21 research outputs found
Ultrafast electron dynamics in metals
During the last decade, significant progress has been achieved in the rapidly
growing field of the dynamics of {\it hot} carriers in metals. Here we present
an overview of the recent achievements in the theoretical understanding of
electron dynamics in metals, and focus on the theoretical description of the
inelastic lifetime of excited hot electrons. We outline theoretical
formulations of the hot-electron lifetime that is originated in the inelastic
scattering of the excited {\it quasiparticle} with occupied states below the
Fermi level of the solid. {\it First-principles} many-body calculations are
reviewed. Related work and future directions are also addressed.Comment: 17 pages, two columns, 13 figures, to appear in ChemPhysChe
Calculated lifetimes of hot electrons in aluminum and copper using a plane-wave basis set
We report about the lifetimes of hot electrons in crystalline aluminum and copper. For aluminum the results agree quantitatively with the experimental results. For copper we get good agreement for quasiparticle energies in the (110) direction above 2 eV which shows that the lifetimes for quasiparticle states above 2 eV are determined by sp bands, explaining the puzzling fact that simple Fermi liquid theory describes Cu in this direction quite well. The calculations were performed within the shielded interaction approximation using a plane-wave basis expansion for the wave functions. We show that for Cu this basis leads to equally good results as the more demanding linearized augmented plane-wave basis
Dynamics of Excited Electrons in Copper: Role of Auger Electrons
Within a theoretical model based on the Boltzmann equation, we analyze in
detail the structure of the unusual peak recently observed in the relaxation
time in Cu. In particular, we discuss the role of Auger electrons in the
electron dynamics and its dependence on the d-hole lifetime, the optical
transition matrix elements and the laser pulse duration. We find that the Auger
contribution to the distribution is very sensitive to both the d-hole lifetime
tau_h and the laser pulse duration tau_l and can be expressed as a monotonic
function of tau_l/tau_h. We have found that for a given tau_h, the Auger
contribution is significantly smaller for a short pulse duration than for a
longer one. We show that the relaxation time at the peak depends linearly on
the d-hole lifetime, but interestingly not on the amount of Auger electrons
generated. We provide a simple expression for the relaxation time of excited
electrons which shows that its shape can be understood by a phase space
argument and its amplitude is governed by the d-hole lifetime. We also find
that the height of the peak depends on both the ratio of the optical transition
matrix elements R=|M_{d \to sp}|^2/|M_{sp \to sp}|^2 and the laser pulse
duration. Assuming a reasonable value for the ratio, namely R = 2, and a d-hole
lifetime of tau_h=35 fs, we obtain for the calculated height of the peak Delta
tau_{th}=14 fs, in fair agreement with Delta tau_{exp} \approx 17 fs measured
for polycrystalline Cu.Comment: 6 pages, 6 figure
Time-dependent screening of a positive charge distribution in metals: Excitons on an ultra-short time scale
Experiments determining the lifetime of excited electrons in crystalline
copper reveal states which cannot be interpreted as Bloch states [S. Ogawa {\it
et al.}, Phys. Rev. B {\bf 55}, 10869 (1997)]. In this article we propose a
model which explains these states as transient excitonic states in metals. The
physical background of transient excitons is the finite time a system needs to
react to an external perturbation, in other words, the time which is needed to
build up a polarization cloud. This process can be probed with modern
ultra-short laser pulses. We calculate the time-dependent density-response
function within the jellium model and for real Cu. From this knowledge it is
possible within linear response theory to calculate the time needed to screen a
positive charge distribution and -- on top of this -- to determine excitonic
binding energies. Our results lead to the interpretation of the experimentally
detected states as transient excitonic states.Comment: 24 pages, 9 figures, to appear in Phys. Rev. B, Nov. 15, 2000, issue
2
Lifetime of d-holes at Cu surfaces: Theory and experiment
We have investigated the hole dynamics at copper surfaces by high-resolution
angle-resolved photoemission experiments and many-body quasiparticle GW
calculations. Large deviations from a free-electron-like picture are observed
both in the magnitude and the energy dependence of the lifetimes, with a clear
indication that holes exhibit longer lifetimes than electrons with the same
excitation energy. Our calculations show that the small overlap of d- and
sp-states below the Fermi level is responsible for the observed enhancement.
