The competence group "spectroscopy with neutrons" deals with the study of the structural and dynamic properties of condensed matter on a microscopic scale using elastic and inelastic neutron scattering. We focus on the determination of the magnetic properties of mostly intermetallic compounds, i.e., the magnetic ordering phenomena and magnetic excitations and fluctuations which are studied in dependence of temperature, external magnetic field, chemical composition or hydrostatic pressure.
Our research program encompasses the magnetic ordering and the magnetic excitation spectrum of strongly correlated electron systems (heavy-fermion systems) in the vicinity of magnetic quantum phase transitions. Due to the quantum fluctuations an unusual low temperature behavior is observed in the critical spin excitations. Their dimensionality directly affects the thermodynamic properties. Although macroscopic measurements such as heat capacity or bulk magnetization are very important in the investigation of quantum phase transitions, only microscopic probes can give insight into the relevant interactions on an atomic level. Especially, magnetic neutron scattering is an ideal probe to study the spin dynamics and the magnetic order in the vicinity of a quantum phase transition. The dimensionality of the critical spin fluctuations can directly be studied by neutron scattering. The interplay between magnetism and superconductivity came into focus, since several compounds show unconventional superconductivity in the vicinity of their quantum phase transition, with a complex interplay between both, superconductivity and magnetic order. Coexistence as well as competition between both phenomena is observed. Recently, we could observe changes in the magnetic excitation spectrum of a cerium-based heavy-fermion superconductor upon entering the superconducting state. The appearance of a spin excitation gap suggests that magnetic fluctuations are involved in the process of Cooper pair formation.
One of the most versatile instruments (which we use quite frequently) for elastic and inelastic neutron scattering experiments to investigate single crystalline samples is the three-axis spectrometer which allows one to select momentum and energy transfer onto the sample and measure magnetic order and spin fluctuations as function of transferred momentum and energy. For investigations in the vicinity of quantum phase transitions very low temperatures are mandatory. Therefore, we use in most of our experiments dilution cryostats together with superconducting magnets. Hence, measurements at temperatures down to T = 30 mK and magnetic fields up to B = 15 T can be performed. Experiments are carried out at different reactor and spallation neutron sources in Europe, including the high-flux reactor at the Institut Laue-Langevin in Grenoble, France, the reactor BER-II at the Helmholtz-Zentrum für Materialien und Energie in Berlin, Germany, the new research reactor Forschungs-Neutronenquelle Heinz Maier-Leibnitz, FRM-II, in Garching, Germany, or the spallation neutron source ISIS at the Rutherford Appleton Laboratory in Didcot, UK.