Low energy spin excitations in doped copper oxide spin chains

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One dimensional cuprates are important model systems which allow for reasonable comparisons of theory and experiment like only few other solid state materials. For undoped systems, the theoretical model is exactly solvable (Bethe-Ansatz), whereas in doped systems numerical methods yield meaningful results. Therefore, many physical properties such as spin excitations are theoretically accessible. It was thus even more surprising when we found evidence for a spin gap in a S=1/2 spin chain doped with S=1 impurities in our preliminary work using nuclear magnetic resonance (NMR), which was never expected or evidenced before in theory or experiment, respectively. There seems to be an unexplored reason for this gap in the spin excitations which will be investigated in the course of this research project. The applicant is convinced that the origin of this spin gap can be unraveled within this project by means of NMR and with the help of national and international cooperation. The results promise important information for the discussion of the physical properties of higher dimensional and more complex materials where it is much more difficult to apply theoretical or numerical methods to explain the origin of gaps in the spin excitation spectra. The project is placed within the fundamental research. Experimental work on model systems should lead to new information which helps to clarify similar issues in other doped antiferromagnets, which are the basis of unconventional superconductors and many other correlated electron systems.