# Theory of quantum materials

Putting a large number of identical quantum particles together induces a wide range of new and often unexpected physical phenomena. Due to their quantum nature, the interactions of these particles lead to the emergence of new states, new phases, new excitations, new physical laws and principles. This is particularly true for correlated electrons in solids, being at the heart of our theoretical research.

We focus on many-body effects, ordering phenomena, and collective excitations in strongly correlated electron compounds with 3d or 4f/5f elements. Of special interest are magnetic ordering and spin dynamics in compounds with low-dimensional effective interactions, such as layered compounds of 3d elements (vanadates, cuprates) and 4f pnictide compounds. Competing Coulomb and exchange interactions often produce frustration of magnetic degrees of freedom, which in turn leads to a large number of low-lying quasi-degenerate states. The high quasi-degeneracy of these states has characteristic traces in various observables, for example the magnetic excitation spectrum or the field-dependent ordered moment. Characteristic features can also be seen in the temperature and field dependence of thermodynamic quantities like heat capacity, magnetic susceptibility, magnetization, and the magnetocaloric effect.

To investigate these effects, we develop and use various theoretical methods, including classical analytical methods of many-body physics such as spin-wave theory, numerical exact diagonalisation of model Hamiltonians on finite tiles and the finite temperature Lanczos algorithm. Our goal is to derive a detailed understanding of the materials we are exploring. To this end we work closely together with our in-house experimental colleagues as well as with theoretical and experimental groups worldwide. A strong overlap and intense cooperation also exists with the theorists from our in-house departments as well as with the neighbouring Max Planck Institute for the Physics of Complex Systems.