Physics

Most people consider being stuck in peak traffic as an inevitable part of everyday life. Insects, however, show that there is another way: ants build trail systems, much like our own motorway networks, which they use to carry food back to the nest. Yet traffic jams never occur on these trails, even in “peak traffic”. Quite the contrary, in fact: surprisingly, ants may even progress faster as traffic increases - because they communicate with each other.
Future vehicles will be equipped with communication devices for wireless networks (approx. 300 m range) so that they can exchange messages, e. g. accident warnings or traffic information. The research group of Professor Michael Schreckenberg is currently working within the “Next Generation Car-2-X Communication” research project on how to transfer swarm intelligence to road traffic by vehicle-to-vehicle communication. Computer simulations based on cellular automata show that the travelling time of all vehicles decreases significantly if only five percent of vehicles communicate with each other. If one in four vehicles is fitted with a communication device, many traffic jams can be hindered almost entirely.
The nanoworld is governed by different laws than the macroscopic world. The research group of Professor Axel Lorke investigates how this manifests itself in the properties of ultra small electronic (semiconductor) devices and how the newly emerging phenomena can be utilised. By way of an example, in “large” wires electrical current flows almost like a charged fluid. In semiconductor nanostructures, however, electrons can also behave ballistically, like freely moving charged particles. This makes it possible to create functional devices – such as rectifiers and diodes – by appropriately shaping the nanostructure and thus guiding the bouncing electrons. The quantum properties of the electrons also play a crucial role on the nanometre scale, and it is possible to fabricate semiconductor islands so small that the electrons confined in them behave according to laws familiar from atomic physics. Yet these ‘artificial atoms’ offer much more flexibility: they can be electrically contacted and their electron number controlled by applying an electric voltage. It has also been possible for the first time for the electronic wave function to be imaged and manipulated in a controlled manner by an external magnetic field so that new quantum states emerge. The aim of these investigations is to use single electrons for information storage and to utilise quantum mechanics to realise entirely new information processing concepts (‘quantum computing’).
A breakthrough was achieved in the group of Professor Heiko Wende for fabrication of materials with extremely high magnetic moments at room temperature. There is a strong demand for these materials for various critical applications such as hard disk write heads and electric motors. The current record was set 75 years ago by a Fe-Co alloy with a magnetic moment of approx. 2.45 µB per atom. Although rare earth metals of the lanthanides such as Gd exhibit much larger magnetic moments, they do not show ferromagnetic ordering at room temperature. The ordering temperature of lanthanides can be enhanced by coupling them with ferromagnets such as Fe. Unfortunately, the coupling between the different metals is typically antiferromagnetic, leading to a drastic reduction of the net moment. A way to circumvent this obstacle was theoretically predicted by the group of Professor Eriksson (Uppsala University) and verified in experiments by Professor Wende’s group: by introducing an intermediate layer of Cr between Fe and Gd layers, an effective ferromagnetic coupling can be forced between Fe and Gd. Temperature-dependent investigations determined a magnetic moment of 5µB in Gd at the interface at room temperature. As forecast, the Gd moment is aligned parallel to the Fe moment, thus adding to the magnetic density.
Graphene is one of the subjects investigated by the research group of Professor Marika Schleberger. Graphene consists of a single layer of carbon atoms and has quite unusual mechanical and electronic properties. So unusual, in fact, that in October 2010 Andre Geim and Konstantin Novoselov 
received the Nobel Prize for Physics for their experiments on graphene. The Schleberger research group is primarily interested in how the material reacts under electronic excitation. One of the peculiarities of graphene is its extremely high electric conductivity, which should in principle prevent permanent material modifications even under intense electronic excitation. However, for many practical applications, it is desirable or even necessary to introduce one-dimensional defects into the otherwise perfect hexagonal lattice. In an initial experiment conducted in collaboration with a French research group, graphene samples were irradiated with 100 MeV ions on the heavy ion accelerator in Caen and subsequently examined with a scanning force microscope. It was found that the graphene shows characteristic modifications after irradiation which are only partially created by the direct energy transfer into the electronic system of the graphene. The aim now is to change the experimental conditions in such a way that one-dimensional defects can be introduced into the lattice without ripping the graphene apart. 
