Quantum phase transitions and collective modes

Phase transitions are one of most fascinating phenomena of nature. Everyday examples of melting, freezing or liquid-vapor transitions in e.g. water represent only the tip of an iceberg. Many materials develop entirely new properties when cooled below a certain temperature, for example they become magnetic or superconducting. The underlying principle of phase transitions is that the random and independent motion of atoms or electrons becomes correlated over long distances a chaotic crowd turns into a well orchestrated ballet with many dancers. To mathematically capture this transition is an outstanding challenge. In classical physics of the 19th century every motion freezes close to absolute zero temperature. However, in the real world described by quantum physics the random motion, so called quantum fluctuations, survive even at absolute zero. As a result, phase transitions induced by changes of pressure or the application of magnetic field exist even at the lowest temperatures. These so-called quantum phase transitions are the central topic of the present project. The project comprises a theoretical and an experimental part. For the former, we will use and further develop advanced theoretical methods, which make use of supercomputers with thousands of processors for studying the multi-faceted aspects of quantum phase transitions. In this way, we will be able to predict changes in physical properties of solids such as the magnetic susceptibility or the electrical conductivity. We will study specific materials as well as simplified models which, while not representing any specific material, might provide a deeper understanding of the underlying processes. In the experimental part of the project, materials of interest will be synthesized and their physical properties will be measured in wide ranges, down to temperatures close to absolute zero. A detailed comparison between theory and experiment will provide the guidance for advancing our understanding. The project will be executed by three theorists and one experimentalist from TU Wien together with their doctoral and undergraduate students. The project is part of the QUAST consortium, that gathers seven research groups from Austria, Germany and Switzerland active in the development of diverse theoretical methods. Three experimental groups are included to ensure the direct connection to the world of real materials. The ultimate goal of this project, and the entire QUAST consortium, is to improve our understanding of quantum materials with strong electronic correlations, to ultimately harness their properties for quantum technology properties for quantum technology.