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!

Thursday, April 9, 2020

Nature Communications published: "Quantum capacitance mediated carbon nanotube optomechanics"

Coupling carbon nanotube motion to microwaves is hard. Why so? Because typical electromagnetic wavelengths are in the millimeter range there, and a typical nanotube device is less than a micrometer long, with mechanical deflections of nanometers or smaller. As a result, the motion of the nanotube just does not modify the electromagnetic field much; the coupling parameters resulting from optomechanical theory are minimal.
Still, achieving such a coupling and controlling it, without resonantly driving the nanotube to large motion amplitudes, is for many reasons an attractive idea. A nanotube is a very good beam resonator, storing energy coherently for a long time; the mechanics could be used to translate quantum information between different quantum mechanical degrees of freedom. And both single electrons trapped within semiconductors (as a carbon nanotube) and superconducting coplanar microwave circuits are hot candidates for quantum computation architectures, and the topic of much research worldwide.
From this background we are excited to present a first optomechanical experiment where the motion of a suspended single carbon nanotube has been coupled to a superconducting coplanar microwave cavity; our work has been published in Nature Communications. Using the quantization of electric charge, we have been able to amplify the interaction between the two systems, vibration and electromagnetic field, by a factor 10000 compared to simple geometric predictions - and this is by far not the limit yet on what is achievable with our method. In addition, the coupling is controllable, and can be switched on and off quickly.
We obtain a so-called dispersively coupled optomechanical system - novel and exciting on one hand because of the miniaturization of the mechanical part and the coherent single electron effects, but well known on the other hand, since a huge body of theoretical and experimental research on larger (up to macroscopic scales) optomechanical systems exists. There, it has been shown that the coupling can be used for cooling of the vibration, for coherent amplification of signals, or even for arbitrary preparation of quantum states. Based on our results, also the quantum control of the string-like nanotube vibration will be reachable in the near future.

"Quantum capacitance mediated carbon nanotube optomechanics"
S. Blien, P. Steger, N. Hüttner, R. Graaf, and A. K. Hüttel
Nature Communications 11, 1636 (2020)

Monday, March 2, 2020

Visiting professor at Aalto University, absence from Regensburg

Since 15 February 2020, I have moved to Finland, following an invitation to the Department of Applied Physics, Aalto University as visiting professor. This is a great chance to make contacts and contribute to projects there, as well as to learn techniques and push our own project planning ahead.
For Regensburg this means that I am currently not accepting any students for thesis projects anymore, and that it may be quite difficult to find time for oral exams (like those "Modulprüfungen").

Monday, August 12, 2019

pssRRL accepted: Coulomb Blockade Spectroscopy of a MoS2 Nanotube

We are happy to be able to announce that our manuscript "Coulomb Blockade Spectroscopy of a MoS2 Nanotube" has been accepted for publication by pssRRL Rapid Research Letters.

Everybody is talking about novel semiconductor materials, and in particular the transition metal dichalcogenides (TMDCs), "layer materials" similar to graphene. With a chemical composition of TX2, where the transition metal T is, e.g., tungsten W or molybdenum Mo, and the chalcogenide X is, e.g., sulphur S or selenium Se, a wide range of interesting properties is expected.

What's by far not so well known is that many of these materials also form  nanotubes, similar to carbon nanotubes in structure but with distinct properties inherited from the planar system. Here, we present first low temperature transport measurements on a quantum dot in a MoS2 nanotube. The metallic contacts to the nanotube still require a lot of improvements, but the  nanotube between them acts as clean potential well for electrons.

Also, our measurements show possible traces of quantum confined behaviour. This is something that has not been achieved yet in planar, lithographically designed devices - since these have by their very geometric nature larger length scales. It means that via transport spectroscopy we can learn about the material properties and its suitability for quantum electronics devices.

A lot of complex physical phenomena have been predicted for MoS2, including spin filtering and intrinsic, possibly topologic superconductivity - a topic of high interest for the quantum computing community, where larger semiconductor nanowires are used at the moment. So this is the start of an exciting project!

"Coulomb Blockade Spectroscopy of a MoS2 Nanotube"
S. Reinhardt, L. Pirker, C. Bäuml, M. Remskar, and A. K. Hüttel
Physica Status Solidi RRL, doi:10.1002/pssr.201900251 (2019); arXiv:1904.05972 (PDF)