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prof. C. Strunk


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PRL accepted: Secondary electron interference from trigonal warping in clean carbon nanotubes   (posted 2016-09-16)

http://www.akhuettel.de/publications/fabryperot.pdf
We're very happy to announce that a few days ago one of our manuscripts, "Secondary electron interference from trigonal warping in clean carbon nanotubes", was accepted for publication in Physical Review Letters

Imagine a graphene "sheet" of carbon atoms rolled into a tube - and you get a carbon nanotube. Carbon nanotubes come in many variants, which influence strongly their electronic properties. They have different diameter, but also different "chiral angle", describing how the pattern of the carbon atoms twists around the tube axis. In our work, we show how to extract information on the nanotube structure from measurements of its conductance. At low temperature, electrons travel ballistically through a nanotube and are only scattered at its ends. For the quantum-mechanical electron wavefunction, metallic nanotubes act then analogous to an optical Fabry-Perot interferometer, i.e., a cavity with two semitransparent mirrors at either end, where a wave is partially reflected. Interference patterns are obtained by tuning the wavelength of the electrons; the current through the nanotube oscillates as a function of an applied gate voltage. The twisted graphene lattice then causes a distinct slow current modulation, which, as we show, allows a direct estimation of the chiral angle. This is an important step towards solving a highly nontrivial problem, namely identifying the precise
molecular structure of a nanotube from electronic measurements alone.

"Secondary electron interference from trigonal warping in clean carbon nanotubes"
A. Dirnaichner, M. del Valle, K. J. G. Götz, F. J. Schupp, N. Paradiso, M. Grifoni, Ch. Strunk, and A. K. Hüttel
accepted for publication in Physical Review Letters; arXiv:1602.03866 (PDF, supplemental information)

Nanotechnology accepted: Co-sputtered MoRe thin films for carbon nanotube growth-compatible superconducting coplanar resonators   (posted 2016-02-04)

http://www.akhuettel.de/publications/remo.pdf
We're happy to be able to announce that our manuscript "Co-sputtered MoRe thin films for carbon nanotube growth-compatible superconducting coplanar resonators" has just been accepted for publication in Nanotechnology.
For quite some time we have been working on techniques to combine ultra-clean carbon nanotubes and their regular electronic spectrum with superconducting material systems. One of our objectives is to perform high-frequency measurements on carbon nanotube nano-electromechanical systems at millikelvin temperatures. With this in mind we have established the fabrication and characterization of compatible superconducting coplanar resonators in our research group. A serious challenge here was that the high-temperature process of carbon nanotube growth destroys most metal films, or if not, at least lowers the critical temperature Tc of superconductors so much that they are not useful anymore.
In the present manuscript, we demonstrate deposition of a molybdenum-rhenium alloy of variable composition by simultaneous sputtering from two sources. We characterize the resulting thin films using x-ray photoelectron spectroscopy, and analyze the saturation of the surface layers with carbon during the nanotube growth process. Low-temperature dc measurements show that specifically an alloy of composition Mo20Re80 remains very stable during this process, with large critical currents and critical temperatures even rising to up to Tc~8K. We use this alloy to fabricate coplanar resonator structures and demonstrate even after a nanotube growth high temperature process resonant behaviour at Gigahertz frequencies with quality factors up to Q~5000. Observation of the temperature dependent behaviour shows that our devices are well described by Mattis-Bardeen theory, in combination with dissipation by two-level systems in the dielectric substrate.

"Co-sputtered MoRe thin films for carbon nanotube growth-compatible superconducting coplanar resonators"
K. J. G. Götz, S. Blien, P. L. Stiller, O. Vavra, T. Mayer, T. Huber, T. N. G. Meier, M. Kronseder, Ch. Strunk, and A. K. Hüttel
accepted for publication in Nanotechnology; arXiv:1510.00278 (PDF)

DFG magazine "german research" publishes article about our research group   (posted 2016-01-26)

http://www.akhuettel.de/publications/german-research.pdf
The 3/2015 edition of the "german research" magazine of the DFG includes an article about the work of our research group! This is a translation of a previous publication in the German language journal "Forschung" of the DFG. Enjoy!

