DFG Heisenberg post-mortem, part 2

This post contains remaining parts of my DFG Heisenberg grant final report, by now long submitted and accepted by the DFG. I hope it will be of use, in particular to future DFG grant candidates. The text also somewhat explains why I'm fed up with academia, Regensburg, and physics right now (in no particular order and to varying degrees). The first part of the report can be found in a previous post. Some personal information has been redacted.

Qualification, career, …

During the Heisenberg period 2021-2026, submitted applications for professorships resulted in 8 invitations for presentation and discussion: [...]

A recurrent topic over all my years in academia was that appointment procedures for professorships either failed or that positions were handed to local Emmy Noether, Heisenberg, Juniorprofessor, etc. candidates – much more than expected from past times when the spectre of unwanted “Hausberufung” (in-house appointment) was looming over such events. From that point of view it was equally disappointing that, once I applied for a position in Regensburg, I was immediately told by a member of the appointment commission that my application would be sorted out for exactly that reason. When I spoke to an external member of the appointment commission for that professorship, that external member did not even know that I had applied for the position. In parallel, somewhat doubtful direct appointments to professorships took place in the department.

Further frustration originated from the fact that the Regensburg physics department decided to rewrite their internal regulations such that even after 6 years of Privatdozentur (senior lecturer with habilitation) they only award the title of Außerordentlicher Professor (associated professor, a title without salary) to candidates with a permanent university position. My corresponding petition never received any written response, only oral comment that this is not done anymore. The motivation behind all this is unclear, but it certainly does not make Regensburg more attractive to (somewhat) junior scientists.

A four-eyes career conversation with Mr. Dean of the Faculty resulted in charming comments such as “the best we can do is half an E13 position in the experimental collection maintenance of the undergraduate lectures in a year or so, maybe you could do some research on the side there” and “as a Heisenberg you are an academic legacy problem” (the original German term was “akademische Altlast”). Plus remarks like “we are not going to hand out any (even unpaid) teaching assignments (“Lehraufträge”) to an external Privatdozent anymore, so you’ll lose that title anyway soon for lack of teaching.”

The final cherry on top was then getting berated by another influential senior faculty member about producing insufficient results and being unable to supervise – which is pretty thick coming from someone who barely finds any MSc students, having a reputation among students for erratic grading there. To quote a colleague from Regensburg with permanent employment who should probably stay anonymous, “Der professorale Dünkel ist stark in der Fakultät” (roughly translated, the professorial sense of self-importance is tantamount in the department).

With all these experiences as background, I strongly recommend to anyone with a grant such as DFG Emmy Noether or Heisenberg not to join the Regensburg physics department, but go somewhere else. You and your work will be valued more, and that by far compensates for any Regensburg “excellence”. [...]

Teaching and scientific events

With already significant teaching experience, I decided to mostly focus on research and supervision during the Heisenberg period – with the exception of a “Low Temperature Physics” course where the usual lecturer was on sabbatical. Regarding scientific events, I had requested a moderate amount of funding for a workshop on inorganic nanotubes in [...], which would have fitted quite well as add-on to the workshops of SFB 1277. This was however declined – accordingly, it also did not have priority for me anymore either.

Perspective

I enjoyed university research and teaching as well as working with students, establishing a research group, quantum device fabrication, and complex measurements very much. With the “orderly shutdown” of the group comes that still some students need to graduate and that some results will still be written up and submitted for publication (at the time of this report two papers have been submitted and another two or three might follow); however, I have cleared out my office the day of the end of my employment contract. 

Out of a certain stubbornness, I will keep teaching my 2SWS per year Titellehre for now – as confirmed by the faculty administration, a teaching appointment (“Lehrauftrag”) is legally not required for that, and as a consequence the students will be treated to a block course “How to write a thesis, present a talk, …” in October. However, after over 15 years in Regensburg I have learned that being nice, friendly, cooperative, and helpful might get you friendly smiles and occasional money there but not any sustainable support, and that the Regensburg physics system prefers “junior scientists” to be subordinates, not colleagues. Of course someone who does not have the visible backing of his current university immediately also has a weaker position when applying elsewhere. 

There’s a lot more I could write, but that would go beyond scope and purpose of this report. Anyway, it is now time to make a bonfire out of lecture notes, to go out into the wide world, and to do something entirely new. Which, barring big surprises, most likely will not involve physics or a university anymore. 

