Research report 2003

  1. Introduction
  2. Electronic properties
    1. Towards coupling of single electron states
    2. Electron-phonon interaction in freely suspended quantum dots
    3. Primary thermometer formed by Coulomb islands in a suspended silicon nanowire
    4. Quantum interference in a one-dimensional silicon nanowire
    5. SAM-assisted assembly of carbon nanotubes on gold electrodes
    6. Nanostructures for testing molecular wires
    7. Low-temperature, in situ tunable, uniaxial stress measurements in Semiconductors using a piezoelectric actuator
  3. Optics with nanostructures
    1. Fine structure of highly charged excitons in semiconductor quantum dots
    2. Stark-shift modulation absorption spectroscopy of single quantum dots
    3. Kondo excitons in self assembled quantum dots
    4. Dynamics of excitons in coupled quantum wells
    5. Quantum well photograph
  4. NEMS
    1. Optically-tunable mechanics of microlevers
    2. Acoustic actuation of nanomechanical resonators
    3. Excitation of nano-electromechanical systems in silicon
    4. Optical detection of quasi-static actuation of nanoelectromechanical systems
  5. Nanodevices and bio-templated nanostructures
    1. Surface acoustic wave studies for chemical sensors and nonlinear surface acoustic wave propagation
    2. Mikro- and nanofluidic on piezoelectric substrates
    3. Patch-clamp-on-a-chip - automated ion channel analysis
    4. DNA-templated materials synthesis
    5. DNA self-assembly and nanostructures
    6. Molecular machinery
  6. Diploma theses in 2003
  7. PhD theses in 2003
  8. Guest scientists in 2003
  9. Group members in 2003
  10. Publications in 2003
  11. Invited talks in 2003
  12. Senior scientists of cooperating groups
  13. Financial support in 2003


Introduction

Physical properties of nanoscale devices and systems are the major theme of our research. In our work we combine the realization and study of top-down fabricated devices employing techniques derived from semiconductor processing with the exploration of bottom-up assembled systems made via chemical and biochemical routes. Our goals range from a deeper fundamental understanding of physical behavior in the nanometer regime to the realization of novel devices and functions developing and employing various nanotechnologies.

The study of electronic properties of nanoscale systems is motivated by the increasing importance of quantum effects as electronic devices decrease in size and the limitations of scaling of traditional electronic devices such as the Metal-Oxide-Semiconductor Field-Effect-Transistor (MOSFET). Our studies of quantum phenomena aim at finding new ways of information processing. One current major research theme are ways to implement and control quantum bits on the basis of solid state electronic and optical devices and thus lay the ground work for a future quantum information technology. In part this research is embedded in the SFB 631 — a long term research platform on “Solid-State Based Quantum Information Processing”, funded by the German Science Foundation (DFG) since July 2003.

Though MOSFET-based Silicon technology will continue to dominate electronics for at least another decade the search for alternative approaches to electronic functions employing molecular scale units and mechanisms of self-assembly is our motivation for new research projects studying molecular electronics with carbon nanotubes and self-assembled monolayers as well artificial molecular networks and machines based on DNA-directed self-assembly. A major portion of this research is part of a DFG-funded Emmy-Noether junior group headed by Friedrich Simmel.

In the nanometer regime electronic, mechanical and optical properties become often inseparably connected. Therefore we have in recent years increased our activities on studying Nano-Electro-Mechanical Systems, in short NEMS, ranging from fundamental studies of the resonant behavior of nanoscale bridges fabricated out of semiconductors to their development as sensing and actuating devices. Ways to efficiently drive such nanoscale resonators by electric, magnetic, mechanic as well as optically induced forces are one current research aspect. Sensing the mechanical motion with nanometer resolution via capacitance, current or optical reflection is another basic theme of ongoing research.

The particular optical properties of nanoscale systems are most dramatically reflected in the discrete optical emission and absorption spectra of quantum dots. There our research is focused on studying individual quantum dots grown by a self-assembling molecular beam epitaxy process in the group of Pierre Petroff, our long-term collaborator and Humboldt award recipient at UC Santa Barbara. In these studies we employ successfully the possibility of changing the occupation of such self-assembled quantum dots with individual electrons by a gate voltage, a technique developed ten years ago in this collaboration. This is combined with high resolution optical techniques to enable the detailed study of the optical absorption and emission of individual quantum dots as a function of charge, magnetic field and temperature. The fundamental aspect of this research carried out jointly with the group of former CeNS member Richard Warburton at Heriot-Watt University in Edinburgh aims at understanding many body interactions in quantum dots. A more application-oriented goal is the realization of single photon emitters that can generate individual photons on demand and are needed for quantum information processing.

The year 2003 has again seen quite a few changes in our group. Five doctoral and one diploma students have successfully completed their degree and left for attractive industrial positions whereas three graduate students, who have finished their diploma have decided to continue as doctoral students in our group. Robert Blick has left for the attractive position of an Associate Professor at the University of Wisconsin and Stefan Ludwig has joined us after a two year postdoctoral period at Stanford, attracted by the possibility of shifting his major research interests from the low-temperature physics of glasses to quantum phenomena in nanoscale electronic and electromechanical systems. Our long-term technical support team Armin Kriele and Klaus Wehrhahn have moved to Nanotools, one of our four successful start-up companies, while Stefan Schöffberger and Alexander Paul have joined us, attracted more by the training possibilities with our clean room tools than by public service salaries. Quite a few members of our group were invited to present 15 talks at international conferences and workshops and we have successfully published 19 papers in internationally acknowledged journals, a number which by chance is well below our long-term yearly average. We are also happy to report that the four start-up companies Advalytix AG, Attocube Systems, Nanion Technologies GmbH and Nanotools GmbH that were formed in recent years by members of our group increase their business success in a highly competitive world and continue to hire graduates from our group.

Finally, after completing a quarter of a century as the “Semiconductor Group”, originally formed at the University of Hamburg in December 1978 we have decided to start the 26th year 2004 with renaming us to “nanophysics group”, adjusting for the continuous change of focus from semiconductor physics to the more general direction of developing systems and studying condensed matter physics in the nanometer regime. We hope that this report finds many interested readers and enjoy to use this opportunity to thank our many cooperating partners worldwide as well as the generous funding bodies that make our effort in research and education possible. Any feedback our readers may have is welcome.