"Carbon Nanotubes: Strong, Conductive and Defect-Free"
Carbon nanotubes are a fascinating material. In experiments at ultra-low temperatures, physicists make their different properties interact with one another - and in so doing find answers to fundamental questions.
Andreas K. Hüttel
german research 3/2015, 24-27 (2015) (PDF)

APL accepted: Liquid-induced damping of mechanical feedback effects in single electron tunneling through a suspended carbon nanotube   (posted 2015-09-14)

Today's good news is that our manuscript "Liquid-induced damping of mechanical feedback effects in single electron tunneling through a suspended carbon nanotube" has been accepted for publication in Applied Physics Letters. So what's it about?
One of the surprises that suspended, clean carbon nanotubes have in store is that they can start vibrating strongly at millikelvin temperatures without any applied radio-frequency driving signal. This was proposed theoretically several years ago by Usmani et al., as a strong feedback between the transversal vibration of the nanotube and the single electron tunneling through it. The effect was identified in measurements, and for example in a previous publication we have shown that damping induced by a magnetic field can suppress it.
Here, we demonstrate how one and the same device behaves distinctly different depending on the environment medium (or lack of the latter): we compare measurements made at the same temperature in a conventional dilution refrigerator, where the chip is placed into a vacuum chamber, and in a so-called top-loading dilution refrigerator, where the chip is inserted into the 3He/4He liquid of the mixing chamber. The overall electronic properties of the device do not change much, even though the thermal cycling could cause a lot of damage and has done so in the past for other devices. We can here even extract a rough estimate of the liquid helium dielectric constant by comparing the slightly shifted Coulomb oscillation positions of the two measurements.
However, a striking difference appears when looking at finite bias conductance and the mechanical feedback effects. In the viscous helium liquid, the resonator is damped and the vibrations are suppressed, and the unperturbed electronic transport spectrum emerges. Such an inert, liquid environment can thus be used to do transport spectroscopy at high transparency of the tunnel barriers and high applied bias voltages - parameter regions interesting for e.g. non-equilibrium Kondo phenomena, where otherwise mechanically-induced features would make data evaluation highly challenging.

"Liquid-induced damping of mechanical feedback effects in single electron tunneling through a suspended carbon nanotube"
D. R. Schmid, P. L. Stiller, Ch. Strunk, and A. K. Hüttel
Applied Physics Letters 107, 123110 (2015); arXiv:1407.2114 (PDF)

PRB accepted: Transport across a carbon nanotube quantum dot contacted with ferromagnetic leads: experiment and non-perturbative modeling   (posted 2015-05-04)

It's time to announce yet another nice result. Our manuscript "Transport across a carbon nanotube quantum dot contacted with ferromagnetic leads: experiment and non-perturbative modeling" has been accepted as a regular article by Physical Review B.
When ferromagnetic materials are used as contacts for a carbon nanotube at low temperature, the current is strongly influenced by the direction of the contact magnetization via the so-called tunneling magnetoresistance (TMR). Since the nanotube contains a quantum dot, in addition its electronic energy levels play an important role; the TMR depends on the gate voltage value and can reach large negative and positive values. Here, in another fruitful joint experimental and theoretical effort, we present both measurements of the gate-dependent TMR across a "shell" of four Coulomb oscillations, and model them in the so-called "dressed second order" framework. The calculations nicely reproduce the characteristic oscillatory patterns of the TMR gate dependence.

