“Energy conversion” and “energy transport” are key words characterizing the research of the theoretical physics group of Professor Wolf. For more than a decade it has been working on basic research themes for engineering and applications. Thermoelectrics is one example: materials that convert temperature differences into electricity directly without detouring via a heat engine gain efficiency if they have a beneficial microstructure. This factor is at least as important as the chemical composition, because it influences the thermoelectric transport coefficients.
A second example is magnetic friction: a magnetic probe scanning a ferromagnetic surface ­decelerates because of the conversion of kinetic energy into magnetic excitation energy. In general, the magnetic friction force depends on the scanning velocity, and at sufficiently high velocities a vortex street forms in the wake of the scanning probe. A particularly important and surprising result here is that, under certain conditions, friction may induce magnetic order.
A further example is Ohm’s law, which does not hold in nanostructures at low temperatures. A novel modelling concept makes it possible to analyze how Ohm’s law emerges at higher temperatures due to the loss of quantum coherence. Further research topics in Professor Wolf’s group include electromigration (the drift of atoms due to an electric current), shear localization, and elasticity in granular media (like sand).
Quantum physics is known to work perfectly for microscopic particles such as atoms and ­photons; however, its predictions turn “classical” everyday experience upside down if it is applied to palpable objects. According to it, for instance, one and the same object should be able to exist simultaneously in different places, and its dynamic behaviour should depend on whether or not it is observed. A main goal in the group of Professor Klaus Hornberger is to investigate systems that belong to the transition region between the quantum regime and classical physics. Based on the theory of open quantum systems, the group is studying to what extent it is possible to understand the emergence of classical physical properties and classical laws if quantum theory is viewed as universally valid.
These questions arise naturally when dealing with the dynamics of ever larger molecules as they interact with their natural environment. The ­increasing complexity of such macromolecules makes a complete microscopic description ­practically impossible and requires identification of the general principles and mechanisms behind the quantum-to-classical transition. In parallel to this, the group is developing experimentally realizable proposals to probe the boundary region between quantum behaviour and classical physics, and to verify quantum phenomena on scales that have not been reached so far. This includes demonstrating the wave nature of ultra-massive particles, such as large metal clusters, by means of a near-field interference effect, as well as proving entanglement, i.e. “spooky action at a distance”, in the properties of mesoscopic systems.
In the research group of Professor Kratzer, the fabrication of semiconductor nanostructures is explored from a theoretical perspective. The aim is to identify the elementary processes on the atomic scale which are active in the deposition of crystalline materials from the gas phase. For example, nanometer-sized gold particles are known to be instrumental in growing thin crystalline needles of the semiconductor gallium arsenide. The group’s calculations provided evidence that the major role of the gold must be seen in capturing and dissociating arsenic molecules from the gas phase. This reactivity of gold comes as a surprise, considering it is a noble metal; however, gallium atoms alloyed in the gold surface layer may help to bind the arsenic atoms more strongly. As a major outcome of this interdisciplinary study, it has been possible to give a precise meaning to the statement that gold acts as a growth catalyst, and thus to make use of known principles in chemical catalysis in this new context. Possible applications of the needle-like semiconductor nanostructures, e.g. for solar cells, are explored by other groups within CENIDE (Center for Nano­integration Duisburg-Essen).
Nanomagnets make advanced data storage possible, serve as contrast media in MRI tests and are used for hyperthermic treatment in cancer patients. These are but a few examples of their applications, yet it is clear to what extent nanomagnets are in demand for so many applications and with so many different properties. In computer memory, for example, nanomagnets have to remain in fixed alignment if they are to store data for any length of time. Hyperthermic treatment of cancer patients, by contrast, requires nanomagnets that can switch polarity very easily. This treatment involves introducing the minute particles directly into the tumour and then rapidly reversing their magnetization using external magnetic fields. The nanomagnets thereby produce heat that locally destroys the surrounding cancer cells.
The research groups of the experimental physicists Professor Heiko Wende and Professor Michael Farle have been working together with the theoretical physicist Professor Peter Entel and have now defined concrete rules by which to precisely determine the properties of nanomagnets during their production. On the experimental side, the groups of Professor Wende and Professor Farle coated the nanoparticles in various metals and then measured the effects of these metals on the magnetic properties of the particles inside. To obtain the most meaningful and detailed data, the researchers used the BESSY II synchrotron ­radiation facility of the Helmholtz Zentrum Berlin. The high brilliancy X-rays deliver information on the magnetic properties of the sample. Dr. Carolin Antoniak, member of Professor Wende’s group, was responsible for coordinating measurement and analysis of the beamtimes. These ­measurements made it possible to distinguish the various types of magnetization. Dr. Markus Gruner from the group of Professor Entel computed the influence of the different coating metals on every single atom inside a nanomagnet. To perform these complex calculations, the theoretical physicist worked on JUGENE, one of Europe’s largest academic research computers, at the Forschungszentrum Jülich research centre.
Both approaches – experimental and theoretical – complement each other ideally here: while the theoretical calculation is extremely precise, it is still based on assumptions, and those assumptions must be confirmed by experiment. The research cooperation is now able to predict which properties can be achieved with which metal coating, meaning that nanomagnets can be tailored to the desired properties during their production. This makes life much easier for all kinds of users. In the future, Professor Wende and Professor Farle plan to coat the nanomagnets with organic materials. Their vision is to modify the properties of these hybrid systems using external stimuli such as light.
