  Electrons in solids have been conventionally classified as either band-like
itinerant ones or atomic-like localized ones depending on their properties. For
heavy Fermion (HF) compounds, however, the f electrons show both itinerant and
localized behaviours depending on temperature. Above the characteristic
temperature T*, which is typically of the order of few K to few tens K, their
magnetic properties are well described by the ionic f-electron models,
suggesting that the f-electrons behave as 'localized' electrons. On the other
hand, well below T*, their Fermi surfaces (FS's) have been observed by
magneto-oscillatory techniques, and generally they can be explained well by the
'itinerant' f-electron model. These two models assume totally different natures
of felectrons, and how they transform between localized and itinerant state as
a function of temperatures has never been understood on the level of their
electronic structures. Here we have studied the band structure of the HF
antiferromagnetic superconductor UPd2Al3 well below and above T* by
angle-resolved photoelectron spectroscopy (ARPES), and revealed the temperature
dependence of the electronic structure. We have found that the f-bands, which
form the FS's at low temperatures are excluded from FS's at high temperatures.
The present results demonstrate how the same f-electrons show both itinerant
and localized behaviours on the level of electronic structure, and provide an
important information for the unified description of the localized and
itinerant nature of HF compounds.