"Transport across a carbon nanotube quantum dot contacted with ferromagnetic leads: experiment and non-perturbative modeling"
A. Dirnaichner, M. Grifoni, A. Prüfling, D. Steininger, A. K. Hüttel, and Ch. Strunk
Phys. Rev. B 91, 195402 (2015); arXiv:1502.02005 (PDF)

PRB accepted: Broken SU(4) symmetry in a Kondo-correlated carbon nanotube   (posted 2015-03-11)

We're happy to be able to announce that our manuscript "Broken SU(4) symmetry in a Kondo-correlated carbon nanotube" has been accepted for publication in Physical Review B.
This manuscript is the result of a joint experimental and theoretical effort. We demonstrate that there is a fundamental difference between cotunneling and the Kondo effect - a distinction that has been debated repeatedly in the past. In carbon nanotubes, the two graphene-derived Dirac points can lead to a two-fold valley degeneracy in addition to spin degeneracy; each orbital "shell" of a confined electronic system can be filled with four electrons. In most nanotubes, these degeneracies are broken by the spin-orbit interaction (due to the wall curvature) and by valley mixing (due to, as recently demonstrated, scattering at the nanotube boundaries). Using an externally applied magnetic field, the quantum states involved in equilibrium (i.e., elastic, zero-bias) and nonequilibrium (i.e., inelastic, finite bias) transitions can be identified. We show theoretically and experimentally that in the case of Kondo correlations, not all quantum state pairs contribute to Kondo-enhanced transport; some of these are forbidden by symmetries stemming from the carbon nanotube single particle Hamiltonian. This is distinctly different from the case of inelastic cotunneling (at higher temperatures and/or weaker quantum dot-lead coupling), where all transitions have been observed in the past.

"Broken SU(4) symmetry in a Kondo-correlated carbon nanotube"
D. R. Schmid, S. Smirnov, M. Marganska, A. Dirnaichner, P. L. Stiller, M. Grifoni, A. K. Hüttel, and Ch. Strunk
Phys. Rev. B 91, 155435 (2015) (PDF)

DFG magazine "forschung" publishes article about our research group (in German)   (posted 2015-01-12)

http://www.akhuettel.de/publications/forschung.pdf
The 4/2014 edition of the "forschung" magazine of the DFG, published just a few days ago, includes an article about the work of our research group (in German)! Enjoy!

"Zugfest, leitend, defektfrei"
Kohlenstoff-Nanoröhren sind ein faszinierendes Material. In Experimenten bei ultratiefen Temperaturen versuchen Physiker, ihre verschiedenen Eigenschaften miteinander in Wechselwirkung zu bringen ? und so Antworten auf grundlegende Fragen zu finden.
Andreas K. Hüttel
forschung 4/2014, 10-13 (2014) (PDF)

NJP accepted: Thermally induced subgap features in the cotunneling spectroscopy of a carbon nanotube   (posted 2014-11-10)

Today's good news is that our manuscript "Thermally induced subgap features in the cotunneling spectroscopy of a carbon nanotube" has been accepted for publication by New Journal of Physics.
In a way, this work is directly building on our previous publication on thermally induced quasiparticles in niobium-carbon nanotube hybrid systes. As a contribution mainly from our theory colleagues, now the modelling of transport processes is enhanced and extended to cotunneling processes within Coulomb blockade. A generalized master equation based on the reduced density matrix approach in the charge conserved regime is derived, applicable to any strength of the intradot interaction and to finite values of the superconducting gap.
We show both theoretically and experimentally that also in cotunneling spectroscopy distinct thermal "replica lines" due to the finite quasiparticle occupation of the superconductor occur at higher temperature T~1K: the now possible transport processes lead to additional conductance both at zero bias and at finite voltage corresponding to an excitation energy; experiment and theoretical result match very well.

"Thermally induced subgap features in the cotunneling spectroscopy of a carbon nanotube"
S. Ratz, A. Donarini, D. Steininger, T. Geiger, A. Kumar, A. K. Hüttel, Ch. Strunk, and M. Grifoni
New J. Phys. 16, 123040 (2014), arXiv:1408.5000 (PDF)

Emmy Noether grant extended   (posted 2014-06-27)



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!

