Reducing the length scales of transistors well below 100 nanometers is one of the present key efforts in semiconductor industry. At the same time such tiny structures allow the observation of phenomena obeying the fundamental laws of quantum mechanics. In our clean room we fabricate a variety of gate-controlled devices by e-beam lithography starting from GaAs/AlGaAs heterostructures that contain a two-dimensional electron system. One of our main efforts aims at possible applications in quantum information technology of interacting zero-dimensional quantum dots (QD). In 2006 we completed an experiment on the Kondo effect of one or two electrons charging a double QD. We are thankful for collaborations with Mikhail Kiselev (Trieste, Italy) and Boris Altshuler (New York, USA), who helped to interpret the observations. In another project we realized a quantum ratchet based on a double QD. The energy source was a strongly biased quantum point cantact in an electrically isolated circuit. Based on our previous work on double QDs, we realized in 2006 a serial triple QD and demonstrated full control of its quantum mechanical states in the regime of few electrons charging the device. A triple QD is an important step towards a full-scale quantum computer. In this still ongoing work we profit strongly from collaborations with the Lloyd Hollenberg group (Melbourne, Australia) and the Andy Sachrajda group (Ottawa, Canada). In an effort to understand the interaction between discrete electronic states and discrete vibrational modes we also explore electronic transport through double quantum dots defined in a mechanical nanoscale bridge. In addition we explore electronic transport properties in molecular devices. In 2006 one aspect were studies of the influence of mechanical strain on the conductance of individual carbon nanotubes. Another topic concerned the conductance mechanisms of organic field effect transistors based on pentacene. In close collaboration with the group of Bert Nickel we studied the interband photoresponse with high spatial resolution to obtain a better understanding of the separation of photoexcited carriers.
Daniel M. Schröer, Andreas K. Hüttel, and Stefan Ludwig,
in collaboration with Boris L. Altshuler, Karl Eberl, Mikhail N. Kiselev
We have investigated the Kondo effect on a double quantum dot (DQD) (Fig. 1b) in the few electron limit (Fig. 1a and c). For a charge of just one electron in the DQD, our measurements display a surprising quasi oscillation of the Kondo current in dependence of a small perpendicular magnetic field of a few mT (Fig. 2). We assign this behaviour to a fine tuning of the single dot eigen-energies. The interdot tunnel coupling and, as a consequence, the Kondo current have a local maximum for perfect alignment of two orbital states of the two adjacent quantum dots. In the two-electron case, transport is determined by symmetric and anti-symmetric combinations of the wave functions (i.e. singlet and triplet). Therefore, the two-electron Kondo conductance is not sensitive to single electron eigen energies and no oscillating behavior appears (Fig. 2 inset).
Vadim S. Khrapai, Stefan Ludwig, and Jörg P. Kotthaus,
in collaboration with Werner Wegscheider
We studied a double quantum dot (DQD) coupled to a strongly biased quantum point contact (QPC), each embedded in independent electric circuits. A SEM-picture of the gate layout is shown in Fig.1a and a sketch of the experiment in Fig.1b. If the QPC is unbiased, we observe a finite current through the DQD only at the triple points of its charge stability diagram, where Coulomb blockade is lifted. This situation is plotted in Fig.1c. For a strong bias on the QPC and weak interdot tunneling of the DQD we observe a finite current flowing even through the Coulomb blockaded DQD (Fig.1d). The direction of the current through the DQD is determined by the relative detuning of the energy levels of the two quantum dots. We interpret the results in terms of a quantum ratchet phenomenon in a DQD energized by a nearby QPC. The QPC emits energy quanta, which can be reabsorbed by the DQD as sketched in the inset of Fig.1d. For the sketched case electrons are pumped through the DQD from the right lead to the left lead.
Daniel M. Schröer, Jörg P. Kotthaus, and Stefan Ludwig,
in collaboration with A.D. Greentree, L.C.L. Hollenberg, L. Gaudreau, and K. Eberl
We have created three tunnel coupled quantum dots in a serial configuration. The quantum dots are defined laterally in a two-dimensional electron gas by applying negative voltages to Ti/Au gate electrodes on the surface of a GaAs/AlGaAs heterostructure (Fig. 1). Each quantum dot is occupied with a well-defined number of electrons that can be changed down to zero by varying gate voltages Vα, Vβ and Vγ. Three nearby quantum point contacts (Fig. 1) allow to monitor changes of the electronic configuration and can be used to map charge stability diagrams of the triple quantum dot (TQD) (Fig. 2). Quantum mechanical ground and excited states can be investigated by driving current through the TQD. The rich set of observed features includes peculiar TQD properties like quadruple points, where four configurations are degenerate allowing resonant transport of electrons through the TQD, combined charge reconfigurations of multiple quantum dots (quantum cellular automata processes), and a bistable region of the stability diagram.
