The group studied heavy fermion effects by means of high resolution photo emission spectroscopy. Heavy fermions are materials with electrons at the Fermi level which exhibit a very large effective mass.
Heavy fermions straddle the boundary between localized and itinerant states in a solid.
The vast majority of heavy fermion compounds contain at least one element from the lanthanide or actinide series of the periodic table. U. These materials are well suited for investigation using a tuneable, high flux, high resolution source of photons to conduct photoelectron spectroscopy measurements. High resolution photoemission can measure the binding energy, linewidth, spectral weight, bandwidth and temperature dependence of the f-levels and thus provides valuable input regarding the applicability of models postulated to explain heavy fermion characteristics. The high flux at high resolution allows data to be collected with sufficient statistics to support quantitative analysis and lineshape fitting while still keeping the acquisition times reasonable for detailed studies requiring large numbers of scans such as angle resolved photoemission. All of the Yb heavy fermions show a non-integer valence which means that the 4f electrons are found in two different configurations within the material.