PRB Rapid Comm. accepted: Sub-gap spectroscopy of thermally excited quasiparticles in a Nb contacted carbon nanotube quantum dot   (posted 2014-05-25)

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)

PRB accepted: Temperature dependence of Andreev spectra in a superconducting carbon nanotube quantum dot   (posted 2014-01-08)

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)

SFB 689 "Spin phenomena in reduced dimensions" funding also renewed for another 4 years!   (posted 2013-11-21)

Yet another piece of exciting news reaches us from Bonn today. After positive evaluation some weeks ago, the collaborative research centre SFB 689 "Spin phenomena in reduced dimensions" has now also officially received a renewal of funding for four more years. Time to celebrate - and then do a lot more of fascinating research!

GRK 1570 "Electronic Properties of Carbon Based Nanostructures" extended!   (posted 2013-11-08)

Today's exciting news is that funding for the the Regensburg DFG research training group "Electronic Properties of Carbon Based Nanostructures" (GRK 1570) has been extended until 2018! The program focuses on the experimental and theoretical investigation of carbon-based nanostructures, i.e. devices based on graphene, carbon nanotubes, aromatic molecules or hybrids of those. A large number of projects is involved, with topics as various as e.g. the transport spectroscopy and analysis of electronic interactions in ultra-clean carbon nanotubes (our group), atomic force microscopy based research on forces in molecular electronics, or femtosecond plasmonics in graphene. More details on the research activities can be found on the projects web page, including direct links to the participating research groups. Thanks a lot to everyone who helped making this happen!

pss(b) accepted: Negative frequency tuning of a carbon nanotube nano-electromechanical resonator under tension   (posted 2013-09-27)

One of the measurements in the MSc project of Sabine Kugler revealed a very particular carbon nanotube nano-electromechanical resonator. Usually the mechanical resonance frequency is tuned up to higher values by applying a gate voltage to the back gate: the larger charge on the nanotube and the larger forces between gate and nanotube cause mechanical tension within the nanotube, and exactly the same way as when tuning a guitar or piano string this translates to a higher resonance frequency. Curiously, here a very strong effect to the opposite can be observed; we can tune the resonance frequency to 75% of its zero-gate voltage value by applying a gate voltage! This is even more astonishing since we can conclude from the higher harmonics of our vibration mode that the nanotube is under tension.
The observed effect can be explained via so-called electrostatic softening of the vibration mode. Let us assume that the carbon nanotube is very close to the gate and vibrates towards and away from it. The capacitance between gate and nanotube varies within one oscillation cycle, and thereby the electrostatic force between these two obtains an additional position-dependent component. This can be seen as an electrodynamic contribution to the spring constant of the resonator; it is negative and thereby decreases the resonance frequency. We can estimate the size of this effect and obtain indeed consistent values for our sample geometry.

"Negative frequency tuning of a carbon nanotube nano-electromechanical resonator under tension"
P. L. Stiller, S. Kugler, D. R. Schmid, C. Strunk, and A. K. Hüttel
accepted for publication by physica status solidi (b), arXiv:1304.5092 (PDF)

PhD position available: A carbon nanotube as a moving weak link   (posted 2013-01-24)

The combination of localized states within carbon nanotubes and superconducting contact materials leads to a manifold of fascinating physical phenomena and is a very active area of current research. An additional bonus is that the carbon nanotube can be suspended, i.e. the quantum dot between the contacts forms a nanomechanical system. In this research field a PhD position is immediately available; the working title of the project is "A carbon nanotube as a moving weak link".

You will develop and fabricate chip structures combining various superconductor contact materials with ultra-clean, as-grown carbon nanotubes. Together with your colleagues, you will optimize material, chip geometry, nanotube growth process, and measurement electronics. Measurements will take place in one of our ultra-low temperature setups.

Good knowledge of superconductivity is required. Certainly helpful is knowledge of semiconductor nanostructures and low temperature physics, as well as basic familiarity with Linux. The starting salary is 1/2 TV-L E13.

