Itinerant to localized transition of f electrons in the antiferromagnetic superconductor UPd2Al3

In heavy-fermion compounds, f electrons show both itinerant and localized behaviour depending on the temperature(1). Above a characteristic temperature, T*, their magnetic properties can be described by completely 'localized' f-electron models. On the other hand, well below T*, their Fermi surfaces can be explained by the 'itinerant' f-electron model(2). These two models assume totally different natures of the f electrons, and how they transform between these states as a function of temperature has never been understood at the level of their electronic structures. Here, we use angle-resolved photoelectron spectroscopy to study the band structure of the heavy-fermion antiferromagnetic superconductor UPd2Al3 below and above T*, and find that the f bands, which form the Fermi surfaces at low temperatures, are excluded from the Fermi surfaces at high temperatures. This result demonstrates how the same f electrons can be both itinerant and localized.