Although there is qualitative good agreement of our theoretical predictions and
the measured lifetimes, there still exist some discrepancies pointing to the
need of a better description of the actual band structure of the solid.Comment: 15 pages, 7 figures, 1 table, to appear in Phys. Rev.
Hole dynamics in noble metals
We present a detailed analysis of hole dynamics in noble metals (Cu and Au),
by means of first-principles many-body calculations. While holes in a
free-electron gas are known to live shorter than electrons with the same
excitation energy, our results indicate that d-holes in noble metals exhibit
longer inelastic lifetimes than excited sp-electrons, in agreement with
experiment. The density of states available for d-hole decay is larger than
that for the decay of excited electrons; however, the small overlap between d-
and sp-states below the Fermi level increases the d-hole lifetime. The impact
of d-hole dynamics on electron-hole correlation effects, which are of relevance
in the analysis of time-resolved two-photon photoemission experiments, is also
addressed.Comment: 4 pages, 2 figures, to appear in Phys. Rev. Let
Anomalous Quasiparticle Lifetime in Graphite: Band Structure Effects
We report ab initio calculation of quasiparticle lifetimes in graphite, as
determined from the imaginary part of the self-energy operator within the GW
aproximation. The inverse lifetime in the energy range from 0.5 to 3.5 eV above
the Fermi level presents significant deviations from the quadratic behavior
naively expected from Fermi liquid theory. The deviations are explained in
terms of the unique features of the band structure of this material. We also
discuss the experimental results from different groups and make some
predictions for future experiments.Comment: 4 pages, 4 figures, submitted PR
Response theory for time-resolved second-harmonic generation and two-photon photoemission
A unified response theory for the time-resolved nonlinear light generation
and two-photon photoemission (2PPE) from metal surfaces is presented. The
theory allows to describe the dependence of the nonlinear optical response and
the photoelectron yield, respectively, on the time dependence of the exciting
light field. Quantum-mechanical interference effects affect the results
significantly. Contributions to 2PPE due to the optical nonlinearity of the
surface region are derived and shown to be relevant close to a plasmon
resonance. The interplay between pulse shape, relaxation times of excited
electrons, and band structure is analyzed directly in the time domain. While
our theory works for arbitrary pulse shapes, we mainly focus on the case of two
pulses of the same mean frequency. Difficulties in extracting relaxation rates
from pump-probe experiments are discussed, for example due to the effect of
detuning of intermediate states on the interference. The theory also allows to
determine the range of validity of the optical Bloch equations and of
semiclassical rate equations, respectively. Finally, we discuss how collective
plasma excitations affect the nonlinear optical response and 2PPE.Comment: 27 pages, including 11 figures, version as publishe
Lifetime of surface states in optically excited Al, Cu, and Ag
In this Letter we report about ab initio calculations of the lifetime of excited electrons in Surface states of Al, Cu, and Ag. We show that at the ab initio level the decay of surface states is very much similar to that of bulk states, because in both cases it is the density of the final bulk states which matters. This shows by way of example that most probably the popular jellium-like models will fail in predicting accurate lifetimes and that the ab initio treatment of surface states is a must
The determination of the lifetime of hot electrons in metals by time-resolved two-photon photoemission: the role of transport effects, virtual states, and transient excitons
Experience has shown that theoretically determined lifetimes of bulk states of hot electrons in real metals agree quantitatively with the experimental ones, if theory fully takes into account the crystal structure and many-body effects of the investigated metal, i.e., if the Dyson equation is solved at the ab initio level and the effective electron– electron interaction is determined beyond the plasmon-pole approximation. Therefore the hitherto invoked transport effect [Knoesel et al.: Phys. Rev. B 57, 12 812 (1998)] does not seem to exist. In this paper we show that likewise neither virtual states [Hertel: et al. Phys. Rev. Lett. 76, 535 (1996)] nor damped band-gap states [Ogawa: et al.: Phys. Rev. B 55, 10 869 (1997)] exist, but that the hitherto unexplained d-band catastrophe in Cu [Cu(111), Cu(110)] can be naturally resolved by the concept of the transient exciton. This is a new quasiparticle in metals, which owes its existence to the dynamical character of dielectric screening at the microscopic level. This means that excitons, though they do not exist under stationary conditions, can be observed under ultrafast experimental conditions