Clemens Rössler, Jörg P. Kotthaus, and Stefan Ludwig,
in collaboration with Max Bichler, Dieter Schuh, and Werner Wegscheider
We employ quantum dots embedded in nanoscale phonon cavities in order to investigate the electron-phonon interaction. Our phonon cavities are nanoscale bridges excavated from an AlGaAs/GaAs heterostructure. The latter contains a two-dimensional electron system (2DES) which is depleted locally by applying a negative voltage to metallic top gates (Fig. 1).
By this method it is possible to define 1D-constrictions (i.e. quantum point contacts) or 0D-systems (quantum dots) within the 2DES. Fig. 2 depicts a measurement of our first successfully realised gate defined freely suspended quantum point contact defined by biasing gates 1 and 2 (Fig.1). The differential conductance G decreases quantized in multiples of the fundamental value of G0 = 2e2/h as the one-dimensional subbands are successively depleted.
Dawid Kupidura , Jörg P. Kotthaus, and Stefan Ludwig,
in collaboration with Werner Wegscheider
We aim towards the coherent manipulation of single electrons in double quantum dots laterally defined by surface gates (expanded region in Fig. 1) in GaAs/AlGaAs heterostructures. A strong magnetic bias field parallel to the sample surface serves to allign the magnetic moment of the electron. A much weaker radio frequency magnetic field perpendicular to the sample surface is produced by the integrated loop antenna shown in Fig.1. The sample holder (photograph in Fig. 1) provides impedance matched coupling of the rf-signal between coax-cable and the sample chip. The design aims at spin-resonance experiments on a single electron (ESR). First measurements performed on a single quantum dot show that the rf antenna and sample holder work.
Daniel Harbusch, and Stefan Ludwig,
in collaboration with Werner Wegscheider,Tobias Zibold and Peter Vogl
We realized a switchable quantum mechanical beamsplitter for ballistic electrons that could be used as a universal logic gate for quantum computing. The system consists of two coupled, gate defined 1D channels in the 2DEG of a GaAs/AlGaAs heterostructure (Fig. 1a). For appropriate geometric properties of the gate structure, electrons coming from one channel (1) can be switched between two exit channels (3 and 4) by changing the fermi energy of the system e.g. via a back gate. With a gate geometry based on numerical calculations with ³NextNano performed by Tobias Zibold and Peter Vogl we performed first switching experiments on such a device. The results, shown in Fig. 1b are in good agreement with theoretic predictions (Fig. 1c). Strong temperature and magnetic field dependences of the switching effect (not shown) exclude purly classical switching.
Christian J.-F. Dupraz, Udo Beierlein, and Jörg P. Kotthaus
Carbon nanotubes are one key building block in nanotechnology, they have gained a tremendous attention in science in the last decade. They show unique material properties: e.g. in mechanics they are known as the stiffest material with an extremely high breaking tension. We combine mechanical manipulation of single wall carbon nanotubes with electrical measurements, to learn how substential strain affects electronic transport. Single wall carbon nanotubes are dispersed on a polyimide film and connected by gold leads. Bending of the substrate exerts strain on the nanotubes and influences the electrical conductance. The bent sections are expected to act as scatterers for the electrons, and hereby the conductance is reduced . The resistance usually increases with larger strain. Some samples show contrary behaviour, with a decrease of resistance followed by a rise (Fig. 1). The findings are in qualitative agreement with theoretical calculations demonstrating a change of the chirality dependent band gap with uniaxial stress .
 Chaplik A.V., JETP Letters 80 (2), 130 (2004).
 Yang L., Anantram M.P, Han J. and Lu J.P., PRB 60 (19), 13874 (1999).
Matthias Fiebig and Jörg P. Kotthaus
In collaboration with Martin Göllner, Stefan Schiefer, Bert Nickel, Christoph Erlen, Paolo Lugli
Performance of organic thin film transistors (OTFTs) has improved in recent years dramatically to a degree where devices become suited for widespread applications. Nevertheless electronic properties are still not fully understood. In order to get further insight in electronic transport of these devices we have performed spatially resolved photoresponse measurement with submicron resolution on our bottom contact pentacene TFTs (Fig. 1). We observe inhomogeneous photoresponse due to varying contact quality. This we consider an important limiting factor for electronic transport in bottom contact pentacene transistors (Fig.2). Moreover, the photoresponse is clearly enhanced if the laser beam is focused close to the positively biased electrode. We believe that this is caused by a limited lifetime of the photogenerated electrons that, compared to holes, have a very small mobility in pentacene.
