Unconventional phases under extreme conditions
New quantum states of condensed matter are a fascinating research area, which is at the basis of modern solid state physics. Unconventional phases, such as unconventional superconductivity and non-Fermi liquid phases, are typically observed in regions of competing energy scales, e.g. in the vicinity of magnetic instabilities. In these regions only small changes of an appropriate external control parameter can change the properties of a material fundamentally.
We use external pressure to deliberately tune the ground-state properties of strongly correlated 4f-electron metals and iron-based high temperature superconductors to study the nature of novel unconventional phases without introducing additional disorder or polarizing the electrons by application of a magnetic field. However, sometimes a smart combination of chemical substitution, magnetic field, and external pressure might lead to a deeper insight and understanding. Even though, pressure allows to easily tune the properties of a material, the study of the physical properties becomes more challenging due to the particular restrictions posed by the high-pressure equipment. What remains essential, though, is to find and develop techniques which allow characterizing the thermodynamic, magnetic, and transporting properties of a material as complete as possible also under external pressure. This is a challenging task requiring a constant effort in developing new and improving already existing experimental setups, which is always driven by the obtained results and newly emerging physical questions.