Fermi-liquid theory is the established context to describe the electronic properties of metals. The scattering of a Fermi liquid should quadratically depend on both temperature and frequency, but the latter prediction, the quadratic frequency dependence, is extremely difficult to observe experimentally. Furthermore, recent solid-state research has found several material classes that do not obey Fermi-liquid theory, i.e. here electrons in metals behave fundamentally different, and it is not clear how one explain this behavior. One hallmark of such “non-Fermi liquids” is a scattering rate that does not depend quadratically on temperature. But the frequency dependence of the scattering rate is much less studied for these materials and not well understood yet.
We want to probe Fermi-liquid and non-Fermi-liquid electrodynamics in heavy-fermion metals to observe and understand their frequency- and temperature-dependent scattering rates. Heavy-fermion materials are model systems for strongly interacting electrons in metals, and they allow convenient tuning of electronic states by changing temperature, magnetic field, or composition. The characteristic energy scales of these materials are very low, corresponding to temperatures around 1 K or lower and to GHz frequencies. Therefore we employ microwave spectroscopy at ultralow temperatures to study the charge dynamics of heavy fermions. In addition we can also probe spin dynamics, which allows another avenue to study the fundamental physics in these materials that strongly depend on the interplay of spin (magnetism) and charge (conduction) degrees of freedom.