We are currently studying lattice dynamics, electronic and magnetic excitations in several strongly correlated systems using the inelastic scattering of photons, both in the visible and x-ray ranges, as our main tool of investigation. More specifically, we focus on unconventional superconductors, mostly high-Tc cuprates, transition metal oxides, like nickelates and vanadates, as well as 4d- and 5d-systems, such as ruthenates and iridates. In all these materials we use inelastic photon scattering to probe the various electronic phases and ordering phenomena with a special interest for the interplay between magnetism, charge order and superconductivity when simultaneously present in the system. We carry out our experiments on single crystals as well as on thin films, heterostructures and superlattices that allow studies as a function of strain and spatial confinement. These projects are carried out in close collaboration with the other groups of our department and within the institute.
In order to gain information about the properties of the above-cited materials we primarily use Raman scattering and X-ray Scattering. Raman scattering has been widely used to study lattice, magnetic and electronic excitations in solids. The use of lasers light in the visible range allows to reach high energy resolution (below 1 meV), whereas the selection of incoming and outgoing polarizations of the light allows to disentangle contributions from different irreducible representations of the space group of the system. Various excitation lines are available in our laboratories, so that we are also able to explore possible resonance effects. Our group operates 4 state-of-the-art high-resolution Raman spectrometers: a single-grating Jobin Yvon LabRAM HR800 and three triple-grating spectrometers (two Jobin Yvon T64000 and a Dylor XY). One of the T64000 and the Dylor permit experiments under high pressures and in magnetic fields up to 16 T, respectively. The LabRAM setup is instead capable of measuring the excitation spectra from very thin films by using a combination of motorized objective lens with short depth-of-focus and a precisely calibrated confocal hole along the scattering path.
Due to the small momentum carried by visible light, the information obtained from Raman scattering experiments is essentially limited to the center of the Brillouin zone. Thus, in order to gain insights about the dispersion of the observed excitations, it is necessary to complement the Raman measurements with scattering probes providing a non-negligible momentum transfer, such as x-rays. To this aim, we take advantage of the latest technical improvements in synchrotron radiation facilities carrying out non-resonant and Resonant Inelastic X-ray Scattering (RIXS) experiments.
In particular RIXS, thanks to its chemical selectivity, is a powerful tool to measure collective excitations of strongly correlated systems and it has allowed a remarkable series of scientic achievements over the last few years, including, for example, the direct observation of both high-energy collective spin excitations and charge density waves in high-Tc superconduting cuprates. In RIXS an incoming photon with energy tuned to an absorption edge of the material hits the sample and excites the system. One observes then the radiative decay of the excited state. The energy of the emitted photons in a given direction is accurately measured with a spectrometer, to obtain the energy and momentum transferred in the scattering event from the photon to the sample. RIXS is thus capable of mapping out dispersive excitations in a wide region of the reciprocal space. Crucially, the high sensitivity of RIXS, due to the resonant nature of the scattering process, allows also the study of ultrathin layers of material (down to a few unit cells), overcoming the technical limitations of alternative techniques like inelastic neutron scattering, which requires instead more massive samples.
Finally, we carry out Raman and x-ray scattering measurements also as a function of uniaxial strain (both tensile and compressive) by means of a device, based on piezo-electrics and developed at the MPI in Dresden, that we integrated into the setups that we use in-house and at the synchrotron.