package has changed once more a bit, and with it the way your measurement scripts should look like. The "hub" package is gone, and the whole XPRESS functionality is now made available by a simple
use Lab::Measurement;
statement at the start of your file. The example scripts (also on the website) have been adapted accordingly.
In addition, we have implemented a generic support for so-called multichannel instruments, as for example multimeters with several input channels. This is a more generic solution compared to earlier multichannel sources, and also ties in with the device cache mechanisms. So far, the drivers for the Oxford Instruments ITC temperature control system and the Agilent 34420A nanovoltmeter are multichannel-aware.
Next, now at the start of any measurement the entire device configuration as far as accessible by the driver is saved into a file for your later perusal. Should you ever try to decipher mystery data months later...
Finally, a driver for the Andeen-Hagerling AH 2700A capacitance measurement bridge has been added, and of course also quite some bugfixes. Enjoy!

Wednesday, January 8, 2014

PRB accepted: Temperature dependence of Andreev spectra in a superconducting carbon nanotube quantum dot

The new year once more brings good news. Our manuscript "Temperature dependence of Andreev spectra in a superconducting carbon nanotube quantum dot" was finally accepted for publication by Physical Review B. So what's this about?
When you place a carbon nanotube at low temperature between contacts made from a superconducting metal, lots of interesting things happen. Strongly simplifying, currents in a superconductor are carried by Cooper pairs of two electrons each, while the localized electronic system in the carbon nanotube is normal-conducting and carries single electrons. One mechanism at a superconductor - normal conductor interface that mediates between these two types of charge transport is so-called  Andreev reflection. An electron from the normal conductor enters the superconductor, at the same time a "missing electron", i.e. a "hole where an electron should be", is sent back into the normal conductor. The total charge passing through the interface is 2e, just right to form a Cooper pair. The superconductor-nanotube-superconductor system consistis of two such interfaces back to back; analogous to box potential, multiple reflections on both sides lead to the formation of bound quantum states within the nanotube, the so-called Andeev bound states (ABS).
So far, all other observations of ABS involved aluminum, which has a fairly low critical temperature and critical field. What is new in our work is that we use niobium as superconducting material, with higher critical temperature and larger energy gap. We can increase the temperature to over 1K and still see the superconductivity plus the ABS in the transport spectrum. This way, we can observe how thermal population of an excited Andreev state takes place. Additionally we observe a second pair of Andreev states in the larger superconducting energy gap, and a surprising multi-loop behaviour. All these effects are successfully modelled by calculations based on the superconducting Anderson model, in a collaboration with Alfredo Levy Yeyati and Alvaro Martin-Rodero from Universidad Autonoma de Madrid.

"Temperature dependence of Andreev spectra in a superconducting carbon nanotube quantum dot"
A. Kumar, M. Gaim, D. Steininger, A. Levy Yeyati, A. Martin-Rodero, A. K. Hüttel, and C. Strunk
Physical Review B 89, 075428 (2014), arXiv:1308.1020 (PDF)