Elementary or Composite ? About Excitations in Correlated Systems

Goetz S. Uhrig

Institute for Theoretical Physics, University of Cologne

D-50937 Cologne, Germany

Before doing a calculation a theoretical model has to be set up. One must specify what the relevant degrees of freedom are. In a quantum mechanical model this means that one chooses which excitations are considered to be the elementary ones: a basis is chosen. The best choice depends on the energy scale that one is interested in.

Strongly correlated systems display a strong interaction between the elementary excitations, for instance in Mott insulators, electrons and holes attract each other strongly and can form fascinating new collective modes.

Very often, several choices of the elementary excitations are possible in strongly correlated systems. For instance, in chains of localized spins either spinons with S=1/2 or triplets (triplons) with S=1 can be chosen. I will show that the latter choice makes it possible to compute the experimentally relevant quantities on all energy scales.

Results are shown for spin chains and spin ladders. The ladders are relevant for the so-called telephone number compounds (Sr,Ca,La)₁₄Cu₂₄O₄₁. The computed spectral densities agree very well with experimental findings (optical absorption, inelastic scattering of light or neutrons), and a novel complex bound state composed of two triplons is identified.