Research
We investigate self-organizing nanostructures after the deposition of atoms or molecules on metal or semiconductor surfaces. The reduced dimensionality of the structures can lead to interesting electronic properties due to electron correlations, such as the formation of a Luttinger liquid in one-dimensional nanostructures. The investigation of the structural and electronic properties is fundamentally exciting but also very relevant for applications with regard to the development of future technologies. Other current research topics include conductivity measurements on molecules and molecules on surfaces.
The investigation of electron correlations and interactions of electrons with lattice vibrations is a highly topical field of solid-state physics. In the theoretical description of solids, various simplifications must be made in order to describe the complex interaction of 1023 electrons with just as many positive atomic bodies (crystal lattice). A very far-reaching simplification that enables the calculation of electronic band structures is the image of independent electrons (one-electron image). Here, the electrons are described independently of each other in the "sea" of other electrons. The interaction of the electrons with each other is only captured by an interaction with the local density of the other electrons: local density approximation (LDA).
Electron correlations are interactions of electrons that go beyond this simple picture and are the cause of many very interesting effects (superconductivity, colossal magnetoresistance, metal-insulator transitions, etc.). If the correlations are weak, the one-electron picture can largely be retained and the many-electron system is described by the Landau theory of Fermi liquids. Here, the electrons are assigned to so-called quasiparticles, which have the same quantum numbers as the real electrons, but have a renormalized mass and a limited lifetime. If the electron correlations are strong, we speak of highly correlated materials, for which other models must be used to describe the electron system.
The coupling of the electrons to the lattice vibrations (phonons) also has a significant influence on the electronic properties. The best known is the temperature dependence of conductivity. Here, the drift speed of the electrons is reduced with increasing temperature due to an increase in collisions with phonons, thus reducing the conductivity. With particularly strong coupling to phonons, however, the opposite effect can also occur. In superconductivity, two electrons with reversed spin and opposite momentum are bound together by phonons and form so-called Cooper pairs. These can move through the crystal without any resistance and without colliding with defects or phonons. Such complex couplings are still not fully understood. In the case of high-temperature superconductors, for example, the coupling mechanism has not yet been clarified and an interplay of electron correlations, electron-phonon coupling and antiferromagnetic couplings is being discussed. Experimental methods for the targeted investigation of electron correlations and electron-phonon coupling are therefore of great value.