Munich, May 20th, 2004
Jörg P. Kotthaus



Electronic properties

Quantum dots defined electrostatically via surface gates in a GaAs-AlGaAs heterostructure are considered promising candidates for the implementation of quantum information registers, so-called qubits, in solids. Our low temperature studies of single electron transport through quantum dots aim at realizing well-defined charge and spin states in which the coupling between adjacent quantum dots as well as coupling to the environment can be manipulated in a controlled fashion. One aspect concerns creating and coupling quantum dots with low electron numbers. A second aspect aims at controlling the coupling of spin and charge states via pulsed high frequency radiation fields. Coupling of the quantum dots to the environment via phonons is studied in suspended devices acting as phonon cavities. There we observe phonon blockade of single electron transport, a new type of current suppression mediated by discrete cavity phonons In suspended Si nanowires we studied the temperature dependence of Coulomb-blockade-induced conductance minima for their possible utilization as a primary electron thermometer. The wave nature of electron transport is reflected in characteristic conductance oscillations studied in very narrow Silicon field-effect controlled Si nanowires resulting from electron interference. Non-linear multi-mode ballistic electron transport through lateral nanowires we now have demonstrated to be responsible for the ballistic rectification observed in asymmetric mesoscopic four-terminal junctions on GaAs-AlGaAs heterostructures. Alternatively to top-down fabricated semiconductor nanostructures there exist many efforts to use bottom-up assembled molecular structures for the study and possible utilization of electron transport in nanoscale systems. We presently explore routes to combine top-down fabrication with self-assembly techniques aiming to create and investigate electronic devices out of individual carbon nanotubes as well as mesoscopic ensembles of self-assembled molecules. Additional insight into mesoscopic transport phenomena is gained by the study of its dependence on in-situ tunable strain.


Towards coupling of single electron states

Andreas K. Hüttel, Clemens Rössler, Stefan Ludwig, Robert H. Blick, and J. P. Kotthaus,
in cooperation with Dieter Schuh and Werner Wegscheider.

One of the most important prerequisites for quantum-dot based quantum computing schemes is the accessibility of single electron states. We have developed and prepared gate geometries designed to reach the limit of one electron per quantum dot or double quantum dot, targeting both charge [1] and spin qubits [2].

Fig. 1 shows a gate electrode geometry defined by electron beam lithography. Two double quantum dots can be defined in separate circuits. Each dot is optimised for minimal electron number. In addition, a quantum point contact as charge detector has been integrated in a third circuit. This is intended to provide a geometry for versatile charge qubit devices.

As a complementing ansatz, we have developed a combined gate – antenna geometry for probing the effect of magnetic RF signals on quantum dots. In Fig. 2, the strip line loop antenna can be seen above the much smaller quantum dot circuit; it is separated from the underlying gate electrodes by an insulating layer.

Figure 1. Gate geometry designed to define two low electron number quantum double dots

Figure 2. Loop antenna for coupling magnetic RF signals.

[1] R. H. Blick and H. Lorenz, Proc. IEEE Symp. Circuits and Systems, II-245 (2000).
[2] D. Loss and D.P. DiVincenzo, Phys. Rev. A 57, 120 (1998).


Electron-phonon interaction in freely suspended quantum dots

E. M. Weig (neé Höhberger), D. Schröer, J. Kirschbaum, T. Krämer, R. H. Blick, and J. P. Kotthaus,
in cooperation with Max Bichler and Werner Wegscheider.

We have integrated lateral quantum dots into freely suspended phonon cavities. A bridge containing a two-dimensional electron system (2DES) is excavated from an AlGaAs/GaAs heterostructure by etching (fig. 1). The integrated 2DES can then be depleted further by applying a negative voltage to a nearby gate to form a quantum dot.

Single electron tunneling and Coulomb blockade have been observed in a similar freely suspended quantum dot at a bath temperature of 10 mK [1]. Surprisingly, an additional blockade mechanism that suppresses single electron tunneling appears for small bias voltages (fig. 2). This phonon blockade provides direct evidence of the modified phonon spectrum in the free-standing membrane, which is characterized by van Hove singularities and thus features an extremely enhanced phonon density of states for certain so-called cavity modes. As a result of the phonon excitation, the chemical potential of the quantum dot drops, leading to a blockade of further single electron tunneling. This is a direct consequence of the Franck Condon principle [2].

Figure 1. Free standing phonon cavity with integrated quantum dots and additional gate electrodes [3].

Figure 2. Coulomb blockade in a suspended quantum dot [3].

[1] "Integrating suspended quantum dot circuits for applications in nanomechanics", J. Kirschbaum, E. M. Höhberger, R. H. Blick, W. Wegscheider, and M. Bichler, Appl. Phys. Lett. 81, 280 (2002).
[2] "Single-Electron-Phonon Interaction in a Suspended Quantum Dot Phonon Cavity", E. M. Weig, R. H. Blick, T. Brandes, J. Kirschbaum, W. Wegscheider, M. Bichler, and J. P. Kotthaus, Phys. Rev. Lett. 92 (4), 046804 (2004)
[3] "Electron–phonon interaction in freely suspended quantum dots", E. M. Höhberger, J. Kirschbaum, R. H. Blick, J. P. Kotthaus, and W. Wegscheider, Physica E 18, 99 (2003)


Primary thermometer formed by Coulomb islands in a suspended silicon nanowire

A. T. Tilke, L. Pescini, H. Lorenz, and R. H. Blick.

We realized thermometers in suspended highly n-doped silicon nanowires with lateral dimensions down to about 40 nm (see Fig. 1). Random dopant fluctuations in the suspended wires lead to the formation of multiple tunnel junctions, utilized for Coulomb blockade thermometry (see Figs. 2 & 3). In the low bias regime, we observe relaxation via discrete acoustic phonon modes to give a lower bound for the sensitivity [1].

Figure 1. Suspended nanowire in a sideview (a) demonstrating complete suspension of the wire. The wire is integrated in a bow-tie antenna (b) for achieving maximal radiation coupling.

Figure 2. Temperature dependence of the conductance dip of the suspended nanowire caused by Coulomb blockade. The lowest trace is taken at 1.5 K and the upper most at 16.5 K.

Figure 3. (a) Temperature dependence of the normalized amplitude of the conductance dip for the suspended and the non suspended wire. Also shown (solid line) is the calculated amplitude (b). Temperature dependence of the peak width (FWHM) dV of the conductance dip for the suspended and the nonsuspended wire. The slopes of the curves for high temperatures are almost identical. (c). Normalized dV as a function of temperature normalized to the effective temperature T / Teff = kB T / EC.

