Theoretical investigations are focused on many-body effects, ordering
phenomena and collective excitations in strongly correlated electron
compounds with 3d or 4f/5f elements. Current research topics include
• Magnetic ordering and spin dynamics in compounds with low dimensional
structures, for example halide or oxide chain, ladder, or layered compounds
of 3d elements (vanadates, manganites, cuprates) or 4f pnictide compounds.
• Frustration of spin and charge degrees of freedom in planar systems and
three dimensional pyrochlore and spinel structures. Frustration of Coulomb
and exchange interactions, partly due to the geometry of the lattice leads
to a high degeneracy of low lying excitations and possibly new quantum
phases. This causes anomalous low temperature and high field behaviour due
to quantum fluctuations.
• Thermodynamic properties of low-dimensional frustrated spin systems. The
large number of low-lying quasidegenerate states inherent to many
frustrated systems often lead to characteristic features in the temperature
and field dependence of thermodynamic and magnetocaloric properties like
heat capacity, magnetic susceptibility, magnetization, and the
magnetocaloric effect.
To investigate these physical effects various methods are used:
• electronic structure calculations to characterize model Hamiltonians,
• classical analytical methods of many-body physics such as spin-wave
theory,
• numerical exact diagonalization of model Hamiltonians on finite tiles and
the finite temperature Lanczos algorithm.