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

Abstract

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 E2g 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 ≈ 50 K the nonthermal electron model needs to be applied. As the lattice temperature increases, the damping of the coherent E2g phonon increases, while the amplitudes of both fast electronic response and the coherent E2g phonon decrease. The temperature dependence of the damping of the E2g phonon indicates that population decay of the coherent optical phonon due to anharmonic phononphonon 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 E2g 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.

Comparing the oscillation phase in optical pump-probe spectra to ultrafast x-ray diffraction in the metal-dielectric SrRuO3/SrTiO3superlattice

“…Raman scattering with real and imaginary tensor contributions is held responsible for the excitation of coherent phonons in metallic systems (Cd, Zn, Zr) as well. 12,13 In the most intensively investigated material, the semimetal Bi, the microscopic interpretation of the excitation is developed in detail. Theory predicts the time-dependent change of the interatomic potential during the relaxation of photoexcited carriers.…”

Ultrafast near-infrared spectroscopic study of coherent phonons in the phase-separated manganiteLa1/4Pr3/8CaJang1, Lim2, Ahn3 et al. 2010Phys. Rev. B11280View full textAdd to dashboardCite

“…Probing coherent phonons or directly tracing particular lattice vibrations in strongly correlated materials enables the characterization of the relaxation dynamics caused by the excited energy carriers. 17 In particular, ultrafast studies of phase-separated manganite may provide a means of achieving optical control over the ground-state competition between the metallic and insulating phases. Generation of coherent phonons has been reported in La 1−x Ca x MnO 3 ͑Ref.…”

Excitation and Time‐Evolution of Coherent Optical Phonons

“…Raman-active phonons in Zn and Cd were studied by means of pump-probe reflectivity measurements [47]. The oscillation in the reflectivity change (Figure 10.15) was attributed to the coherent E 2g phonons.…”

“…(10.4), respectively. Reprinted with permission from[47]. Copyright 2005 by the American Physical Society.…”