Jonas Fransson, Physics, particularly theory of strongly correlated materials

Photo: Mikael Wallerstedt

Strong correlations in materials mean that the electrons are highly interdependent and theoretically must be treated accordingly. However, strong correlations can give rise to properties such as magnetism. Magnetism and magnetic interactions are particularly interesting in molecular compounds because some can be controlled by external influences, such as electric currents and temperature variations. Magnetically active components can thus be used to open or block the electric currents through the molecular structure, which can be used for more thorough studies of the same. Theoretical aspects of magnetism and molecules under non-equilibrium can be treated with quantum field theory and quantum statistical methods. The emphasis is on the huge amount of electrons involved, which leads to reasoning in terms of the density the electrons have, and with which the magnetic structure can be described. While correlated (independent) electrons can be given a simple and consistent description, strong correlations usually lead to contradictory results and ambiguous interpretations. Comparisons with experimental observations provide guidance on how the theory should be formulated. Temperature is often an additional aspect in this context. Low temperatures can lead to considerably increased or decreased electrical conductivity or magnetic stability, while these properties completely disappear at high temperatures. A theoretical description that is too simple typically lacks these qualities, which again challenges the theoretical approaches used.