We work on the exploration of semiconductors regarding fundamental physical phenomena as well as the development of innovative components. There are two main work fields: molecular beam epitaxy (MBE) to produce semiconductor heterostructures and focused ion beam implantation (FIB) for material handling.
We use bandgap engineering to realise high-quality low dimensional semiconductors in AllnGaAs with mono layer resolution via MBE. These are two-dimensional electron layers like the ones occurring in field-effect transistors, one-dimensional quantum wires or zero-dimensional quantum dots which will form elementary functional units of future supercomputers.
The spatial limits lead to quantum phenomena like the quantum Hall effect and the quantisation of conductivity. We measure our semiconductors optically and electrically – even at low temperatures. These serve as a base for cutting-edge research in world-wide collaborations. The limits of contemporary lithography are foreseeable which is why we implemented a mask less technique for structuring via FIB without any chemistry involved. This technique allows to produce lateral structures down to the nanometre scale. New individual components (e.g. in-plate gate transistors) and nano structures to investigate quantum phenomena are thus made possible.
We investigate the geometric, electronic, vibronic and magnetic structures of nano structures and thin layer systems on the surfaces of semiconductors and oxides. The dynamics of the evolutionary processes of the structures are of particular interest, e.g. growth, adsorption and diffusion processes.
The technological goal is to produce layers or nano structures with predefined structural, magnetic or chemical properties. The main measurement technique is scanning tunnelling microscopy (STM). Images of the sample with atomic resolution are taken in an ultra-high vacuum during the evolution of the nano structures or ultra-thin layers. Thus, a time-resolved observation of the growth of the surface is possible to directly follow the growth and structure building processes.
We gain chemical information on the condition of the surface with high-resolution electron spectroscopy methods. Oscillation modes of molecules bounded at the surface together with the results from scanning tunnelling microscopy allow for an exact characterisation of the bonding conditions of molecule layers on surfaces. Magnetic properties of nano structures are investigated directly while they are being produced using the magneto-optical Kerr effect (MOKE). This way it is possible to observe the behaviour of magnetic domains on a sub-micrometre scale.
In solids, numerous physical processes occur within few nanoseconds or picoseconds, for example recombination of charged particles, magnetisation or spin relaxation. The group of Spectroscopy of Condensed Matter visualises such processes using laser spectroscopy. Short pulse lasers enable the investigation of these rapid phenomena with a time resolution of about 100 femtoseconds.
One focus of our experimental work is the development of materials and structures with long spin lifetimes that can be used in semiconductor spin electronics. Furthermore, the group contributes to the growing field of fluctuation spectroscopy. We develop methods to measure and characterise random thermal movements of electron spins in solids or the stochastic transport of charged particles in resistors. For this purpose, a system was developed which processes data streams of 0.4 GB/s in real time and uncovers correlations that are concealed by noise. The so-called correlation spectroscopy is used e.g. for the investigation of critical dynamics at phase transitions, the quantum dynamics of mesoscopic systems and the tracking of systematic disturbances in highly sensitive measurements.
Theoretical physics of condensed matter is a rich and complex field thanks to the fact that it deals with the quantum theory of many body systems. This theory describes a huge number of atoms or ions and their mutual interactions (or correlations), where phenomena of only quantum mechanical origin play a crucial role.
In our group, we use various theoretical methods to study the electrical and magnetic properties of strongly correlated low-dimensional systems, including the unconventional high-temperature superconduction that forms in many of these systems. The particularity of these systems lies in the electron-electron correlations which are strongly enforced by the effect of low dimensionality and the rivalry of spin, charge and orbital degrees of freedom. It is furthermore important to keep in mind that the ground state of such systems can change drastically as a function of a single parameter.
Additionally, our group is concerned with strongly correlated low-dimensional systems which exhibit a geometrical frustration of the magnetic order and are responsible for many exotic quantum states of matter.
We study quantum materials whose properties are dominated by quantum mechanical effects. For example, the interaction of electrons with each other and between the electrons and lattice in a crystal leads to rich phenomena, including magnetic order, charge-density waves and superconductivity.
We grow crystals of these often intermetallic compounds via solution growth techniques. Their frequently still unknown structural and electronic properties are studied with a variety of techniques, ranging from structure determination and chemical analysis via electrical transport and thermodynamic properties to x-ray and neutron diffraction.
We investigate a wide temperature range from approx. 1 K - 400 K and high magnetic fields up to 17 T. The investigation of effects caused by uniaxial pressure or strain plays an important role. We use high-resolution thermal expansion and elastic modulus measurements and elastotransport to approach this question.
We aim to understand and control the properties of complex materials. For example, a change of composition or (anisotropic) pressure may induce new phases and lead to functional properties. We search especially for new quantum materials with exotic properties with the methods of material's design.