Interested? Contact Andreas K. Hüttel (e-mail: andreas.huettel@ur.de, web: http://www.physik.uni-r.de/forschung/huettel/ ) for more information!

PhD position available: Gigahertz nanomechanics with carbon nanotubes   (posted 2013-01-24)

We are currently working on integrating carbon nanotube nanomechanical systems into superconducting radio-frequency electronics. Overall objective is the detection and control of nanomechanical motion towards its quantum limit. In this project, we've got a PhD position with project working title "Gigahertz nanomechanics with carbon nanotubes" available immediately.

You will design and fabricate superconducting on-chip structures suitable as both carbon nanotube contact electrodes and gigahertz circuit elements. In addition, you will build up and use - together with your colleagues - two ultra-low temperature measurement setups to conduct cutting-edge measurements.

Good knowledge of electrodynamics and possibly superconductivity are required. Certainly helpful is low temperature physics, some sort of programming experience, as well as basic familiarity with Linux. The starting salary is 1/2 TV-L E13.

Interested? Contact Andreas K. Hüttel (e-mail: andreas.huettel@ur.de, web: http://www.physik.uni-r.de/forschung/huettel/ ) for more information!

JAP accepted: Transversal Magnetic Anisotropy in Nanoscale PdNi-Strips   (posted 2013-01-01)

Right at the start the new year 2013 brings the pleasant news that our manuscript "Transversal Magnetic Anisotropy in Nanoscale PdNi-Strips" has found its way into Journal of Applied Physics. The background of this work is - once again - spin injection and spin-dependent transport in carbon nanotubes. (To be more precise, the manuscript resulted from our ongoing SFB 689 project.) Control of the contact magnetization is the first step for all the experiments. Some time ago we picked Pd0.3Ni0.7 as contact material since the palladium generates only a low resistance between nanotube and its leads. The behaviour of the contact strips fabricated from this alloy turned out to be rather complex, though, and this manuscript summarizes our results on their magnetic properties.
Three methods are used to obtain data - SQUID magnetization measurements of a large ensemble of lithographically identical strips, anisotropic magnetoresistance measurements of single strips, and magnetic force microscopy of the resulting domain pattern. All measurements are consistent with the rather non-intuitive result that the magnetically easy axis is perpendicular to the geometrically long strip axis. We can explain this by maneto-elastic coupling, i.e., stress imprinted during fabrication of the strips leads to preferential alignment of the magnetic moments orthogonal to the strip direction.

"Transversal Magnetic Anisotropy in Nanoscale PdNi-Strips"
D. Steininger, A. K. Hüttel, M. Ziola, M. Kiessling, M. Sperl, G. Bayreuther, and Ch. Strunk
Journal of Applied Physics 113, 034303 (2013); arXiv:1208.2163 (PDF[*])
[*] Copyright American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.

Lecture announcement: Low Temperature Physics   (posted 2012-10-14)

It's a real pleasure to do the Low Temperature Physics lecture again this winter term. This is a so-called "Modulvorlesung" and "Wahlpflichtfach" (compulsory optional subject?), meaning you'll be able to pick it as examination subject for the MSc physics (possibly also for other courses as e.g. chemistry). Slides are in English, the lecture itself will be in German unless the audience requests otherwise.  In short, we'll be covering the following topics: 
  • properties of cold gases and liquids
  • cryotechnology
  • properties of cold solids (all except superconductivity)
  • superconductivity
  • low-temperature nanophysics
A lot more information can be found on the homepage of the lecture itself. For legal reasons, the slides are only available to participants; I'll tell you the password during the first lecture. So, see you on tuesday morning 8:00ct, PHY 5.0.21!

