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As the name suggests, semiconductors are materials that are located between metals and insulators. Therefore, their electrical and optical properties can be easily manipulated even by small external influences, e.g. tiny electrical voltages.

In a modern microprocessor, for example, certain semiconductor components are switched back and forth between an electrically conductive and an insulating state several billion times a second. The demands on the material, nowadays mostly silicon, are extreme. In addition, semiconductors are finding ever wider applications, e.g. as lamps, lasers, solar cells, photonic crystals, etc.

In university semiconductor research, therefore, materials and concepts are being sought for the computers and data networks of the day after tomorrow: ever faster, ever smaller and ever more versatile.
New semiconductor materials

In Theoretical Physics I we are interested in semiconductor-based nanostructures and unconventional materials such as organic semiconductors or the strongly polar III-V and II-VI semiconductors (In)GaN or ZnO.

For this purpose we calculate the electronic band structures and wave functions (see figure), simulate growth and structure formation as well as charge and excitation transport and analyze the ultrafast kinetics after optical excitation.

If you have any questions or are interested in a bachelor, master, or doctoral thesis in this field, please contact Prof. Dr. Erich Runge.

Today, a wide variety of methods can be used to produce and study structures that are only a few tens to hundreds of nanometers in size. Even at room temperature, quantum effects then become important and must be taken into account in the theoretical description. For structures smaller than the wavelength of light, geometric optics and wave optics cannot be applied easily. Rather, evanescent near fields must be explicitly taken into account. Even the mechanics familiar to us from everyday life lead to surprising results, for example when nano-tuning forks oscillate in the GHz range.

In the Theoretical Physics I department, optical properties of metallic and semiconductor-based nanostructures are mainly studied, as they can be examined by near-field microscopy (SNOM). The main focus is on excitons, biexcitons and plasmons enclosed in quantum wells. In addition to the confinement by changing the material in a vertical direction (e.g. AlGaAs/GaAs/ALGaAs), the varying width of the quantum well also leads to a lateral localization of the excitations (Anderson localization).

If you have questions or are interested in a bachelor, master or doctoral thesis in this field, please contact Prof. Dr. Erich Runge.

We use quantum chemical methods such as time-dependent density functional theory, self-consistent Hartree Jerk calculations including configuration interaction, semi-Empire and multi-reference methods to calculate organic semiconductors such as conjugated polymers, molecule stacks with overlapping π orbitals, dendrimers, fullerenes and their derivatives, and carbon nanotubes. We are particularly interested in their suitability as materials for organic solar cells. Here we work closely together with the department Experimental Physics I.

To simulate molecular structure and dynamics, we use advanced programs developed by us and in the Department of Theoretical Physics II, some of which are based on quantum-chemically calculated potentials for the mono- and oligomers. For example, the structure of polyparaphenylenes can be derived from the torsion of biphenyl.

Another important problem in connection with organic semiconductors that we investigate is charge transport and transfer (see figure).

W. J. D. Beenken, Excitons in conjugated polymers: Do we need a paradigm change?
pss(a) 206 (2009) 2750

If you have questions or are interested in a bachelor, master or doctoral thesis in this field, please contact Dr. Wichard J. D. Beenken .

Organic ionic liquids are novel materials with amazing properties. Their electrical properties are comparable to those of inorganic molten salts, yet they are still liquid at room temperature. However, they have such a low vapor pressure that they do not evaporate even in ultra-high vacuum, which qualifies them as lubricants in this UHV range.

To better understand the basic properties of ionic liquids, we perform quantum chemical calculations for the individual ions as well as for dimeric complexes of cation and anion (see figure on the right). From these calculations, element-specific densities can be determined, which can be used to interpret photoelectron spectra (XPS, UPS and MIES). Here we are working intensively with the Research Group Surface Physics of Functional Nanostructures.

If you have questions or are interested in a bachelor, master or doctoral thesis in this field, please contact Dr. Wichard J. D. Beenken.

