We present a detailed theoretical study of the ultrafast quasiparticle relaxation dynamics observed in normal metals and heavy fermion materials with femtosecond time-resolved optical pump-probe spectroscopy. For normal metals, a nonthermal electron distribution gives rise to a temperature (T) independent electron-phonon relaxation time at low temperatures, in contrast to the T^{-3}-divergent behavior predicted by the two-temperature model. For heavy fermion compounds, we find that the blocking of electron-phonon scattering for heavy electrons within the density-of-states peak near the Fermi energy is crucial to explain the rapid increase of the electron-phonon relaxation time below the Kondo temperature. We propose the hypothesis that the slower Fermi velocity compared to the sound velocity provides a natural blocking mechanism due to energy and momentum conservation laws.
@article{arxiv.cond-mat/0308213,
title = {Ultrafast quasiparticle relaxation dynamics in normal metals and heavy fermion materials},
author = {K. H. Ahn and M. J. Graf and S. A. Trugman and J. Demsar and R. D. Averitt and J. L. Sarrao and A. J. Taylor},
journal= {arXiv preprint arXiv:cond-mat/0308213},
year = {2007}
}