New dilution refrigerator arriving!   (posted 2012-10-01)

We've been waiting patiently for this for quite some time, but now, it seems, our brand new Oxford Instruments Kelvinox HA400 dilution refrigerator is about to arrive in our lab. Yay! The first two boxes are already there... and another seven, I've been told, are on the way from the UK.
The first picture on the right shows the main insert assembly, which is later dipped into a liquid helium-4 bath for precooling. (Remember, we're going to the millikelvin range, so liquid helium-4 at 4.2K is pretty hot.) The lower, copper-coated part of the insert is a vacuum can, the so-called IVC, and inside there, thermally shielded by the vacuum, all the ultra-low temperature cooling goes on. The lower end of this can finally with a slender tail fits into the 3" central bore of a small superconducting magnet. Some experiments which do not need a magnetic field can be conducted directly at the last cooling stage, others are mounted at the center of this tail, i.e. in the center of the magnet.
The second picture shows the actual main cooling circuit, the so-called dilution unit, which will be mounted into the vacuum can when the system is undergoing final assembly. The cooling process is based on the quantum mechanical properties of liquid helium-3 and helium-4 at temperatures below 0.9K; this particular model can reach temperatures down to 7mK (yes that's 0.007 degrees above the absolute zero of temperature). A simplified description of the process can be found on the dilution refrigerator wikipedia page. We'll post some more and nicer pictures once the final assembly is on the way...

NJP accepted: Magnetic damping of a carbon nanotube NEMS resonator   (posted 2012-07-26)

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)

Top position in condensed matter physics   (posted 2012-06-16)

While everyone in the German research landscape was following the selection of the "Elite Universities" during the last days, we've had some remarkable news here ourselves: the DFG, which awards all the federal research funding in Germany, published the statistics of the last years. The full "Förderatlas" can be found on the website of the DFG, here's the interesting bit (self-made translation):



In the comparison of all German universities, Regensburg managed to get to rank 4 in physics in total, and to a clear number one position in the field of condensed matter physics! Actually we've gotten more funding in CM than TU München and LMU München together! Now if that's not something to celebrate...
Summarizing, now you know where to go for top-notch physics research. :)

Open post-doc position: carbon nanotube-based NEMS   (posted 2012-02-21)

We're still looking for some support in the Regensburg carbon nanotube nanomechanics team! The job description can be found below; please e-mail me if interested! The position is available immediately, however I'm travelling during March and may be hard to reach, so arranging a meeting and a presentation may take a bit of time...

You have already been working successfully with millikelvin RF equipment in your PhD research, and have a good understanding of low temperature physics as well as gigahertz technology? Ideally, you are coming from a research group specialized in superconductor-related mesoscopic physics, quantum information, or cavity QED? You are interested in contributing to a young and dynamic team, trying to push the limits of what is doable in nano-electromechanical systems?
Then you might be just about right here. Your will conduct measurements on coupled superconductor-carbon nanotube nano-electromechanical systems, with a low-temperature high frequency measurement setup in a state-of-the-art dilution refrigerator. Our NEMS team consists at the moment of one PhD student and two MSc students (who you'll help supervise). We expect your work to lead to exceptional publications!
Your salary will be based on the German TV-L E13 (info in German). Regensburg university has a strong focus on nanophysics, in particular on spin phenomena and carbon-based systems. The natives are friendly, and while our university buildings feature classic 1965 concrete, the medieval city of Regensburgis a jewel on its own, with a vibrant young atmosphere. Both mountains and Munich airport are not far away.
Interested? Have a look at our web pages, and contact Andreas K. Hüttel (e-mail: andreas.huettel@physik.uni-r.de) for more information!