[1] A. T. Tilke, L. Pescini, H. Lorenz, and R. H. Blick: "Fabrication and transport characterization of a primary thermometer formed by Coulomb islands in a suspended silicon nanowire " Appl. Phys. Lett. 82, 3773 (2003).


Quantum interference in a one-dimensional silicon nanowire

A. T. Tilke, F. C. Simmel, H. Lorenz, R. H. Blick, and J. P. Kotthaus.

We studied the electronic transport within a lithographically defined silicon nanowire for zero and finite bias. The 10-nm wide and 500-nm long nanowires have been fabricated by advanced electron-beam lithographic techniques (see Fig. 1). Transport experiments reveal clear quantum size effects in the conduction through the wire. Energy quantization within the wire leads to a shift in conduction threshold (see Fig. 2). Quantum interference effects cause an oscillatory pattern in the conductance (see Fig. 3). At low source-drain bias, transport is dominated by shallow tunneling barriers. At higher bias, additional nanowire modes are found to contribute to the conductance [1].

Figure 1. Sketch of the SOI nanowire: a metallic top gate is separated from the silicon quantum wire by a 55 nm silicon oxide. SEM micrograph of the nanowire defined in Calixarene with a width below 10 nm and a length of 500 nm.

Figure 2. Top gate voltage dependence of the conductance of a 25 nm wide and a 10 nm wide silicon nanowire. The conductance in the narrow wire starts to rise at much higher gate voltage values than in the wide wire. This offset is due to the lateral energy quantization in the narrow wire. The asterisk marks a gate voltage value above which a strong increase in conductance can be noticed. This threshold behavior is caused by shallow tunnel barriers at the wire openings or within the wire. Inset: Magnified view of the conductance trace for the 10 nm wire between VTG = 8.5 and 10.5 V. The first resonances appear at VTG = 8.9 V which is taken as the onset gate voltage. The conductance peaks in this region remind of Coulomb blockade resonances. The mean spacing between adjacent conductance peaks corresponds roughly to the charging energy for the whole wire.

Figure 3. Resonant structure in the conductance of the narrow wire. Here the conductance is plotted as a function of the longitudinal wave number kx of the electrons along the transport direction. The maxima and minima are marked with vertical bars. In the inset, the positions of these extrema are plotted against their position number starting arbitrarily at 0 for the feature with the lowest kx value. The maxima and minima are well fit by a straight line corresponding to a resonant length which is roughly the wire length.

[1] A. T. Tilke, F. C. Simmel, H. Lorenz, R. H. Blick, and J. P. Kotthaus "Quantum interference in a one-dimensional silicon nanowire" Phys. Rev. B 68, 075311 (2003).


SAM-assisted assembly of carbon nanotubes on gold electrodes

Christian J.-F. Dupraz, Udo Beierlein, and J.P. Kotthaus.

An attempt was made to decrease the contact resistance of CNT placed on top of a pair of gold electrodes. Instead of assembling the CNT`s on a substrate and depositing gold contacts afterwards, the electrodes were first optically defined. Similar to the attachment of DNA on gold [1], a monolayer of cysteamine was formed on the surface. The pretreated hydrophobic CNT-material, that is negatively charged favourably sticks to the now hydrophilic positively charged goldsurface. AFM-pictures (Fig. 1) show rather than bridging the contacts freely suspended the nanotubes prefer to adhere to the substrate. This could be due to vdW-interaction. Both types of nanotubes (MWNT and SWNT) are deposited and studied in their electrical transport properties. In both cases at 4.2 K the conduction is in the Coulomb blockade regime. Coulomb diamonds are visible, in the case of SWNT excited states are apparent (Fig. 2). The sharpness of the features in the diamonds of the SWNT, in contrast to measurements from MWNT, results from a further reduced size of the coulomb blockade islands. The room-temperature resistance of the MWNT-devices lies normally between 10 kOhm and several 100 kOhm, while the resistance of the SWNT reaches up to MOhm. The role of the cysteamine-molecules need not be a better coupling of the nanotube to the electrode, but could result in a cleaning of the goldsurface from adsorbates.

Figure 1. AFM-Picture of a MWNT lying over the contacts (scale-unit µm)

Figure 2. Conductance [S] of a SWNT at 4.2 K, excited states are visible

[1] G. Maubach, A. Csáki, D. Born and W. Fritzsche. Nanotechnology 14, 546-550 (2003)


Nanostructures for testing molecular wires

Michael Lambacher, Christian J.-F. Dupraz, Udo Beierlein and Jörg P. Kotthaus,
in collaboration with Philip Andres and Ulrich S. Schubert.

In order to continue the miniaturization of electronic elements to the nanometer scale, perhaps even to the molecular scale, researchers are investigating several alternatives to the transistor for ultra-dense circuitry. Many of these advances have been made possible by using the self-assembly of molecules on microfabricated semiconductor and/or metallic structures. In order to measure the electronic transport properties of self-assembled mono- and multilayers of thiol- and isocyanide-terminated molecules, we developed three different contact geometries [1]. E.g., pairs of gold electrodes with contact distances of a few nanometers can be fabricated by using a thin Al2O3 layer as a spacer between the electrodes. This can be achieved by optical and electron beam lithography and subsequent evaporation of gold, Al2O3 and gold (Fig. 1). Single layers of molecules or multilayers can then be inserted between the contacts where the oxide was partly removed in a wet etch process.

With all of these techniques, we observe peaks in the conductance versus source-drain-voltage curves which could arise from coherent transport through the molecular orbitals or from the coupling to the vibrational modes of the molecules (Fig. 2).

Figure 1. SEM image of an electrode pair and two gate electrodes for conductance measurements of self-assembled mono- and multilayers.

Figure 2. Current as a function of source-drain voltage before (red) and after insertion of chains of 1,4 phenylenediisocyanide (black curve) at 290 K. Inset: Conductance vs. VSD at low temperature.

[1] Dupraz C.J.-F., Beierlein U., Kotthaus J.P., CHEMPHYSCHEM 4 (11), 1247, 2003.


