Photoexcited electron dynamics in Kondo insulators and heavy fermions

We have studied the photoexcited carrier relaxation dynamics in the Kondo insulator SmB6 and the heavy fermion metal YbAgCu4 as a function of temperature and excitation level. The dynamic response is found to be both strongly temperature dependent and nonlinear. The data are analyzed with a Rothwarf-Taylor bottleneck model, where the dynamics are governed by the presence of a narrow gap in the density of states near the Fermi level. The remarkable agreement with the model suggests that carrier relaxation in a broad class of heavy electron systems (both metals and insulators) is governed by the presence of a (weakly temperature dependent) hybridization gap.Comment: accepted for publication in Physical Review Letter

Ultrafast dynamics of coherent optical phonons and nonequilibrium electrons in transition metals

The femtosecond optical pump-probe technique was used to study dynamics of photoexcited electrons and coherent optical phonons in transition metals Zn and Cd as a function of temperature and excitation level. The optical response in time domain is well fitted by linear combination of a damped harmonic oscillation because of excitation of coherent $E_{2g}$ phonon and a subpicosecond transient response due to electron-phonon thermalization. The electron-phonon thermalization time monotonically increases with temperature, consistent with the thermomodulation scenario, where at high temperatures the system can be well explained by the two-temperature model, while below $\approx$ 50 K the nonthermal electron model needs to be applied. As the lattice temperature increases, the damping of the coherent $E_{2g}$ phonon increases, while the amplitudes of both fast electronic response and the coherent $E_{2g}$ phonon decrease. The temperature dependence of the damping of the $E_{2g}$ phonon indicates that population decay of the coherent optical phonon due to anharmonic phonon-phonon coupling dominates the decay process. We present a model that accounts for the observed temperature dependence of the amplitude assuming the photoinduced absorption mechanism, where the signal amplitude is proportional to the photoinduced change in the quasiparticle density. The result that the amplitude of the $E_{2g}$ phonon follows the temperature dependence of the amplitude of the fast electronic transient indicates that under the resonant condition both electronic and phononic responses are proportional to the change in the dielectric function.Comment: 10 pages, 9 figures, to appear in Physical Review

Ultrafast electron dynamics in silver nanoparticle arrays

We present the results of ultrafast optical experiments on silver quantum dot superlattices. Dramatic changes in the electron dynamics occur as a function of interparticle spacing due to enhanced dipolar coupling and, most importantly, electron delocalization

Far-infrared carrier dynamics in superconducting MgB[sub 2]

We have performed optical-pump terahertz-probe measurements in transmission on MgB{sub 2} thin films in the superconducting state. The initial optical perturbation of the superconducting condensate and subsequent pair recovery display a strong fluence dependence. In conclusion, we have performed the first time-resolved far-infrared studies on the new superconductor MgB{sub 2}. We have observed a strongly fluence dependent pair recovery time that, at the lowest fluences used, is approximately 500 ps. In addition, at the lower excitation fluences where superconductivity is not destroyed, a risetime in the dynamics is observed that may be due to photoinduced gap fluctuations

Hot electron relaxation in the heavy-fermion Yb1−xLuxAl3 compound using femtosecond optical pump-probe spectroscopy

Femtosecond time-resolved optical spectroscopy was used to systematically study photoexcited carrier relaxation dynamics in the intermediate-valence heavy-fermion system Yb1-xLuxAl3 (0x1). Given the demonstrated sensitivity of this experimental technique to the presence of the low-energy gaps in the charge excitation spectrum, the aim of this work was to study the effect of dilution of the Kondo lattice on its low-energy electronic structure. The results imply that in Yb1-xLuxAl3 the hybridization gap, resulting from hybridization of local moments and conduction electrons, persists up to 30% doping. Interestingly, below some characteristic, doping dependent temperature T*(x) the relaxation-time divergence, governed by the relaxation bottleneck due to the presence of the indirect hybridization gap, is truncated. This observation is attributed to the competing ballistic transport of hot electrons out of the probed volume at low temperatures. The derived theoretical model accounts for both the functional form of relaxation dynamics below T*(x), as well as the doping dependence of the low-temperature relaxation rate in Yb1-xLuxAl3

Snapshots of cooperative atomic motions in the optical suppression of charge density waves

Macroscopic quantum phenomena such as high-temperature superconductivity, colossal magnetoresistance, ferrimagnetism and ferromagnetism arise from a delicate balance of different interactions among electrons, phonons and spins on the nanoscale(1). The study of the interplay among these various degrees of freedom in strongly coupled electron-lattice systems is thus crucial to their understanding and for optimizing their properties. Charge-density-wave (CDW) materials(2), with their inherent modulation of the electron density and associated periodic lattice distortion, represent ideal model systems for the study of such highly cooperative phenomena. With femtosecond time-resolved techniques, it is possible to observe these interactions directly by abruptly perturbing the electronic distribution while keeping track of energy relaxation pathways and coupling strengths among the different subsystems(3-7). Numerous time-resolved experiments have been performed on CDWs(8-13), probing the dynamics of the electronic subsystem. However, the dynamics of the periodic lattice distortion have been only indirectly inferred(14). Here we provide direct atomic-level information on the structural dynamics by using femtosecond electron diffraction(15) to study the quasi two-dimensional CDW system 1T-TaS2. Effectively, we have directly observed the atomic motions that result from the optically induced change in the electronic spatial distribution. The periodic lattice distortion, which has an amplitude of similar to 0.1 angstrom, is suppressed by about 20% on a timescale (similar to 250 femtoseconds) comparable to half the period of the corresponding collective mode. These highly cooperative, electronically driven atomic motions are accompanied by a rapid electron-phonon energy transfer (similar to 350 femtoseconds) and are followed by fast recovery of the CDW (similar to 4 picoseconds). The degree of cooperativity in the observed structural dynamics is remarkable and illustrates the importance of obtaining atomic-level perspectives of the processes directing the physics of strongly correlated systems