Strong correlations in surfaces and interfaces

In recent years it became possible to fabricate complex materials in a very high quality due to improved experimental techniques. This enables researchers to investigate completely new systems and questions that were not studied so far. Two examples for such novel compounds are the so-called adatom-systems, as well as heterostructures. The first ones consist of a semiconductor surface coated with metal atoms, which therefore form a two-dimensional system. The second ones are built up by different layers of transition-metal oxides, and at the interface of the adjacent layers they also show two-dimensional behavior. Particularly interesting is the case when one of the components of the heterostructure belongs to the family of high-temperature-superconducting materials. In this project we want to investigate these two classes of materials, the adatom-systems and superconducting heterostructures, with theoretical calculations. One of the fundamental questions that shall be answered is whether the physical properties known from bulk materials also show up in these two-dimensional surfaces and interfaces. Can they become superconducting? Is there some magnetism? Which other phases can emerge? The focus of the project lies on answering these questions. From different studies it is known that in these systems the charge carriers interact very strongly. For this reason conventional methods of theoretical physics cannot be used any more. Instead, it is necessary to apply numerical methods for the investigation of the abovementioned questions. To be specific, we intend to combine first-principle calculations (density-functional theory) with modern methods of many-body physics. This procedure unifies the advantages of these two approaches, namely doing calculations very close to the real materials on the one hand, and an accurate treatment of the strong interactions between the charge carriers on the other hand. In this way it is possible to calculate the phase diagram of the compounds as function of different parameters such as charge carrier concentration or temperature, and compare the results to experimental data. It should be noted that the combination of first-principle calculations and methods of many-body physics, as proposed in this project, has not been performed so far for the abovementioned materials. However, from other investigations it is known that for the correct description of the competition between different phases it is important to include the strong correlations in a proper way, which will be done in this research work. We therefore expect that the results of these studies can contribute to the understanding of fundamental questions of solid-state physics. Furthermore, the results are not only interesting from an academic point of view, but they will also be important for applications of these novel compounds in nanoscaled devices, such as fast electronic switches.