Low-temperature, in situ tunable, uniaxial stress measurements in Semiconductors using a piezoelectric actuator

M. Shayegan, K. Karrai, and S. Manus,
in cooperation with Y. P. Shkolnikov, K. Vakili, and E. P. De Poortere.

We demonstrated the use of a piezoelectric actuator to apply, at low temperatures, uniaxial stress in the plane of a two-dimensional electron system confined to a modulation-doped AlAs quantum well. Via the application of stress, which can be tuned in situ and continuously, we control the energies and occupations of the conduction-band minima and the electronic properties of the electron system. We also reported measurements of the longitudinal and transverse strain versus bias for the actuator at 4.2 K, both of which are nearly linear and shows very little hysteresis with the applied bias.

Figure 1. Longitudinal strain vs piezo bias, measured at 4.2 K, with a strain gauge (SGW) glued on top of a 0.1 mm thick GaAs wafer that is in turn glued to the piezo Inset: Data are shown for up and down directions of piezo bias sweeps.

Figure 2. (a) Change in resistance of an AlAs 2D electron system at filling factor n=14 (B=1.78 T) and at T=50 mK as a function of bias applied to the piezo rod. The vertical arrow indicates the position of zero stress. (b) Schematic energy level diagram for the AlAs 2D electrons in the presence of a fixed B, ignoring B-induced enhancement of valley splitting.

[1] M. Shayegan, K. Karrai, Y. P. Shkolnikov, K. Vakili, E. P. De Poortere and S. Manus, “Low-temperature, in situ tunable, uniaxial stress measurements in Semiconductors using a piezoelectric actuator”, Appl. Phys. Lett., 83, 5235 (2003).



Optics with nanostructures

Excitons in self-assembled quantum dots constitute another atomic-like solid state system ideally suited to study quantum properties and attractive for storing qubits. Here we combine our know-how of filling quantum dots with well-defined exciton and electron occupation with the expertise developed to perform a range of luminescence and absorption experiments on individual quantum dots. Electron-occupation dependent fine-structure of the luminescence nicely reflects the quantum nature of the charged excitonic states as well as their interaction with a nearby two-dimensional electron system. Absorption spectroscopy on individual quantum dots enabled by Stark-shift modulation spectroscopy yields quantitative insight lifetime broadening and oscillator strength of excitonic transitions and charge-tunable exchange coupling. In an international collaboration our experimental studies are complemented by theoretical efforts aiming to understand the interaction between excitons in the dot and a nearby Fermi sea. We also develop methods to trap long-living spatially indirect excitons in a double quantum well into defined lateral potential geometries ranging from 1D stripes to fully confining excitonic traps. The experiments aim at a better understanding of the temporal and spatial dynamics of such long-living excitons. In similar devices containing a single quantum well we have utilized the reversible spatial separation of optically generated electron-hole pairs via two-dimensional electrostatic potentials to create a photonic camera. This enables us to store and modify whole pictures for times up to seconds before being reemitted as excitonic luminescence.


Fine structure of highly charged excitons in semiconductor quantum dots

B. Urbaszek, and K. Karrai,
in cooperation with R.J. Warburton, B. Gerardot, P. M. Petroff, and J. M. Garcia.

An exciton in a symmetric semiconductor quantum dot has two possible states, one dark and one bright, split in energy by the electron-hole exchange interaction. We demonstrate that for a doubly charged exciton, there are also two states split by the electron-hole exchange, but both states are now bright. We also uncover a fine structure in the emission from the triply charged exciton. By measuring these splittings, and also those from the singly charged and doubly charged biexcitons, all on the same quantum dot, we show how the various electron-hole exchange energies can be measured without having to break the symmetry of the dot.

Figure 1. Gray-scale plot of the photoluminescence versus gate voltage for a single dot from sample A at 4.2 K. White corresponds to 0 counts, black to 4000 counts on the detector. The excitons are labeled with X standing for exciton, 2X for biexciton, with excess charge as the suffix. Dotted lines mark charging events. The unlabeled emission at 1.288 eV is related to 2X0, but its exact origin is presently unknown. The power of the 850 nm laser was chosen so that the biexciton features are clearly visible. Above a voltage of about 0.05 V, the wetting layer (WL) loads with electrons.

[1] B. Urbaszek, R.J. Warburton, K. Karrai, B. Gerardot, P. M. Petroff, and J. M. Garcia: “Fine structure of highly charged excitons in semiconductor quantum dots”, Physical Review Letters 90, 247403-1 (2003).


Stark-shift modulation absorption spectroscopy of single quantum dots

B. Alèn, F. Bickel, K. Karrai,
in cooperation with R. J. Warburton, and P. M. Petroff.

Excitonic interband optical transitions within single InAs self-assembled quantum dots have been directly observed in a transmission experiment at 4.2 K. Using Stark shift, the excitonic energy levels of a single quantum dot are tuned into resonance with a narrow-band laser line. The Stark shift is also modulated at low frequencies. Relative changes in transmission can be detected this way down to one part per million. The oscillator strength as well the homogeneous linewidth of the transition is obtained.

Figure 1. (a) Transmission measurement schematics (to scale). The Ge detector is placed directly behind the sample at 4.2 K. (b) The optical absorption creates an exciton with its electron and hole in a p level of the InAs quantum dot. Here, we illustrate two electrons filling the s shell.

Figure 2. Top: Differential transmission obtained by Stark-shift modulation of the excitonic dot absorption. The multiple lines originate from different excitonic transitions in several dots. Bottom: Modulated excitonic absorption line from a single optical transition in a single quantum dot.

[1] B. Alèn, F. Bickel, K. Karrai, R. J. Warburton and P. M. Petroff, „Stark-shift modulation absorption spectroscopy of single quantum dots”, Appl. Phys. Lett 83, 2325 (2003).


Kondo excitons in self assembled quantum dots

K. Karrai,
in cooperation with R. J. Warburton, and A. O. Govorov.

We describe excitons in quantum dots by allowing for an interaction with a Fermi sea of electrons. We argue that these excitons can be realized very simply with self-assembled quantum dots, using the wetting layer as host for the Fermi sea. We show that a tunnel hybridization of a charged exciton with the Fermi sea leads to two striking effects in the optical spectra. First, the photoluminescence lines become strongly dependent on the vertical bias. Second, if the exciton spin is nonzero, the Kondo effect leads to peculiar photoluminescence line shapes with a linewidth determined by the Kondo temperature.

Figure 1. (a) Schematic of the heterostructure with a quantum dot embedded between front and back gates; d<<l. (b) The band diagram of a quantum dot and associated wetting layer showing also the energies typical to self-assembled quantum dots.