Metastable Induced Electron Spectroscopy (MIES) allows an extremely surface sensitive electronic characterization of liquid and solid surfaces by excitation of metastable noble gas atoms. We have developed a quantum chemical approach for the ab initio calculation of MIES spectra that goes beyond previous approaches, e.g. by Kantorovich et al. [1]. In contrast to these approaches, the anisotropy of the MIES spectra is not only represented with respect to the direction of incidence of the metastable helium atoms but also with respect to the direction of electron detection. Since we can specify both in our simulation, we are able to reconstruct MIES spectra for any experimental setup.

For the time being, our method is limited to the calculation of MIES spectra of molecules physiosorbed on solid surfaces where direct eye excitation (AD process) is predominant. However, we want to further develop the method in close cooperation with the Department of Technical Physics 1 (Prof. Krischok ).

1] L.N. Kantorovich, A.L. Shluger, P.V. Sushko, A.M. Stoneham, "The prediction of metastable impact electronic spectra (MIES): perfect and defective MgO(001) surfaces by state-of-the-art methods," Surface Science 444, 31 (2000).

If you have questions or are interested in a bachelor, master or doctoral thesis in this field, please contact Dr. Wichard J. D. Beenken.

Dendrimers are high-molecular organic or inorganic compounds which, in contrast to polymers, consist of chains (dendrons) branching out further and further around a core.

Our research activity is focused on the calculation of the structure and optical properties of dendrimers, where dendrons of conjugated organic molecules originate from a tetrapyrrolic core, e.g. a porphyrin or corrol. Since both the dendrons and the core of the dendrimer have an extended π electron system, both absorb and emit light in the visible spectral range. If their spectra overlap, excitonic interaction between dendrons and nucleus occurs, which leads either to excitation energy transfer (weak coupling), which causes an extinction of the shorter-wavelength fluorescence band in the common emission spectrum, or to delocalization of the excitation (strong coupling) with spectral shifts of the absorption bands. Since the strength of the excitonic coupling depends on the position of the dendron relative to the nucleus (distance and orientation), spectroscopic methods can be used to determine the conformation of the dendrimer, which can be manipulated by changing the chemical-physical environment.

Dendrimers can thus also be used as environmental detectors, especially by using functional, environmentally sensitive groups at the ends of the dendrons. However, to determine the relationship between dendrimer conformation and spectrum as early as possible before the synthesis of such a complex molecule, extensive quantum chemical calculations such as those we perform are extremely helpful. We can draw on many years of experience in calculating the excitonic coupling in conjugated polymers.

If you have questions or are interested in a bachelor, master or doctoral thesis in this field, please contact Dr. Wichard J. D. Beenken.

Every one of us has had the experience of behaving differently in the crowd than when we are on our own. This is also true for electrons, for example, in solids. Just as the l'ola in one stage can only establish itself in a system of many people, there are also effects in the theory of condensed matter that only manifest themselves in a many-body system. From a quantum theoretical point of view, these lead to new so-called quasi-particles, which have effective masses and charges that differ considerably from those of the particles on which they are based. There may even be completely new types of quasiparticles that have no equivalent among the elementary particles.

Members of the Theoretical Physics I group have contributed to many aspects of many-particle physics:

    For example, the extreme increase of the effective mass of highly correlated electrons in f-bands has been investigated, which leads to the localization of the charge carriers, analogous to the small polaron. For the calculation of these materials called heavy fermion systems, conventional LDA programs are combined with special approaches for the self-energy of the highly correlated f-electrons within the framework of renormalized band structure calculations.
    Motivated by the crystal structure of the heavy-fermion compound LiV2O4 (see figure), another exciting phenomenon has been theoretically predicted: quasi-particles with a half-integer effective charge in frustrated systems. In the borderline case of large mutual repulsion, the Verwey-Anderson tetrahedron rule applies in such systems, which states that only two electrons per tetrahedron occupying the corner places form a half-filled band. In this band, however, the motion of the electrons is so highly correlated by the above rule that it effectively appears to be the motion of quasiparticles with a half-integer charge.
    Of particular interest in connection with nanostructured, amorphous and organic semiconductors is the description of their transport properties, e.g. by means of non-equilibrium green functions.
    In addition, we deal with fundamental questions of density functional theory and the further development of numerical methods in many-particle physics.

If you have questions or are interested in a bachelor, master, or doctoral thesis in this field, please contact Prof. Dr. Erich Runge.