The optical manipulation of electronic excitations in nano-structures is at the center of our research. To this end, optically generated excitons are studied in various solid state environments. A. Gärtner et al. developed a new method to store long-living excitons in coupled GaAs quantum wells. The excitonic traps are electrostatically controllable und they rely on the quantum confined Stark effect. A. Högele et al. demonstrated how the dispersion and absorption of self assembled InAs and InGaAs quantum dots corresponding to the real and imaginary part of the dielectric susceptibility can be measured by means of a high-resolution laser spectroscopy. Generally, excitons in self-assembled quantum dots constitute an atomic-like solid state system ideally suited to study quantum properties, while it is attractive for realizing qubits. S. Seidl et al. showed how the exciton fine structure in self-assembled quantum dots can be tuned via uniaxial strain. To this end, S. Seidl et al. developed an experimental technique to apply the strain in-situ by a piezo-driven stage. B. Zebli et al. developed a chemical scheme to self-assemble hybrid structures for optoelectronic studies. The hybrid structures are made out of single wall carbon nanotubes, proteins, and nanocrystals.
Andreas Gärtner, Alexander W. Holleitner, and Jörg P. Kotthaus,
in collaboration with Dieter Schuh
Trapping and confining long-living excitons attracts much scientific interest especially in the field of Bose-Einstein condensation (BEC) of excitons. The creation of high exciton densities which are suitable for BEC requires efficient trapping techniques. Whereas the motion of excitons can be manipulated in detail [1-3], exciton confinement still remains a challenge. We developed a new method for storing long-living excitons in line-shaped traps (Fig. 1a) [4,5]. The trap for excitons is generated via two metallic surface gates which are in close vicinity (Fig 1b). The semi-transparent gates were micro-structured photo-lithographically on top of a GaAs/AlGaAs structure containing coupled quantum wells. Both surface gates differ by the presence/absence of a SiO2 layer which is sandwiched between the gate and the semiconductor. A line-shaped in-plane harmonic trap for excitons is formed along the interface of both gates. The trap can be switched „off“ and „on“ electrically via the gate voltages. The measured trapping potential is approximated by a harmonic function with spring constants of about k = 11 keV/cm2 [4,5].
 A. Gärtner et al., Physica E 32, 195 (2006).
 A. Gärtner et al., Appl. Phys. Lett. 89, 052108 (2006).
 J. P. Kotthaus, Phys. stat. sol (b) 243, 3754 (2006).
 A. Gärtner et al. (in preparation).
 A. Gärtner, PhD thesis (2006).
Alexander Högele, Martin Kroner,Stefan Seidl, and Khaled Karrai
in collaboration with B. Alén, R. J. Warburton, A. Badolato, G. Medeiros-Ribeiro and P. M. Petroff
The dielectric susceptibility of a single quantum dot was determined experimentally. Both the dispersion and absorption of self assembled InAs and InGaAs quantum dots corresponding to the real and imaginary part of the dielectric susceptibility were measured with high-resolution laser spectroscopy in a Fabry-Perot setup at 4.2 K. In this experiment, the weak electromagnetic fields scattered by the excitonic transitions in the quantum dot are made to interfere with the reference laser excitation field .
 B. Alén, et al., Appl. Phys. Lett. 89, 123124 (2006).
Stefan Seidl, Martin Kroner, Alexander Högele,
and Khaled Karrai
in collaboration with R. J. Warburton, A. Badolato and P. M. Petroff
The biexciton-exciton decay cascade in a neutral quantum dot is a possible source for single polarisation entangled photon pairs. However, the fine structure of the exciton destroys this property. Therefore we investigated the fine structure of the neutral exciton in a single self-assembled InGaAs quantum dot under the effect of an applied uniaxial stress. The spectrum of the excitonic Rayleigh scattering was measured in reflectivity using high-resolution laser spectroscopy while the sample was exposed to a tunable uniaxial stress along its  crystal axis. We show that using this stretching technique, the quantum dot potential is elastically deformable such that the exciton fine structure splitting can be substantially reduced.
