Sunday, March 14, 2021

Gentoo AMD64 Handbook "Preparing the disks" section reworked

Since the text was becoming more and more outdated and also more and more convoluted, I have completely reworked the "Preparing the disks" section of the Gentoo AMD64 handbook

  • Since fdisk supports GUID partition tables (GPT) for a long time now, references to parted have been dropped.
  • The text restricts itself now to the combinations 1) UEFI boot and GPT and 2) BIOS / legacy boot and MBR. While mixing and matching is here certainly possible, we should treat it out of the scope of the manual.
  • Hopefully the terminology regarding the boot partition, UEFI system partition, and BIOS boot partition is more clear now (it was horribly mixed up before).

Please proofread and check for mistakes! I'll drop the "work in progress" label in a few days if nothing comes up.

Sunday, January 31, 2021

New Gentoo riscv (and arm) stages

With the help of our infrastructure team, I've finally managed to integrate the riscv stage builds with our signing and mirroring system. So now we have a riscv tab on the installation media download page, and the mirrors carry weekly signed stage3 archives for riscv64-lp64d and riscv64-lp64, in both openrc and systemd variants.

Using the same build infrastructure based on qemu, there are now also slowly updated stages for all arm variants coming to the mirrors. Please test them, and if anything does not work as expected, file bugs! The qemu-based builds are here a temporary measure; Matt Turner (mattst88) is preparing a fast multi-core arm64 machine, where this task will move to soon.

Thursday, November 26, 2020

Walter Schottky prize; return to Regensburg with DFG Heisenberg grant

The last weeks have brought excellent news in more than one way. First of all, it's now official that I'll return to University of Regensburg starting 1 March 2021 with a Heisenberg grant of the Deutsche Forschungsgemeinschaft (DFG). Named for the physics Nobel laureate and co-founder of quantum mechanics (and Maximilianeum scholarship recipient) Werner Heisenberg, this is the most renowned DFG grant line for researchers of all subjects who already fulfill the requirements for a tenured professorship. The grant with official title "Quantum transport in nanotubes: Single electron optomechanics and novel materials" has a duration of five years, with funding of approximately € 750.000 within the first three years. It is essentially a research fellowship, combined with money for equipment and employing students.

As an important consequence, I am looking for PhD students. Two positions and projects are available:

In case you are interested, please have a look at the announcements, see the links above or our jobs page, and send me an e-mail!

The second phantastic news is that I've been awarded the Walter Schottky Prize 2021 for our results on microwave optomechanics with a carbon nanotube. This is a a scientific prize awarded annually by the German Physical Society for outstanding research work of young academics in the field of solid-state physics. The prize is named after Walter Schottky, one of the pioneers of electronics and in particular semiconductor devices. Obviously this is a great honour, but also a great encouragement to build on and expand our exciting nano-electromechanics research.

Further information: 

Monday, October 19, 2020

PRB published: "Magnetic field control of the Franck-Condon coupling of few-electron quantum states"

The second-lowest vibration mode of carbon nanotubes is the so-called longitudinal or stretching mode; here the vibrational direction is along the axis of the carbon nanotube. The vibration frequency f and with it the harmonic oscillator quantum hf is already much higher than for the transversal motion; it scales with the length L of the vibrating nanotube segment as 1/L, and is for 100nm < L < 1000nm in the range 0.1meV < hf < 1meV.

With such a large frequency, the harmonic oscillator is at typical dilution refrigerator temperatures T < 100mK fully quantized. The mechanics now becomes visible in the transport spectrum of the quantum dot within the carbon nanotube as so-called Franck-Condon sidebands: For current to pass through the nanotube, an electron has to tunnel onto the quantum dot and then off it again. In our nanomechanical system, however, the mechanical equilibrium position depends on the electrostatic forces on the nanotube, and thus on the charge on it - the equilibrium position for N electrons is different from the one for N+1 electrons. This means that tunneling in is suppressed by a geometric factor describing this coupling, i.e., the limited overlap between the macromolecule wavefunction in both situations. If we provide enough energy to reach excited vibrational states, this suppression is partially lifted. Thus, as function of applied bias voltage, we see a series of steps in the current or lines in the differential conductance.

In our article, we demonstrate for the first time Franck-Condon sidebands in a clean carbon nanotube quantum dot with known absolute number of trapped electrons. We evaluate the coupling parameter and see that it depends on a magnetic field, but also on the precise electronic state that the electrons tunnel through. The so-called valley quantum number turns out to be crucial here; it is related to the angular momentum of the electron. Comparing our evaluation results with our previous calculations on the distribution of electrons along the nanotube axis, we propose a model that describes the coupling parameter as function of magnetic field for different quantum states. While the model is a simplification, it nevertheless is clearly able to qualitatively reproduce our experimental results of a tunable electron-vibron coupling.

"Magnetic field control of the Franck-Condon coupling of few-electron quantum states"
P. L. Stiller, A. Dirnaichner, D. R. Schmid, and A. K. Hüttel
Physical Review B 102, 115408 (2020); arXiv:1812.02657 (PDF)

Top Alexander von Humboldt foundation ranking for Regensburg University

After the top position in the Nature Index 2019 and repeated great rankings in physics in the DFG Förderatlas (2012, 2018), there's another excellent news for Regensburg. In the so-called Humboldt-Ranking 2020, listing where most of the foreign scholars with an Alexander von Humbold foundation scholarship go to pursue their research in Germany, Regensburg is at position 1 of the natural sciences! The Alexander von Humboldt foundation sponsors advanced career stages, from post-doc all the way to professor. Congratulations everyone!

Monday, July 20, 2020

Updated Gentoo RISC-V stages

I finally got around to updating the experimental riscv stages. You can find the result on our webserver. All stages use the rv64gc instruction set; there is a multilib stage with both lp64 and lp64d support, and there are non-multilib stages for both lp64 and lp64d ABI. Please test, and report bugs if anything doesn't work.
As for the technical details, the stages are built using qemu-user on a big and beefy Gentoo amd64 AWS instance. We are currently working on automating that process, such that riscv (and potentially also arm and others) get the same level of support as amd64 and friends. Thanks a lot to Amazon for the credits via their open source promotial program!