Figure 2. Contributions to the initial state (b) Calculated emission spectrum of the X2- exciton with the triplet final state configuration;

[1] A. O. Govorov, K. Karrai, and R. J. Warburton, “Kondo excitons in self assembled quantum dots”, Phys. Rev. B67, 241307R (2003).


Dynamics of excitons in coupled quantum wells

Andreas Gärtner, and Jörg P. Kotthaus,
in cooperation with Dieter Schuh and Max Bichler.

The experiments to learn more about motion and interaction of excitons are carried out in semiconductor heterostructures at low temperatures (4K). The heterostructures contain two quantum wells (QWs), which are coupled by a thin tunneling barrier. Patterned gate structures applied to the samples allow impressing electrical fields on the QWs.

Via the electrical field applied perpendicularly to the QWs, we can prolongate the lifetime of excitons by orders of magnitude to 250 ns (see Fig.1). The long life time of these so-called spatially indirect excitons allows us to study excitonic drift properties. Utilizing the Quantum Confined Stark Effect (QCSE) of excitons, a lateral potential landscape for excitons Uexc is created in the QW plane [1,2]. This we use to externally control the drift of the electrically neutral excitons. The black curve in Fig.2 shows maxima and minima of the emitted PL in an excitonic potential with a lateral periodicity of 4µm created by an interdigitated gate structure. Reversing the excitonic potential by swapping the gate voltages, the PL replies by moving half a period (red curve). Reversing the voltages again, the spatial PL emission is shifted again to its initial distribution (green curve).

Specially designed gate patterns allow a wide range of lateral modulations of Uexc and, ultimately, the creation of artificial traps confining long-living indirect excitons in all spatial dimensions.

Figure 1. Left: Spectrally resolving the PL emitted by the coupled QWs at 4K allows to distinguish between short-living direct excitons (black curve, measured during 680nm-laser illumination) and long-living indirect excitons (colored curves, taken at various time intervals after shutting off the excitation laser). In this experiment, the indirect excitons show a lifetime of 250ns, which is deduced from the exponential decrease of their PL intensity (right).

Figure 2. Changing the modulation of the excitonic potential (different curves) causes variations in the lateral distribution of the emitted PL.

[1] S. Zimmermann et al., Phys. Rev. B 56, 13414 (1997)
[2] S. Zimmermann et al., Appl. Phys. Lett. 73, 154 (1998)


Quantum well photography

Jan Krauß, Achim Wixforth, and Jörg P. Kotthaus,
in cooperation with Micah Hanson, Dan C. Driscoll, Arthur C. Gossard, Dieter Schuh, and Max Bichler.

We have realized the capture of optical images in the plane of a semiconductor quantum well and the subsequent re-emission of this image in form of a two-dimensional photon flux. Spatially resolved storage of photonic signals in an electrostatically induced two-dimensional potential landscape in the plane of the quantum well is employed to imprint optical images in form of trapped photogenerated charges into the solid. The two-dimensional potential modulation leads to very long storage times by a deliberate spatial separation of photo­generated electron-hole pairs. Once the potential modulation is lifted, radiative electron-hole recombination restores the initial optical pattern and the photographed image is released in a flash of light as demonstrated in Fig. 1

Figure 1: Images projected onto a quantum well laterally modulated by a two-dimenional electrostrostatically generated potential of 2mm x 4mm period (top) and released as light via excitonic recombination after a storage time of 3.8 msec (bottom). The active device area is 400mm x 500mm containing 200x125 pixels.

[1] J. Krauß, A. Wixforth, J. P. Kotthaus, M. Hanson, C. Driscoll, C. Gossard, D. Schuh, and M. Bichler, "Quantum well photography", Appl. Phys. Lett. in press (2004)



NEMS

Nanoelectromechanical systems (NEMS) are both fascinating as objects for fundamental studies of electromechanical coupling on the nanoscale and promise a large variety of applications as sensitive sensors and actuators. A still unresolved problem is how to easily excite and control motion of nanomechanical elements. Though still on a micromechanical scale we have performed experiments on tuning the mechanics of microlevers with light that open new routes for driving and controlling optically nanomechanical systems. Alternatively we demonstrate with nanoscale beams fabricated out of a GaAlAs heterostructure that one can drive NEMS with surface acoustic waves utilizing parametric excitation. NEMS devices carved out of Silicon on insulator (SOI) are studied with the aim of controlling the coupling between adjacent resonators, understanding and decreasing the damping mechanism as well as driving them more efficiently via Coulomb forces. Electrostatic actuation of a nanoelectromechanical systems aimed at making a nano-tweezer can also be made visible via a newly developed optical detection method.


Optically-tunable mechanics of microlevers

M. Vogel, C. Mooser, and K. Karrai,
in cooperation with R. J. Warburton.

We show how the mechanical rigidity of a slightly detuned miniature Fabry–Perot cavity can be modified with light. We use a microcavity in which one of the mirrors is a soft compliant microlever optimized to detect bolometric forces. The static compliance can either be decreased to zero or increased considerably depending on the detuning of the light with respect to the cavity resonance.

Figure 1. (a) Schematics of the compliant micro-FP cavity ~cavity length 20 µm. The fiber diameter is 125 µm and its mode diameter is 5 µm. The Si lever is Au coated. The potential U serves to tune the cavity length. (b) Reflected light as a function of U2. Here, Kl=80 nN. The full line through the data is the best fit of a FP reflectivity model.

Figure 2. (a) Change in the effective static spring constant as a function of the light intensity. The lower (upper) branch corresponds to a softening (stiffening) of the spring constant. (b) The corresponding frequency response measured at P0 /Pmax= 0.3.

[1] M. Vogel, C. Mooser, R. J. Warburton and K. Karrai: “Optically-tunable mechanics of microlevers”, Appl. Phys. Lett. 83, 1337 (2003).


Acoustic actuation of nanomechanical resonators

Florian W. Beil, Robert H. Blick, and Achim Wixforth,
in cooperation with Werner Wegscheider and Max Bichler.