Thanks to Duong Thi Hai Yen on vecteezy.com for the tomato.

Advanced Materials published: "Non-destructive low-temperature contacts to MoS2 nanoribbon and nanotube quantum dots"

Molybdenum disulfide MoS₂ is often discussed as a groundbreaking material for electronics applications. As bulk crystal or as 2D layer material similar to graphene, it is an excellent semiconductor with strong spin-orbit interaction. At strong doping it even becomes intrinsically superconducting, with possibly interesting topological properties.

So how about using MoS₂ for quantum electronics, single electron devices, or (charge, spin, or valley) qubits? There's already enough science fiction literature out there mentioning "molycircuits"... Well, a small problem stands in the way.  It turns out that the electronic band structure of the material makes reaching quantum confinement, i.e., discrete, addressable electronic quantum states, quite hard. The chip structures need to be built at very small scale, difficult to reach with MoS₂ flakes on a chip surface, and so far no research group has managed to do this in a controlled way.

That's exactly why we are interested in MoS₂ nanotubes. They can be grown clean and straight, with diameters down to 20nm - which automatically confines charge carriers in one more direction compared to a two-dimensional MoS₂ flake. What remains is to restrict motion of the charge carriers along the nanotube axis and to make good metallic contacts. However, it turns out that making good contacts to MoS₂ is hard - either your metals form a Schottky barrier, or they react with the MoS₂ and destroy its crystal structure.

In our work, we show that this problem has now been solved. Using a thin layer of the semimetal bismuth, we obtain contacts which remain transparent even at cryogenic (millikelvin) temperatures and leave the structure of MoS₂ below and next to them intact. This makes transport spectroscopy measurements on a MoS₂ quantum dot forming within the nanotube possible. We indeed observe quantum dot behaviour; the temperature dependence of the Coulomb oscillations of conductance indicates that we reach electronic tunneling through single, discrete quantum states below a temperature of T=100mK. 

With this, we have made a big step towards controllable charge, spin, or even valley quantum states in MoS₂. Next we need to miniaturize our devices even further, and increase our measurement resolution to be able to analyze the electronic level spectrum in magnetic fields.

"Non-destructive low-temperature contacts to MoS₂ nanoribbon and nanotube quantum dots"
R. T. K. Schock, J. Neuwald, W. Möckel, M. Kronseder, L. Pirker, M. Remskar, and A. K. Hüttel
accepted for publication by Advanced Materials (doi:10.1002/adma.202209333); arXiv:2209.15515 (PDF, supplementary information)

Video from Walter Schottky Prize award ceremony now online

 The video recording of the Walter Schottky Prize award ceremony in June is finally online. Enjoy :)

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:

• "Microwave optomechanics of the transversal carbon nanotube vibration"
• "Transport spectroscopy and hybrid devices of transition metal dichalcogenide nanotubes" 

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: 

• press release of the German Physical Society (in German)
• press release of University of Regensburg regarding the Walter Schottky Prize (in German)
• list of Walter Schottky Prize recipients (in German)
• press release of University of Regensburg regarding the Heisenberg grant (in German)
• home page of my research group
• my homepage
  

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)

Press release (in German) on our recent Nature Communications

We've published a press release (in German) on our recent Nature Communications publication "Quantum capacitance mediated carbon nanotube optomechanics".

Read it on the university web page!

(A summary in English can be found in a previous blog post.)

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)

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)

Press release (in German) on our recent PRL

Regensburg University has published a press release (in German) on our recent Physical Review Letters "Editor's Suggestion" publication, "Shaping Electron Wave Functions in a Carbon Nanotube with a Parallel Magnetic Field". Read it on the university web page!

(A summary in English can be found in a previous blog post.)

PRL published: Shaping electron wave functions in a carbon nanotube with a parallel magnetic field

We're happy to announce that our manuscript "Shaping electron wave functions in a carbon nanotube with a parallel magnetic field" has been published as Editor's Suggestion in Physical Review Letters.

When a physicist thinks of an electron confined to a one-dimensional object such as a carbon nanotube, the first idea that comes to mind is the „particle in a box“ from elementary quantum mechanics. A particle can behave as a wave, and in this model it is essentially a standing wave, reflected at two infinitely high, perfect barrier walls. The mathematical solutions for the wave function describing it are the well-known half-wavelength resonator solutions, with a fundamental mode where exactly half a wavelength fits between the walls, a node of the wave function at each wall and an antinode in the center.

