AbstractsChemistry

Heavy fermion materials are intermetallic compounds whose electronic properties exhibit a variety of anomalous effects at low temperatures. These effects emerge from the interplay between strong magnetic moments of localized f-electrons and the conduction electrons of the compound. Due to complex many-body interactions, these electron systems are said to be strongly correlated. At low temperatures the wave functions of the conduction electrons and the f-electrons hybridize, giving rise to a 'heavy' charge carrier with an effective mass even a hundred or a thousand times the free electron mass. Magnetic interactions of the electrons create a stage of magnetic ordering or disordering, superconductivity in some compounds, quantum critical phase changes and experimentally observed deviations from the so-called Fermi liquid theory that describes strongly interacting fermions at low temperatures. In fact, deviations from the Fermi liquid theory have for long been an interest in solid state physics, since the underlying mechanism is not understood. One of the main motivators for understanding unconventional behaviour of fermionic ensembles has been the discovery of high-temperature superconductivity, as well as explaining electronic correlations and quantum critical behaviour emerging from magnetic interactions. Two heavy fermion compounds were examined in this thesis: CeCu6 and YbRh2Si2. CeCu6 is an archetypal heavy fermion metal with well-established Fermi liquid behaviour of its valence electrons. YbRh2Si2, on the other hand, exhibits quantum criticality between magnetic order and disorder, and pronounced deviations from the Fermi liquid characteristics have been shown to occur in the vicinity of the quantum critical region. The compounds were studied with optical spectroscopy, utilizing a superconducting stripline resonator in the GHz frequency range. The method addresses the skin effect of these materials, and in this manner information regarding scattering mechanisms can be obtained. A correction for the theory of skin effect in heavy fermion materials is suggested in this thesis, as these compounds can no longer be treated as conventional metals. In this thesis it is shown that correlation effects in heavy fermion compounds cause deviations to the description of the normal skin effect, and the method is used further to understand the connections between theoretical predictions and experimental observations. All in all, both the theoretically and experimentally conducted method for investigating scattering mechanisms in heavy fermion compounds via the skin effect offers a new approach for understanding correlated electronic transport and the physics behind it. Raskasfermionimetallit ovat metallien yhdisteitä, joiden elektronisilla ominaisuuksilla on lukuisia anomaalisia fysikaalisia ilmiöitä matalissa lämpötiloissa. Nämä ilmiöt aiheutuvat lokalisoituneiden f-elektronien vahvojen magneettisten momenttien ja johtavuuselektronien välisestä vuorovaikutuksesta. Monimutkaisesta monen kappaleen ongelmasta johtuen…