 S. Seidl, et al., Appl. Phys. Lett. 88, 203113 (2006).
Bernd Zebli, Markus Mangold, Alexander W. Holleitner,
and Jörg P. Kotthaus
in collaboration with I. Carmeli, C. Carmeli, S. Richter
In this project, hybrid systems made of individual colloidal nanocrystals, proteins and single wall carbon nanotubes are synthesized via a self-assembly technique, taking advantage of the strong non-covalent biotin-streptavidin-hydrogen bond interaction. To this end, the single wall carbon nanotubes are first functionalized with biotin molecules and then bound to chemically functionalized proteins or nanocrystals which are covered with streptavidin molecules. Individual hybrid systems are adsorbed onto an insulating SiO2 wafer, analyzed by atomic force microscopy, and finally contacted by electrodes using electron beam lithography. The hybrid systems are characterized using optoelectronic charge transport measurements under resonant optical excitation of the constituents of the hybrid nanosystem.
Nanomechanical systems are realized by lithographic techniques that are capable to create freestanding objects in silicon and other materials, with thickness and lateral dimensions down to about some 10 nanometers. Shrinking mechanical devices in thickness, width and length leads to reduced mass, increased resonant frequency, and lowered force constants of these systems. Therefore nanomechanical systems are both fascinating objects for fundamental studies in the quantum regime and promising for a large variety of applications such as extremely sensitive sensors and actuators. Advances in the field include improvements in fabrication processes, new methods for actuating and detecting motion at the nanoscale. A still challenging problem is how to easily excite nanomechanical elements and control their motion. In both areas we achieved substantial progress.
Stephan Camerer, Daniel R. König, and Jörg P. Kotthaus
in collaboration with Theodor W. Hänsch, Jakob Reichel, Philipp Treutlein
Object of this work is the coupling of Bose-Einstein condensed atoms to a nanomechanical resonator (Figure1). By optimizing the fabrication process freely suspended nanosturctures with magnetic islands on top were realized (Figure2). The mechanical characteristics were determined by studying the Brownian motion of the cantilevers. The measured quality factor is in the range between 2000 and 3000 and the resonance frequency around 500 kHz. The magnetic structures were characterized by scanning force spectroscopy. The magnetic trap for the Bose-Einstein can be generated by a current through 1 µm thick gold wires.
Ivan Favero, Constanze Metzger, Stephan Camerer, Daniel R. König, Heribert Lorenz, Jörg P. Kotthaus, and Khaled Karrai
We investigated the passive optical cooling of the Brownian motion of a cantilever suspended micromirror (Fig. 1a, 1b). We show that laser cooling is possible for a mirror of size in the range of the diffraction limit (at λ = 1.3 µm) (Fig. 2). This represents the tiniest mirror optically cooled so far, with a mass of 11.3 pg, more than 4 orders of magnitude lighter than current mirrors used in cavity cooling. The reciprocal effect of cooling is also investigated and opens the way to the optical excitation of MHz vibrational modes under continuous wave laser illumination .
 Favero I., Metzger C., Camerer S., König D.R.,
Lorenz H., Kotthaus J.P., Karrai K.
Appl. Phys. Lett. 90, 104101, 2007
Quirin Unterreithmeier, Daniel R. König, and Jörg P. Kotthaus
The motion of nanostrings under high tensile stress (1.2 GPa) is found to have a Q-factor up to 200000 (low internal damping) at room temperature . Often sensitivity scales with the Q-factor. We study the effect of tunable tensile stress on frequency and Q-factor. The resonators are fabricated using standard top down lithography and subsequent RIE- and wet-etching methods (Fig. 1). Resonators made from silicon nitride were processed, with lengths ranging from 13 to 40µm. width and height are 200 nm. The resonators are driven using a piezo actuator. A static tensile stress of 1.4 GPa resulting from the LPCVD process during wafer fabrication leads to high Q-factors (Fig. 2).
 Verbridge S. et al., J. Appl. Phys. 99, 124304 (2006)
Philipp Paulitschke, and Heribert Lorenz
in collaboration with Achim Wixforth
We succeeded in fabricating large arrays of micro- and nano-pillars by conventional optical and electron beam lithography followed by RIE-etching or wet chemical etching (Fig. 1). The samples are excited by piezo transducers  or by surface acoustic waves. They are investigated in an interferometric set-up as well as in the SEM (Fig. 2). The high aspect ratio pillars show eigenfrequencies of the fist harmonic mode ranging from 100 kHz up to 10 MHz with quality factors of about 2000-3000 (Fig. 3). The measured eigenfreqencies and the simulations are in good agreement.