One of the unsolved problems of miniaturized mechanical systems is the lack of an appropriate driving mechanism. When a mechanical setup is scaled down to the nanometer range the surface tension becomes the dominant force, whereas e.g. capacitive or Lorentzian forces become small. An improved driving mechanism for nanomechanical systems (NEMS), which avoids the unfavorable scaling behavior of conventional forces relies on parametric effects, mediated by a strong surface acoustic wave (SAW) in interaction with the NEMS. Surface acoustic waves are acoustic modes propagating along the surface of a crystal with frequencies in the MHz regime, and wavelengths of a few microns. The SAW is easily generated by comblike structures called IDTs [see Fig. 1(a)] and interacts with the nanomechanical beam resonator via the motion of the beam’s suspension points [see also Fig. 1(a)]. As the length of the beam is matched to half the wavelength of the SAW, the acoustically induced motion of the beam’s clamping points will result in a SAW-periodic longitudinal stress in the beam, and therefore a SAW-periodic modulation of the beam’s eigenfrequencies. Matching the SAW frequency to an eigenfrequency of the beam is accomplished by finite element simulations as shown in Fig. 1(b). In Fig. 1(c) we show the measured bistable range of the parametrically excited beam resonator. The frequency of the parametrically excited mode was located around 411 MHz, whereas the excitation of the frequency matched mode was measured via standard magneto impedance spectroscopy of the beam’s first eigenmode. As the parametrically excited eigenmode is a symmetric mode it contributes to the magneto impedance signal of the first eigenmode. This acoustic excitation mechanism will allow the integration of nanomechanical systems in low cost, room temperature applications for information processing and sensors.

Figure 1. (a) Setup: The Saw propagates along the crystals surface (GaAs) and interacts with a nanomechanical beam resonator. (b) Finite element simulation of the beam’s eigenmode spectrum together with the SAW dispersion. For a 3 mm long beam the SAW is matched to an higher eigenmode. (c) Bistability range for the parametrically excited beam. When increasing the power of the SAW the frequency range of the amplified higher harmonic increases.


Excitation of nano-electromechanical systems in silicon

Dominik V. Scheible, Daniel R. König, Robert H. Blick, and J. P. Kotthaus,
in cooperation with Christoph Weiss.

Established methods of exciting nano-electromechanical systems (NEMS) usually require high magnetic fields and consequently elaborate cryogenic cooling. We have studied alternative excitation mechanisms of NEMS in the material system Si-SiO2-Si, employing the concept of electron shuttling. Here, reduction of mechanical losses is crucial for excitation, since the attainable forces in NEMS are limited. A shuttle device in a tuning-fork configuration (see Fig. 1) prooved to achieve an enhancement of the mechanical quality factor Q [1, 2]. The mechanical transfer of charge does not only allow an increase of the precision of the device itself [3], but also constitutes a way of driving the shuttle via the force exerted on the charged shuttle island [4] (see Fig. 2). Furthermore, we have demonstrated field emission in our NEMS in order to increase the transported net charge, and hence the force [5]. This current enhancement also dramatically reduces the complexity of the external circuitry of such a device within applications.

Figure 1. The electron shuttle in a tuning-fork configuration. This design reduces mechanical losses within the resonator.

Figure 2. Excitation of a NEMS electron shuttle via a resonant Coulomb force (RCF) exerted on a charged shuttle.

[1] D. V. Scheible, A. Erbe, and R. H. Blick, Appl. Phys. Lett. 82, 3333 (2003).
[2] D. V. Scheible, C. Weiss, and R. H. Blick, preprint (2004).
[3] C. Weiss and W. Zwerger, Europhys. Lett. 47, 97 (1999).
[4] D. V. Scheible and R. H. Blick, Appl. Phys. Lett. (accepted 2004).
[5] D. V. Scheible, C. Weiss, J. P. Kotthaus, and R. H. Blick, preprint (2003).


Optical detection of quasi-static actuation of nanoelectromechanical systems (NEMS)

Ch. Meyer, H. Lorenz, and K. Karrai.

An all optical method designed to test the functionality of nanoelectromechanical systems is presented. Silicon tweezers consisting of freestanding nanometer-sized prongs are prepared using electron beam lithography (see Fig. 1). Images of the tweezers structures are taken by scanning confocal microscopy while the prongs are electrostatically actuated under a low frequency ac voltage. The images, which are demodulated at the actuation frequency and its higher harmonics, clearly resolve the actuating parts of the tweezers (see Figs. 1 & 2). An actuation amplitude down to 6 pm (rms)/ sqrt(Hz) can be detected. [1].

Figure 1. FIG. 1. SEM micrographs of nanotweezers. The devices are defined by electron beam lithography and reactive ion etching on silicon-on-insulator (SOI) material. After removing the oxide layer, the structures are dried in a critical point dryer. (a) An example of three parallel tweezers structures with 4 µm long prongs; (b) single pair of tweezers (3 mm long, 200 nm wide, and about 150 nm thick) with electron beam deposited tips on the prongs. Since the latter are insulating, gripped conducting objects should not short-circuit the tweezers.

Figure 2. (a) Scanning confocal reflectivity micrography; (b) corresponding SEM image, showing a device with three tweezers (3 µm long prongs); (c) superposition of (a) and (b) images. (d) The reflected image of the signal amplitude, taken simultaneously with (a), demodulated at twice the voltage excitation frequency f 57 kHz. The tweezers were actuated under an ac voltage of urms = 2.3 V (128 x 128 pixels, time constant 3 ms). The power of the incident signal is about 35 mW. The white boxes indicate the prongs’ positions as determined from (c).

Figure 3. (a) Scanning confocal reflectivity image of the tweezers shown in Fig. 1(a); (b) corresponding image of the signal demodulated at the tweezers actuation frequency f = 710 Hz (integration time constant 10 ms). Here, the right prongs were biased against the substrate with urms = 2 V. The white boxes symbolize the prongs’ positions. It is clearly seen that the floating left prongs show no signal.

[1] Ch. Meyer, H. Lorenz, and K. Karrai, "Optical detection of quasi-static actuation of nanoelectromechanical systems" Appl. Phys. Lett. 83, 2420 (2003).



Nanodevices and bio-templated nanostructures

Surface acoustic wave (SAW) devices utilizing “nanoquakes” for chemical sensors and microfluidic actuators continue to be one aspect of our research though this effort is increasingly shifting to the new group of Achim Wixforth at the University of Augsburg. In a cooperation with the group of Thomas Bein at the chemistry department of LMU and the Advalytix AG we are studying chemical gas sensors employing nanoporous films as selective absorbers and sensing the gas absorption via the modified propagation of SAW in a delay line. In cooperation with Advalytix SAW devices are successfully employed for microfluidics both as pumps and as fluidic sensors aiming towards lab-on-a-chip applications. Another fruitful lab-on-a-chip oriented project has been continued with the young start-up Nanion in the area of improving patch-clamp techniques. Here single ion channel recordings have been successfully performed on a glass chip developed in this cooperation. Whereas the above projects aim at employing solid state device techniques for biomedical applications we are also happy to host the Emmy-Noether young investigator group of Fritz Simmel which is using biophysical and bioinspired techniques to create new electronic and mechanical functions in a bottom-up approach. To this end, the unique molecular recognition properties of DNA molecules are utilized to build complex supramolecular networks or to direct the synthesis of functional materials, but also to construct artificial molecular machines. For example, DNA could be shown to be able to direct the synthesis of chains of copper sulfide nanoparticles, in bulk a semiconducting compound. In another project, an artificial molecular device based on a DNA aptamer was developed with the functionality of a “molecular hand”.


