Spectral densities of disordered quantum magnets

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Disorder is inevitable in condensed matter physics and is known to be the source of interesting novel physics, e.g. it can change quantum criticality or even destroy quantum phase transitions. At the same time it is typically hard to treat disorder theoretically which is especially true for dynamical correlation functions. The latter are desired quantities, since high-resolution spectroscopy on materials with intentional doping are becoming a focus experimentally. In this project we apply white-graph expansions in the framework of perturbative continuous unitary transformations to treat gapped phases of disordered quantum magnets. Disorder averages are taken either for large finite systems or directly in the thermodynamic limit, so an effective quasi-particle picture is derived which goes well beyond mean-field approaches for gapped quantum phases.