A new experiment is currently being set up for this purpose on the heavy ion accelerator at the GSI in Darmstadt. The new experiment is funded by the BMBF and scheduled to start in 2011.
The research team of Professor Andreas Wucher is investigating the game of billiards on an atomic scale. Atoms or ions are fired onto a solid surface, thereby initiating a sequence of complex collisions which lead to the ejection of surface particles (atoms or molecules). This “sputtering” process is used in many ways, for instance by depositing the sputtered material to form a thin coating film or by mass spectrometric analysis in order to determine surface composition. Using a finely focused ion beam, a high resolution image of the surface chemistry can be obtained this way. Continuing bombardment for a longer period of time leads to surface erosion. If the bottom of the eroded crater is analysed as a function of crater depth, a three-dimensional image of the surface chemistry can be obtained. In close cooperation with a research group at Pennyslvania State University, it was recently demonstrated that this type of nanoscale 3D analysis is feasible for molecular films if cluster projectiles such as C60 are used as primary projectile ions.
Basic research in this area primarily focuses on studying the processes and pathways governing the dissipation of kinetic energy imparted to the surface by the impinging projectiles. By combining experimental data and molecular dynamics computer simulations, it was possible to show that – unlike in billiards – a large proportion of this energy is initially transferred to the electronic system of the bombarded solid immediately on impact, thereby generating substantial “kinetic” excitation. As a consequence, the surface emits electrons and particles in excited or even ionised state, a process which is extremely important for surface analysis applications.
Professor Uwe Bovensiepen and his team closed an important gap in scientific research in cooperation with research partners in San Sebastian, Spain. They were able to experimentally confirm theoretical results for the lifetimes of quasiparticles at the interface layer between semi conductors and metals. This is relevant in the production of ever smaller structures, for example in microprocessors.
Electrons interacting with other electrons are called quasiparticles. If a laser pulse is directed onto the surface of a wafer-thin lead film, the electrons at the lead surface are excited and reach a higher oscillation frequency. If the substrate of the lead film is electrically conducting like copper, the electrons migrate to this adjacent substrate. In visual terms this means that they break away from the formation of electrons in the lead before they relax. This drain of electrons into an adjacent substrate distorts the results by shortening the observed lifetime of the quasiparticle.
The scientists have now come up with a way of preventing this drift. It works by using semiconductive substrates that are nonconductive below a certain temperature. If a semiconducting material like silicon is used as a substrate, it is possible to create an “energy window” in which the theory of the lifetime of quasiparticles is validated.
The group of Professor Hermann Nienhaus is in search of hot electrons generated in metal surfaces by reactions with gas particles. The released reaction energy is partly transferred to the electronic system and not directly converted into heat. The lifetime of the electronic excitation is extremely short, typically just a few 10 fs (1 fs = one billionth of a microsecond). To enable its detection, a method was developed using special, self-made electronic devices such as Schottky diodes with nm-thick metal films. An electric current (chemi-current) is observed in these components during the surface reaction. Using this method it was possible for the first time to detect hot charge carriers during homoepitaxy, i.e., the deposition of Mg atoms on a Mg film. This surprising result considerably broadens the view of dynamic processes during metal epitaxy. Current research also focuses on the practical application of chemicurrents for micro gas sensing, reaction monitoring and direct chemical-electrical energy conversion.
Professor Gerhard Wurm’s research group studies planet formation and the surface and 
atmosphere of Mars using somewhat unconventional means, both in laboratory experiments on the ground and under microgravity. One focus of interest is on how micron-sized particles grow into larger bodies. The group examines different processes such as collisions from mm/s to 200 km/h, the erosion of surfaces by gas drag at low pressure or erosion by solar irradiation. The latter in particular was discovered by chance but proved to be extremely important, as it allows particles to be levitated in the laboratory. It may also contribute to the unsolved riddle of dust transport in the Martian atmosphere or explain particle recycling in the early solar system. The effect is heavily dependent on gravity, as demonstrated in parabolic flights. The transport of particles by solar radiation was also investigated and quantified for extraterrestrial material under microgravity in drop tower experiments. This research as a whole will help to provide a better understanding – or an understanding at all – of how individual dust and ice grains grew into planets like Earth or Jupiter in just a few million years.