Surface acoustic wave studies for chemical sensors and nonlinear surface acoustic wave propagation

Alexander Müller, Alex Darga, and Achim Wixforth.

The design of our device for chemical sensors consists of three identical interdigital transducer (IDT) structures forming two SAW delay lines. One delay line is covered with a monolayer of 100nm small zeolite crystals having pores with a diameter of only a few Å. This layer is designed such that it absorbs molecules of well defined type. The other delay line serves as reference line (inset fig.1). A setup with five mass flow controllers and a reaction chamber has been built. In the reaction chamber, the sensor can simultaneously be exposed to different mixtures of gases, controlled and read out by high frequency signals to the transducers, and heated up to temperatures of over 200°C. A first experiment with a nano-porous zeolite ZSM-5 (pore size 5.6 Å) demonstrates and proofs the performance of the sensor chip presented to a nitrogen carrier gas with and without a specific amount of iso-butane (cross section 5.5 Å). The phase difference between sensor and reference SAW is used as the useable signal and is shown in fig.1.

The effective electro mechanical coupling coefficient Keff of a piezoelectric substrate defines the difference between the SAW velocity on a free (open) and a metallized (short) surface. Keff is inversely proportional to the dielectric constant e. Barium titanate has a relatively high e which, moreover, depends among other factors also on the applied electric field and temperature. At the 2nd order phase transition from the ferro- to the paraelectric state e diverges and the crystal symmetry changes. We measure this change in terms of the capacitance of an interdigitated electrode pair, onto which the BaTiO3 has been deposited. The ac capacitance of an IDT with and without barium titanate on top is shown in fig. 2(a). The dependence of the capacitance on the temperature is as expected. However, the BaTiO3 on top of the SAW delay line affects its transmission properties more than purely due to the electrical part, as can be seen from fig.2(b). A phase transition induced change of the dielectric constant cannot explain the observed phase shift in the SAW phase due to the interaction with the dielectric on top of the delay line. Further experiments are needed to investigate the mechanical and electrical influence of the phase transition of the BaTiO3-crystal on the surface acoustic wave.

Figure 1. Phase shift of the SAW sensor delay line relative to the reference delay line. The sensor delay line consists of ZSM-5 zeolite. The carrier gas nitrogen is either pure or contains a small amount (1250ppm) of iso-butane.

Figure 2. (a) Capacitance of the interdigital transducer (IDT) with and without BaTiO3 on top vs. temperature. At the Curie temperature a phase transition between ferro- and paraelectric state occurs. The dielectric constant diverges and therefore affects the capacitance of the IDT. (b) Change of the SAW phase due to BaTiO3 on top of the SAW sensor delay line.


Mikro- and nanofluidic on piezoelectric substrates

C. J. Strobl and A. Wixforth,
in cooperation with Advalytix AG, University of Augsburg and University of Bayreuth.

The handling of liquids in the submilliliter scale is usually governed by a wealth of different difficulties. The use of a surface acoustic wave (SAW) device in a Rayleigh wave mode as a microfluidic pump promises a lot of advantages. First, SAW can be used to efficiently agitate smallest amount of fluids, as a Rayleigh type SAW is strongly absorbed by a liquid, where it is converted in an acoustically induced internal streaming. Secondly, at high enough SAW amplitudes, the fluid can be moved as a whole, even small droplets can be actuated along predefined fluidic trajectories. SAW devices as an approach towards a lab on a chip are demonstrated for various applications. The usage of shear wave devices on the other hand opens the possibility to agitate droplets in addition to analyse it. One analytical application is to examine the salt concentrations in a droplet. This investigation allows for an estimate of the interaction range of the travelling piezoelectric fields by employing a modified relaxation model. As shear waves are not as strongly absorbed by a fluid as Rayleigh type waves, many different droplet can be agitated in parallel.

Both for shear wave and Rayleigh wave devices we have studied the conversion of acoustical modes for our lab-on-a-chip applications: In both cases also bulk modes exist to which the SAW can be coupled. The energy coupling in this mode is strong enough to use it for stirring droplets on the back sides of the devices. This effect allows to spatially separate the electrical contacts of the device from the liquid. Furthermore, this type of mode conversion is used to also employ sound induced effects in a non piezoelectric substrate like, e.g, silicon.

The interaction between the piezoelectric fields of a SAW and charged or polarizable particles within a fluid has been studied by using liquid crystal samples, which have been labelled by fluorescence markers. In preliminary experiments, we find a strong interaction which for instance manifests itself in a clearly visible standing wave pattern in the liquid crystal sample as it is observed in a fluorescence micrograph.

Figure 1. The figure shows the result of the measurement of the salt concentration in a droplet as it is detected by a shear wave. The transmission of the shear wave is attenuated by the ionic conductivity in the droplet (green dots). For a specific salt concentration, the attenuation exhibits a maximum. The red solid curve shows the result of our modified relaxation model.


Patch-clamp-on-a-chip - automated ion channel analysis

A. Brueggemann, M. George, M. Klau, M. Beckler, J. Steindl, J.C. Behrends, and N. Fertig.

Electrophysiology (i.e. patch clamping) remains the gold standard for pharmacological testing of putative ion channel active drugs, but suffers from low throughput. Nanion, a spin off company from the CeNS, developed a new ion channel screening technology based on microfabricated glass chip substrates. Whole-cell voltage clamp [1] as well as single channel recordings [2] from mammalian cell lines have been performed in an automated device.

The planar chip greatly enhances the accessibility of the ion channel containing membrane and can serve as a workbench for experiments on single ion channels using combinations of patch clamp current recording with other single molecule techniques[3]. In an collaborative effort embedded in a BMBF project (Nanobiotechnology) with the group of Dr. Fritz Simmel, the chip based technology is further developed in combination with means for microwave probing of the ion channel proteins.

