Ultrafast quasiparticle dynamics in the heavy-fermion compound YbRh2Si2

Understanding strongly correlated rare-earth intermetallic compounds requires knowledge of the nature of the fermionic quasiparticles in states near the Fermi level EF. We report on a pump-probe experiment using femtosecond time- and angle-resolved photoemission spectroscopy to determine lifetimes of hot quasiparticles in the heavy-fermion compound YbRh2Si2. An unoccupied band with electronlike dispersion and a band bottom 0.2 eV above EF was identified at Γ¯¯¯, in agreement with band structure calculations for the subsurface region. Hot quasiparticle lifetimes from 30 to 80 fs were found for energies between 0.4 and 0.1 eV above EF. These lifetimes generally follow the typical monotonous increase towards EF, in agreement with earlier studies on Yb and Rh elemental metals. However, at normal emission the lifetimes at around 0.2 eV exceed this trend by about +20 fs. This difference decreases with increasing photoemission angle and can be assigned to the particular band that is probed in YbRh2Si2. Potential microscopic scenarios are discussed.