In this publication, we show how wrong this first idea can be, and what impact that has. In a carbon nanotube as quasi one-dimensional system, an electron is not in free space, but confined to the lattice of carbon atoms which forms the nanotube walls. This hexagonal lattice, the same that also forms in planar form graphene, is called bipartite, since every elementary cell of the lattice contains two carbon atoms; one can imagine the nanotube wall as being built out of two sub-lattices, with one atom per cell each, that are shifted relative to each other. Surprisingly, the hexagonal bipartite lattice does not generally support the half-wavelength solutions mentioned above, where the electronic wave function becomes zero at the edges. In each sublattice, we can only force the wave function to zero at one end of the nanotube "box"; its value at the other end remains finite. This means that the wave function shape for each of the two sublattices is more similar to that of a quarter wavelength resonator, where one end displays a node, the other an antinode. The two sublattice wave functions are mirrored in shape to each other, with node and antinode swapping position.

When we now apply a magnetic field along the carbon nanotube, a magnetic flux enters the nanotube, and the boundary conditions for the electron wave function change via the Aharonov-Bohm effect. Astonishingly, its shape along the carbon nanotube can thereby be tuned between half-wavelength and quarter-wavelength behaviour. This means that the probability of the trapped electron to be near the contacts changes, and with it the tunnel current, leading to a very distinct behaviour of the electronic conductance. It turns out that our measurement and the corresponding calculations are agreeing very well. Thus, our work shows the impact of a non-trivial host crystal on the electronic behaviour, important for many novel types of material.

"Shaping electron wave functions in a carbon nanotube with a parallel magnetic field"
M. Marganska, D. R. Schmid, A. Dirnaichner, P. L. Stiller, Ch. Strunk, M. Grifoni, and A. K. Hüttel
Physical Review Letters 122, 086802 (2019), Editor's Suggestion; arXiv:1712.08545 (PDF, supplementary information)

PRL accepted: Nanomechanical characterization of the Kondo charge dynamics in a carbon nanotube

Today's great news is that our manuscript "Nanomechanical characterization of the Kondo charge dynamics in a carbon nanotube" has been accepted for publication by Physical Review Letters.

The Kondo effect is a many-body phenomenon at low temperature that results from a quantum state degeneracy, as, e.g., the one of spin states in absence of a magnetic field. In its simplest case, it makes a quantum dot, in our case a carbon nanotube with some trapped electrons on it, behave very different for an even and an odd number of electrons. At an even number of trapped electrons, no current can flow through the nanotube, since temperature and applied bias voltage are too low to charge it with one more elementary charge; this phenomenon is called Coulomb blockade. Strikingly, at odd electron number, when two degenerate quantum states in the nanotube are available, Coulomb blockade seems not to matter, and a large current can flow. Theory explains this by assuming that a localized electron couples to electrons in the contacts, forming a combined, delocalized singlet quantum state.
What carries the Kondo-enhanced current, and how does the electric charge now accumulate in the carbon nanotube? We use the vibration of the macromolecule to measure this. As also in the case of, e.g., a guitar string, the resonance frequency of a nanotube changes when you pull on it; in the case of the carbon nanotube this is sensitive enough to resolve fractions of the force caused by a single elementary charge. From the vibration frequency, as function of the electrostatic potential, we calculate the average number of electrons on the nanotube, and can then compare the odd and even number cases.
A surprising result of our evaluation is that the charge trapped on the nanotube behaves the same way in the even and odd occupation case, even though the current through it is completely different. Sequential tunneling of electrons can model the charge accumulation, and with it the mechanical behaviour. The large Kondo current is carried by virtual occupation of the nanotube alone, i.e., electrons tunneling on and immediately off again so they do not contribute to the charge on it.

"Nanomechanical Characterization of the Kondo Charge Dynamics in a Carbon Nanotube"
K. J. G. Götz, D. R. Schmid, F. J. Schupp, P. L. Stiller, Ch. Strunk, and A. K. Hüttel
Physical Review Letters 120, 246802 (2018); arXiv:1802.00522 (PDF, HTML, supplementary information)

PRL accepted: Secondary electron interference from trigonal warping in clean carbon nanotubes

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

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 T_c 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 T_c~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

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

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

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

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)

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

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)

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)