Based on the proprietary patch-clamp-on-a-chip technology Nanion offers the worlds smallest patch clamp rig, the Port-a-Patch, and develops higher throughput electrophysiological screening systems for ion channel active drugs. Our high information content screening technology increases the speed and efficiency of ion channel research and drug discovery [4,5] (www.nanion.de).

[1] Fertig, N., R.H. Blick, and J.C. Behrends. 2002. Whole cell patch clamp recording performed on a planar glass chip, Biophys. J. 82(6):3056-3062
[2] Fertig, N., M. Klau, M. George, R.H. Blick, and J.C. Behrends. 2003.Activity of Single Ion Channel Proteins Detected with a Planar Microstructure. Appl. Phys. Lett. 81(25):4865-67
[3] V. Borisenko, T. Lougheed, J. Hesse, E. Füreder-Kitzmüller, N. Fertig, J. C. Behrends, G. A. Woolley, and G. Schütz. 2003. Simultaneous Optical and Electrical Recording of Single Gramicidin Channels. Biophys. J. 84(1):1–11
[4] Fertig, N., C. Meyer, A. Tilke, M. George, M. Klau, C. Sobotta, R. H. Blick, and J.C. Behrends. 2003. Microstructured apertures in planar glass substrates for ion channel research. Channel and Receptors 9(1):29-40
[5] A. Brüggemann, M. George, M. Klau, M. Beckler, J. Steindl, J.C. Behrends, and N. Fertig. 2003. High Quality Ion Channel Analysis on a Chip with the NPC© Technology. ASSAY and Drug Development Technologies 1(5):656-673


DNA-templated materials synthesis

Wendy U. Dittmer, Patrick Nickels, and Friedrich Simmel.

Although its unique molecular recognition properties render DNA an extremely promising material for the self-assembly of nanoelectronic circuits, its intrinsic conduction properties are too poor to make it useful as an electronic component itself. Nevertheless, DNA can serve as a template for the directed synthesis of functional materials with improved electronic properties. Until now, we were able to synthesize metals, semiconductors and conducting polymers along the backbone of DNA. Here we utilize the highly charged sugar-phosphate backbone of DNA for the localization of charged species which serve as precursors for subsequent chemical steps such as reduction or oxidative polymerization. This leads to preferential materials deposition along DNA strands. Among others, we synthesized conducting gold nanowires, copper sulphide nanoparticles (fig. 1), and polyaniline wires and nanostructures (fig. 2) along DNA. Electronic characterization of these materials is in progress.

Figure 1. TEM micrograph of copper sulphide nanoparticles synthesized on DNA. The size of the frame is 0.9µm x 0.9 µm. Individual particles are about 10 nm in diameter.

Figure 2. Self-organized cloverleaf structures occuring during DNA-templated synthesis of polyaniline. The size of the AFM micrograph is 25 µm x 10 µm.

[1] Wendy U. Dittmer, Friedrich C. Simmel, „Chains of semiconductor nanoparticles templated on DNA“, submitted (2003).


DNA self-assembly and nanostructures

Stefan Beyer, and Friedrich Simmel.

In this project, the base-pairing interactions of DNA are utilized for the construction of complex DNA networks. Two molecules of DNA will bind to each other if their base sequences are exactly complementary – they will not bind if they are not. Based on this “simple” rule we designed basic motifs such as linear structures, three-arm and four-arm junctions which form the building blocks of more complex molecular networks. These constitute the basis of DNA-based supramolecular structures which can be used for the arrangement of functional nanoscale components such as nanoparticles or quantum dots and even for the assembly of nanoelectronic circuits.

As building blocks for these DNA networks we used short randomly generated DNA sequences with designed hybridization sections. Longer stretches of DNA were synthesized using various PCR techniques. The DNA networks were characterized for different DNA densities and reaction conditions using gel electrophoresis and scanning force microscopy. Depending on the conditions, we found complex structure formation behavior influenced by specific and non-specific binding events as well as DNA-substrate interactions.

To achieve better control of the formation and deposition of DNA-based nanostructures, microfluidic devices were constructed using soft lithographic techniques. Microfluidic channels were molded using SU-8 stamps and the elastomer PDMS. To facilitate well-defined incorporation of supramolecular DNA structures into lithographically defined micro- and nanostructures, extensive experiments on electronic and dielectrophoretic manipulation of DNA were performed.

Figure 1. AFM micrographs of artificial DNA networks formed by self-assembly of short oligonucleotides at different densities. The basic building blocks of these networks are four-arm junctions sticking together with various connectivities. Micrograph size is 2 µm x 2µm.


Molecular machinery

Andreas Reuter, Wendy U. Dittmer, and Friedrich C. Simmel.

In an effort to construct novel functional DNA nanodevices, we combined the operation principles of DNA nanomechanical devices with the binding properties of DNA aptamers. These are short DNA sequences which specifically bind to proteins or small molecules. As a first example, the well-known anti-thrombin aptamer 5’-GGTTGGTGTGGTTGG-3’ was incorporated as a binding unit into a DNA nanomechanical switch. This yields an artificial molecular device which can be selectively instructed to grab or release a protein (a “molecular hand”). The principle of the release process is displayed in Fig. 1. The function of the device was extensively tested in gel electrophoresis and Förster resonance energy transfer (FRET) experiments (Fig. 2).

Figure 1. A fuel strand F binds to an aptamer based DNA device M and triggers the release of a protein P (thrombin). This process can be reversed in a subsequent step, enabling cyclical operation.

Figure 2. Kinetics of protein release and binding followed by FRET. Without protein (filled circles), the transition between binding and non-binding conformation of the device is fast. Upon binding to the protein (open circles) this process is significantly slowed down.

[1] Wendy U. Dittmer, Andreas Reuter, Friedrich C. Simmel, „A DNA-based machine that can cyclically bind and release thrombin“, Angew. Chem. Int. Ed., in press (2004).



Diploma theses in 2003


PhD theses in 2003


Guest scientists in 2003


Group members in 2003

Senior scientists

Secretary

Technical staff

Ph.D. students

Diploma students


Publications in 2003


Invited talks in 2003

Udo Beierlein

Florian Beil

Robert Blick

Andreas Hüttel

Khaled Karrai

Jörg P. Kotthaus

Bert Lorenz

Eva Weig

Achim Wixforth


Senior scientists of cooperating groups


Financial support in 2003

Funding of this work via the following agencies is gratefully acknowledged: