The department uses neutron and X-ray diffraction and spectroscopy as well as optical spectroscopy and Raman scattering to explore the structure and dynamics of materials with strong electron correlations. We also have a strong effort in the development of new spectroscopic methods. As the close collaboration between experimentalists and theorists is essential for the progress in this field, a small theory group operates within the department.
First Light at IRIXS
On March 28, 2017, we saw "first light" at our new IRIXS spectrometer at the PETRA-III synchrotron in Hamburg. IRIXS stands for "Intermediate-energy Resonant Inelastic X-ray Scattering", a method that will allow energy- and momentum-resolved measurements of electronic modes in quantum materials with 4d valence electrons. The construction of the unique instrument is made possible by an Advanced Grant from the European Research Council. The photo shows Hlynur Gretarsson sharing a drink to celebrate this event with Markus Tischer, whose group designed the undulators that supply photons to the spectrometer. See also the time lapse video of him, Simon Mayer, Hasan Yavas and others assembling the spectrometer.
Neutron scattering experiments on the two-dimensional antiferromagnet Ca2RuO4 revealed a well-defined, dispersive "Higgs" mode that modulates the amplitude of the ordered moment. The mode quickly decays into transverse spin waves at the antiferromagnetic ordering wavevector, and through a complete mapping of the transverse modes in the reciprocal space we were able to obtain a quantitative understanding of the decay process. The results establish a novel condensed matter platform for research on the dynamics of the Higgs mode.