The group of Professor Marika Schleberger is working on the controlled modification of graphene. This special material has been of interest to many research groups worldwide since its discovery in 2004 by the Nobel Prize winners A. Geim and K. Novoselov. Because of its unique electronic, mechanical and thermal properties, the ultrathin carbon layers are envisaged to play an important role in many future applications. For many of these, the controlled manipulation of graphene would be highly desirable. Professor Schleberger’s group uses ionizing particle radiation to achieve this goal. With this technique the group has been able to achieve a number of modifications, such as the introduction of defects and foreign atoms but also graphene folding. These graphene origami are not only aesthetic nano objects, they are also predicted to have special transport properties and may therefore be of interest in applications.
In the group of Professor Rolf Möller, processes at surfaces are studied with very high ­spatial resolution to show the individual atoms and molecules. Electronic transport through small structures is analyzed in detail, providing basic information for the development of future electronics. Here it was possible, for example, to determine the electric resistivity of individual atomic steps for a layer of silver on silicon (Si(111)-- Ag). In another experiment, what is known as ‘ballistic’ transport of electrons, or transport without significant loss of energy and change of momentum, is studied for single molecules, revealing different paths for electronic transport. If the electrons lose energy within the molecule, this may lead to the molecule moving slightly on the surface. Jumps between three ­different positions are observed for a copper phthalocynine molecule. In collaboration with the group of Professor Nicolàs Lorente from ­Barcelona, who has performed theoretical calculations for the behaviour of the molecule on the surface, the observed fluctuations were able to be explained in detail.
Professor Michael Horn-von Hoegen and his group are experts in the growth of ultrathin crystalline and epitaxial films. Recently, the lattice expansion of graphene on a supporting iridium substrate has been determined with an unmatched resolution of 1/3000 of the atomic distance. A second important research topic is the investigation of the ultrafast dynamics of electrons and atoms at surfaces. The electron dynamics is measured in a directly imaging photo electron emission microscope with femtosecond time resolution and attosecond phase stability: the motion of a plasmon, which propagates with half the speed of light in a 10 µm large silver particle, is displayed in extreme slow motion in a film lasting only 8 femtoseconds. Such an electronic excitation is transferred in picoseconds to the lattice of atoms, which triggers either motion of the atoms or a phase transition. Such structural transitions at surfaces are studied using ultrafast electron diffraction – a unique experiment that has produced exciting results over the last two years.
Ultrafast phenomena in solids and at their interfaces are the focus of research carried out in the group of Professor Uwe Bovensiepen. Femtosecond time-resolved optical, electron and X-ray spectroscopy and diffraction in combination with short-pulse laser systems are used to study transient changes in the electronic and atomic structure of materials. The aim is to gain an ­understanding of the microscopic interaction processes and the dynamics of elementary excitations. For example, work on the relaxation of excited carriers in iron-based superconductors revealed that the magnetic interactions between electrons lead to a significantly slower relaxation of the excited electrons compared to the magnetically non-interacting holes. Other work, in collaboration with scientists from the SLAC National Accelerator Laboratory and Lawrence Livermore National Laboratory, addressed the transient lattice response of ferro-electric materials after laser excitation, as well as the dynamics of fast order-disorder transitions (melting and plasma formation) in carbon, which are induced by intense femtosecond X-ray pulses. In the semimetal bismuth it was found that irradiation with femtosecond laser pulses leads to a direct light-induced excitation of the lattice.
Planet formation is at the heart of research carried out by Professor Gerhard Wurm and his group. More than 800 extrasolar planets are known to date. How they – and our own solar system – were formed is one of the most topical research fields in astrophysics. Within this field, Duisburg is one of the few locations worldwide where the process of planetary growth is studied in laboratory experiments – earthbound, under microgravity in parabolic flights and in the drop tower. One special aspect of the work here is to understand how ­micron-size dust particles grow to larger km-size planetesimals. The group was able to show that growth of dusty matter is possible even at 180 km/h. This is the current experimental basis for explaining the first steps of planet formation. Other research includes interaction of stellar (and solar) radiation with dusty surfaces. Particle ejections due to this interaction provide an explanation for processes, such as planet-encircling dust storms on Mars, that up to now have remained unexplained. To also give students better access to ­observational astronomy, a 14” telescope has been installed on top of the university building.
Research on physics education above all describes and explains subject-specific teaching and learning processes and classroom interaction for all types of schools from primary school to university. The focus is on analysis and optimization of teaching and learning processes in physics lessons and the development of quality criteria and quality assessment for teaching and teacher education in physics.
The head of DFG Research Unit 511, Professor Hans E. Fischer, and his group are working on physics competence among students in upper and lower secondary school in order to analyze the effect of specifically designed lesson plans and modifications. A number of projects funded by the German Federal Ministry of Education (BMBF) deal with the coherence between teachers’ professional knowledge and their activities during lessons; their aim is to draw conclusions for new or modified content of teacher training at universities. The differences between Finnish and German teachers and their activities in the classroom are also analyzed in order to better understand the PISA ranking of Finnish students and optimize German physics education.
One of the topics in the research group of Professor Heike Theyßen concerns the experimental skills of lower secondary school students. Up to now, experimental skills have usually been assessed in large-scale studies like TIMSS and PISA using paper and pencil tests. However, these instruments do not account for process aspects of experimental skills. In a cooperation project funded by the BMBF, a PC-based test instrument is being developed for process-based analysis of experimental skills in large-scale assessments.