45 research outputs found
Photoinduced Fano-resonance of coherent phonons in zinc
Utilizing femtosecond optical pump-probe technique, we have studied transient
Fano-resonance in zinc. At high excitation levels the Fourier spectrum of the
coherent E phonon exhibits strongly asymmetric line shape, which is well
modeled by the Fano function. The Fano parameter (1/Q) was found to be strongly
excitation fluence dependent while depending weakly on the initial lattice
temperature. We attribute the origin of the Fano-resonance to the coupling of
coherent phonon to the electronic continuum, with their transition
probabilities strongly renormalized in the vicinity of the photoinduced
structural transition.Comment: 5 pages, 3 figures, to be published in Physical Review
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
Combined investigation of collective amplitude and phase modes in a quasi-one-dimensional charge-density-wave system over a wide spectral range
We investigate experimentally both the amplitude and phase channels of the
collective modes in the quasi-1D charge-density-wave (CDW) system, K0.3MoO3, by
combining (i) optical impulsive-Raman pump-probe and (ii) terahertz time-domain
spectroscopy (THz-TDS), with high resolution and a detailed analysis of the
full complex-valued spectra in both cases. This allows an unequivocal
assignment of the observed bands to CDW modes across the THz range up to 9 THz.
We revise and extend a time-dependent Ginzburg-Landau model to account for the
observed temperature dependence of the modes, where the combination of both
amplitude and phase modes allows one to robustly determine the bare-phonon and
electron-phonon coupling parameters. While the coupling is indeed strongest for
the lowest-energy phonon, dropping sharply for the immediately subsequent
phonons, it grows back significantly for the higher-energy phonons,
demonstrating their important role in driving the CDW formation. We also
include a reassessment of our previous analysis of the lowest-lying phase
modes, whereby assuming weaker electronic damping for the phase channel results
in a qualitative picture more consistent with quantum-mechanical treatments of
the collective modes, with a strongly coupled amplitudon and phason as the
lowest modes
Highly anisotropic transient optical response of charge density wave order in ZrTe
Low dimensionality in CDW systems leads to anisotropic optical properties, in
both equilibrium and non-equilibrium conditions. Here we perform polarized
two-color pump probe measurements on a quasi-1D material ZrTe, in order to
study the anisotropic transient optical response in the CDW state. Profound
in-plane anisotropy is observed with respect to polarization of probe photons.
Below both the quasi-particle relaxation signal and amplitude
mode (AM) oscillation signal are much larger with
nearly parallel to axis () than for
parallel to axis (). This reveals that signal is
much more sensitive to the variation of the CDW gap. Interestingly, the
lifetime of the AM oscillations observed with is longer than . Moreover, at high pump
fluence where the electronic order melts and the AM oscillations vanish for
, the AM oscillatory response still
persists for . We discuss possible origins
that lead to such unusual discrepancy between the two polarizations.Comment: 6 pages, 4 figure
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 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 phonon
increases, while the amplitudes of both fast electronic response and the
coherent phonon decrease. The temperature dependence of the damping of
the 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 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
Collective modes in the charge-density wave state of KMoO: The role of long-range Coulomb interactions revisited
We re-examine the effect of long-range Coulomb interactions on the collective
amplitude and phase modes in the incommensurate charge-density wave ground
state of quasi-one-dimensional conductors. Using an effective action approach
we show that the longitudinal acoustic phonon protects the gapless linear
dispersion of the lowest phase mode in the presence of long-range Coulomb
interactions. Moreover, in Gaussian approximation amplitude fluctuations are
not affected by long-range Coulomb interactions. We also calculate the
collective mode dispersions at finite temperatures and compare our results with
the measured energies of amplitude and phase modes in KMoO. With
the exception of the lowest phase mode, the temperature dependence of the
measured mode energies can be quantitatively described within a multi-phonon
Fr\"{o}hlich model neglecting long-range Coulomb interactions
Dynamics of collective modes in an unconventional charge density wave system BaNi2As2
BaNi 2As 2 is a non-magnetic analogue of BaFe2 As2 , the parent compound of a prototype
pnictide high-temperature superconductor, displaying superconductivity already at ambient
pressure. Recent diffraction studies demonstrated the existence of two types of periodic
lattice distortions above and below the triclinic phase transition, suggesting the existence of
an unconventional charge-density-wave (CDW) order. The suppression of CDW order upon
doping results in a sixfold increase in the superconducting transition temperature and
enhanced nematic fluctuations, suggesting CDW is competing with superconductivity. Here,
we apply time-resolved optical spectroscopy to investigate collective dynamics in BaNi 2 As 2.
We demonstrate the existence of several CDW amplitude modes. Their smooth evolution
through the structural phase transition implies the commensurate CDW order in the triclinic
phase evolves from the high-temperature unidirectional incommensurate CDW, and may
indeed trigger the structural phase transition. Excitation density dependence reveals excep-
tional resilience of CDW against perturbation, implying an unconventional origin of the
underlying electronic instability
Tracking the surface atomic motion in a coherent phonon oscillation
X-ray photoelectron diffraction is a powerful tool for determining the
structure of clean and adsorbate-covered surfaces. Extending the technique into
the ultrafast time domain will open the door to studies as diverse as the
direct determination of the electron-phonon coupling strength in solids and the
mapping of atomic motion in surface chemical reactions. Here we demonstrate
time-resolved photoelectron diffraction using ultrashort soft X-ray pulses from
the free electron laser FLASH. We collect Se 3d photoelectron diffraction
patterns over a wide angular range from optically excited BiSe with a
time resolution of 140 fs. Combining these with multiple scattering simulations
allows us to track the motion of near-surface atoms within the first 3 ps after
triggering a coherent vibration of the A optical phonons. Using a
fluence of 4.2 mJ/cm from a 1.55 eV pump laser, we find the resulting
coherent vibrational amplitude in the first two interlayer spacings to be on
the order of 1 pm