Quasiparticle Relaxation Dynamics in Heavy Fermion Compounds

We present the first femtosecond studies of electron-phonon ($e$-ph) thermalization in heavy-fermion compounds. The $e$-ph thermalization time ${\tau }_{ep}$ increases below the Kondo temperature by more than 2 orders of magnitude as $T=0\mathrm{K}$ is approached. Analysis using the two-temperature model and numerical simulations based on Boltzmann’s equations suggest that this anomalous slowing down of the $e$-ph thermalization derives from the large electronic specific heat and the suppression of scattering between heavy electrons and phonons.

Figure 1

Normalized PI reflectivity data (solid symbols) on (a) ${\mathrm{L}\mathrm{u}\mathrm{A}\mathrm{g}\mathrm{C}\mathrm{u}}_4$ and (b) ${\mathrm{Y}\mathrm{b}\mathrm{A}\mathrm{g}\mathrm{C}\mathrm{u}}_4$ at various $T$, together with best fits (see text) to the data—solid lines. The data have been vertically shifted for clarity.

Figure 2

(a) $T$ dependence of relaxation time ${\tau }_r$ on ${\mathrm{L}\mathrm{u}\mathrm{A}\mathrm{g}\mathrm{C}\mathrm{u}}_4$ (solid circles), together with the TTM prediction (dashed line) and the result of the numerical simulation (open circles) including the nonthermal electron distribution. Inset: $C_e$ (dashed line) and $C_l$ (solid line) of ${\mathrm{L}\mathrm{u}\mathrm{A}\mathrm{g}\mathrm{C}\mathrm{u}}_4$.

Figure 3

$T$ dependence of ${\tau }_r$ on ${\mathrm{Y}\mathrm{b}\mathrm{A}\mathrm{g}\mathrm{C}\mathrm{u}}_4$ (solid circles), together with the corresponding ${\tau }_{ep}$ (multiplied by 10 for presentation purposes) from simple TTM prediction (dashed line). Assuming suppressed scattering of heavy electrons by phonons due to $v_F<v_s$ condition, very good agreement with the data is obtained: open circles present the result of numerical simulation, while solid line presents the TTM simulation—see text. Inset: experimentally determined $C_e(T)$ for ${\mathrm{Y}\mathrm{b}\mathrm{A}\mathrm{g}\mathrm{C}\mathrm{u}}_4$ (open circles), together with calculated $C_e(T)$ based on the model $D_e(ϵ)$—solid line. $C_e(T)$ of ${\mathrm{L}\mathrm{u}\mathrm{A}\mathrm{g}\mathrm{C}\mathrm{u}}_4$ (dashed line) is added for comparison.