UC Irvine

We report an optically induced reduction of the f-d hybridization in the prototypical heavy-fermion compound YbRh2Si2. We use femtosecond time- and angle-resolved photoemission spectroscopy to monitor changes of spectral weight and binding energies of the Yb 4f and Rh 4d states before the lattice temperature increases after pumping. Overall, the f-d hybridization decreases smoothly with increasing electronic temperature up to ∼250K but changes slope at ∼100K. This temperature scale coincides with the onset of coherent Kondo scattering and with thermally populating the first excited crystal electrical field level. Extending previous photoemission studies, we observe a persistent f-d hybridization up to at least ∼250K, which is far larger than the coherence temperature defined by transport but in agreement with the temperature dependence of the noninteger Yb valence. Our data underlines the distinction of probes accessing spin and charge degrees of freedom in strongly correlated systems.