Tuning the dispersion of 4f bands in the heavy-fermion material YbRh2Si2

Localized Yb 4 f and itinerant Rh 4 d states are subject to substantial hybridization effects in the heavy-fermion material YbRh 2 Si 2 . The proximity to the Fermi level and the high anisotropy in k space naturally raise questions regarding the role of these hybridization effects for the observed, unusual physical properties. Using angle-resolved photoemission spectroscopy (ARPES) we found that the non-dispersive behavior of the localized Yb f states is broken around the Γ point due to interaction with approaching Rh 4 d bands. The intriguing point here is that the hybridization strength turns out to be systematically tunable by electron doping of the material. Gradual deposition of silver atoms onto the atomically clean, silicon terminated surface of YbRh 2 Si 2 leads to transfer of Ag 5 s charge into the Rh 4 d bands. This substantially changes the energy overlap, and thus the hybridization strength, between the interacting Yb 4 f and Rh 4 d bands in the surface and subsurface region. The shown possibility to control the variation of the f – d hybridization at the surface of heavy-fermion materials may also be helpful for other ARPES studies on the diverse phenomena in electron-correlated materials.