The Regensburg nanotube team is growing!   (posted 2011-11-14)

A lot of things have happened recently in our nanotube / nanomechanics research group in Regensburg... First of all, I'd like to congratulate Peter Stiller for finishing off his Diploma thesis and thereby his degree. Peter is immediately continuing as a PhD student, however switching topic from nanomechanics to charge qubits in carbon nanotubes - a newly founded project in the SFB 631. Here we plan to couple electronic quantum states in carbon nanotube double quantum dots to the electric field of a coplanar microwave resonator.
Then, straight from München and the research group of Jan von Delft, Alois Dirnaichner will join us soon as PhD student to work on experimental and theoretical characterization of few-electron states in ultraclean suspended carbon nanotubes. This is a project pursued together with the groups of Milena Grifoni and Christoph Strunk; we hope that the high quality of our carbon nanotubes enables us to do fundamental observations and analysis on unperturbed electronic multi-particle states.
Next, Sabine Kugler joins the nanomechanics team for her MSc thesis project. She will continue the development of chip geometries and materials suitable for combining carbon nanotubes with complex electronics, and help us with the characterization measurements.
Finally, last but not least, Hermann Kraus starts in december as a Diploma student, and will focus on high-frequency electronics at very low temperatures and superconducting nanocircuitry. Time to get these electrons rock'n'roll!

PhD position available: Transport spectroscopy and theoretical analysis of few-carrier systems in carbon nanotubes   (posted 2011-09-01)

We're currently planning a research project in close collaboration with the theory group Prof. M. Grifoni, with working title "Transport spectroscopy and theoretical analysis of interacting few-carrier systems in semiconducting and small-bandgap carbon nanotubes". It combines equal parts of experimental work and theoretical data analysis and modelling. You've already done an excellent solid-state physics theory Diploma or MSc thesis and liked it, but would like to get your hands dirty as well? Then you're maybe the perfect candidate!

Interested? Please have a look at the PDF file with more details, at our web pages (group Prof. M. Grifoni, group Prof. C. Strunk, group Dr. A. K. Hüttel), and contact Andreas K. Hüttel (e-mail: andreas.huettel@physik.uni-r.de) for more information!

PRL accepted: Universality of the Kondo effect in quantum dots with ferromagnetic leads   (posted 2011-09-01)

I'm very glad to be able to report that our manuscript "Universality of the Kondo effect in quantum dots with ferromagnetic leads", describing results that we've been working on during the last months, was just accepted for publication in  Physical Review Letters.
So what is it about, in a few simple words?
In general, much of our work is about charges trapped inside carbon nanotubes at very low temperatures (0.05K). Such a trap for e.g. electrons is called a quantum dot; similar to the electron shell of an atom or molecule, the laws of quantum mechanics force the electrons to occupy specific discrete levels, or quantum states. By looking at a tiny tunnel current through a quantum dot we can characterize its quantum mechanical properties; this is called transport spectroscopy.
The Kondo effect is a special case, as it is caused by strong interaction between localized charges inside the quantum dot and charges in the leads that we attach to the quantum dot. Whenever the localized charge can assume either of two (or more) states with equal energy ("degenerate states") and these states all couple to the leads, the Kondo effect causes an extra electrical conductance through the system. This is one of the simplest many particle effects in quantum mechanics and has fascinated researchers for quite some time; its behaviour is called universal, as it is independent of many detailed properties of the system at hand. In a non-magnetic system, the degenerate states are usually given by different directions of the electron internal magnetic moment, its spin.
Now, we contact our nanotube quantum dot with magnetic contacts. In these contacts, 1) the different directions of spin can couple differently to the quantum dot, and 2) the number of charges with one spin direction differs from the other (that's just what makes them magnetic). Among other things, we've been able to show that all this modification just acts on the Kondo effect the same way as an (imagined) magnetic field, so by applying the reverse magnetic field with an external magnet coil, we can restore the universal behaviour as known from non-magnetic systems. This makes the system much easier to describe, and will, we hope, be useful for future work in spintronics, where the magnetic moments are to be used for information processing.

"Universality of the Kondo effect in quantum dots with ferromagnetic leads"
M. Gaass, A. K. Hüttel, K. Kang, I. Weymann, J. von Delft, and Ch. Strunk
accepted for publication by Physical Review Letters; arXiv:1104.5699 (PDF)

Note: the Wikipedia articles "quantum dot" and "Kondo effect" are not wrong, but describe special uses of these terms and not the most general case as known today. Unfortunately this makes them completely useless as references here...
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