 Jänchen G., Hoffmann P., Kriele A., Lorenz H., Kulik A.J., Dietler G. Appl. Phys. Lett. 80,
Tim Liedl, Michael Olapinski, and Friedrich C. Simmel
Artificially constructed DNA-based nanodevices can cyclically undergo large conformational changes in response to the presence of certain trigger molecules such as other DNA strands or to changes of the environmental conditions. So far, in most cases DNA-based devices have been controlled by an external operator, who added the effector DNA strands or changed the buffer conditions manually. As an approach towards autonomous motion of molecular machines, our group had previously demonstrated a pH-dependent DNA switch in solution, which reacted on the pH changes of a chemical oscillator run in a semibatch reactor . By attaching the pH-dependent DNA switch to a solid substrate an run the pH oscillator in a continuously stirred tank reactor (CSTR), we were recently able to overcome an essential drawback of the semibatch setup - the damping of the oscillations - and achieve continuous opening and closing of the surface-bound DNA switch over many undamped oscillations .
 T. Liedl and F.C. Simmel, Nano Letters 5, 1894-1898 (2005).
. T. Liedl, M. Olapinski, F. C. Simmel, Angew. Chem. Int. Ed. 45, 5007-5010 (2006).
Stefan Beyer, and Friedrich C. Simmel
DNA-based nanotechnology provides the possibility to combine simple information processing tasks with mechanical or chemical action. An example for such a combination is the thrombin aptamer, which binds to the protein "thrombin" in a sequence-specific manner. The addition of a DNA strand complementary to the sequence of the aptamer can induce the release of the protein by a conformational change of the aptamer strand . In order to uncouple the input sequence from the sequence needed for protein release we designed and successfully employed a signal translator based on branch migration and the action of the endonuclease FokI . For future applications the input strand could be derived from disease related DNA sequences, which then would trigger the release of a therapeutic protein.
 W. U. Dittmer, A. Reuter, and F. C. Simmel, Angew. Chem Int. Ed. 43, 3550-3553 (2004).
 S. Beyer and F. C. Simmel, Nucleic Acids Research 34, 1581-1587 (2006).
Barbara Müller, Andreas Reuter, Friedrich C. Simmel, and Don Lamb
Single molecule studies of biological molecular machines reveal many interesting features which are usually missed in bulk experiments.
Bulk experiments characterize the average behavior of a large number of molecules, whereas single molecule studies also provide information
on individual properties. In a CeNS-collaboration with Don Lamb and Barbara Müller from the LMU chemistry department, we recently used
single-pair fluorescence resonance energy transfer (spFRET) experiments to study one of the prototype DNA-based molecular machines - the .
In spFRET experiments, the energy transfer between fluorophores is measured on a single molecule level and the results are evaluated
statistically. FRET provides information on the distance between two fluorophores and can therefore be used to study the conformation of
biomolecules. In contrast to earlier bulk experiments, spFRET characterization of DNA tweezers revealed the presence of subpopulations
which can be attributed to incorrectly assembled molecular devices.
It is expected that single molecule techniques will become increasingly important for the future development of more complex artificial molecular machines.
 B. Müller, A. Reuter, F. C. Simmel, and D. C. Lamb, "Single-pair FRET characterization of DNA tweezers", Nano Letters 6, 2814-2820 (2006).
Michael Olapinski, Niels Fertig, and Friedrich C. Simmel
Many biological molecules and structures exhibit large dipolar moments and are strongly influenced by electrical fields. In particular in bilayer membranes and ion channels, electrical fields contribute actively to their biological function. In a cooperation with Nanion Technologies GmbH, we investigate the interaction between external electrical fields of frequencies extending to the Gigahertz range and bilayer or cell membranes containing pore forming peptides or ion channels, in order to examine the dynamic behavior of these systems and to identify methods to externally control their biological function. We recently succeeded in the detection of supported lipid bilayer formation and subsequent incorporation of pore forming Alamethicin peptides in the Gigahertz transmission signal of a lithographically defined coplanar waveguide . In another approach, whole cell currents under the influence of high-frequency (HF) irradiation are investigated, using a planar patch-clamp setup, pulsed HF and lock-in demodulation of the current variations. One major challenge is the discrimination of the thermal influence of HF electrical fields from specific dipolar effects on ion channels. We are currently studying the effects with different pulse modulation frequencies (see fig. 1, manuscript in preparation).
 Olapinski M., Manus S., George M., Brüggemann A., Fertig N. and Simmel F. C., „Detection of lipid bilayer and peptide pore formation at gigahertz frequencies“, Appl. Phys. Lett. 88, 013902 (2006).
Funding of this work via the following agencies is gratefully acknowledged: