Dynamics of photoexcited quasiparticles in heavy electron compounds

Abstract

Femtosecond real-time spectroscopy is an emerging new tool for studying low energyelectronic structure in correlated electron systems. Motivated by recent advances inunderstanding the nature of relaxation phenomena in various correlated electron systems(superconductors, density wave systems) the technique has been applied to heavy electroncompounds in comparison with their non-magnetic counterparts. While the dynamics intheir non-magnetic analogues are similar to the dynamics observed in noble metals (onlyweak temperature dependences are observed) and can be treated with a simpletwo-temperature model, the photoexcited carrier dynamics in heavy electron systems showdramatic changes as a function of temperature and excitation level. In particular, belowsome characteristic temperature the relaxation rate starts to decrease, dropping by morethan two orders of magnitude upon cooling down to liquid He temperatures. Thisbehaviour has been consistently observed in various heavy fermion metals as well as Kondoinsulators, and is believed to be quite general. In order to account for the experimentalobservations, two theoretical models have been proposed. The first treats theheavy electron systems as simple metals with very flat electron dispersion near theFermi level. An electron–phonon thermalization scenario can account for theobserved slowing down of the relaxation provided that there exists a mechanism forsuppression of electron–phonon scattering when both the initial and final electronicstates lie in the region of flat dispersion. An alternative scenario argues that therelaxation dynamics in heavy electron systems are governed by the Rothwarf–Taylorbottleneck, where the dynamics are governed by the presence of a narrow gapin the density of states near the Fermi level. The so-called hybridization gapresults from hybridization between localized moments and the conduction electronbackground. Remarkable agreement with the model suggests that carrier relaxation in abroad class of heavy electron systems (both metals and insulators) is governed bythe presence of a (weakly temperature dependent) indirect hybridization gap.Here we review the experimental results on a variety of heavy electron compounds, pointout the common features as well as the peculiarities observed in some compounds, andcompare the data with existing theoretical models.