369 research outputs found
Spin- and time-resolved photoemission studies of thin Co2FeSi Heusler alloy films
We have studied the possibly half metallic Co2FeSi full Heusler alloy by
means of spin- and time-resolved photoemission spectroscopy. For excitation,
the second and fourth harmonic of femtosecond Ti:sapphire lasers were used,
with photon energies of 3.1 eV and 5.9 eV, respectively. We compare the
dependence of the measured surface spin polarization on the particular
photoemission mechanism, i.e. 1-photon-photoemission (1PPE) or 2-photon
photoemission (2PPE). The observed differences in the spin polarization can be
explained by a spin-dependent lifetime effect occurring in the 2-photon
absorption process. The difference in escape depth of the two methods in this
context suggests that the observed reduction of spin polarization (compared to
the bulk) cannot be attributed just to the outermost surface layer but takes
place at least 4-6 nm away from the surface.Comment: 7 pages, 3 figures; submitted to Journal of Magnetism and Magnetic
Material
Experimental time-resolved photoemission and ab initio study of lifetimes of excited electrons in Mo and Rh
We have studied the relaxation dynamics of optically excited electrons in
molybdenum and rhodium by means of time resolved two-photon photoemission
spectroscopy (TR-2PPE) and ab initio electron self-energy calculations
performed within the GW and GW+T approximations. Both theoretical approaches
reproduce qualitatively the experimentally observed trends and differences in
the lifetimes of excited electrons in molybdenum and rhodium. For excitation
energies exceeding the Fermi energy by more than 1 eV, the GW+T theory yields
lifetimes in quantitative agreement with the experimental results. As one of
the relevant mechanisms causing different excited state lifetime in Mo and Rh
we identify the occupation of the 4d bands. An increasing occupation of the 4d
bands results in an efficient decrease of the lifetime even for rather small
excitation energies of a few 100 meV.Comment: 8 pages, 10 figure
Love kills: Simulations in Penna Ageing Model
The standard Penna ageing model with sexual reproduction is enlarged by
adding additional bit-strings for love: Marriage happens only if the male love
strings are sufficiently different from the female ones. We simulate at what
level of required difference the population dies out.Comment: 14 pages, including numerous figure
Deterministic spatio-temporal control of nano-optical fields in optical antennas and nano transmission lines
We show that pulse shaping techniques can be applied to tailor the ultrafast
temporal response of the strongly confined and enhanced optical near fields in
the feed gap of resonant optical antennas (ROAs). Using finite-difference
time-domain (FDTD) simulations followed by Fourier transformation, we obtain
the impulse response of a nano structure in the frequency domain, which allows
obtaining its temporal response to any arbitrary pulse shape. We apply the
method to achieve deterministic optimal temporal field compression in ROAs with
reduced symmetry and in a two-wire transmission line connected to a symmetric
dipole antenna. The method described here will be of importance for experiments
involving coherent control of field propagation in nanophotonic structures and
of light-induced processes in nanometer scale volumes.Comment: 5 pages, 5 figure
Spin-flip processes and ultrafast magnetization dynamics in Co - unifying the microscopic and macroscopic view of femtosecond magnetism
The femtosecond magnetization dynamics of a thin cobalt film excited with
ultrashort laser pulses has been studied using two complementary pump-probe
techniques, namely spin-, energy- and time-resolved photoemission and
time-resolved magneto-optical Kerr effect. Combining the two methods it is
possible to identify the microscopic electron spin-flip mechanisms responsible
for the ultrafast macroscopic magnetization dynamics of the cobalt film. In
particular, we show that electron-magnon excitation does not affect the overall
magnetization even though it is an efficient spin-flip channel on the sub-200
fs timescale. Instead we find experimental evidence for the relevance of
Elliott-Yafet type spin-flip processes for the ultrafast demagnetization taking
place on a time scale of 300 fs.Comment: 12 pages, 3 figures; accepted by Physical Review Letter
Dynamics of Excited Electrons in Copper and Ferromagnetic Transition Metals: Theory and Experiment
Both theoretical and experimental results for the dynamics of photoexcited
electrons at surfaces of Cu and the ferromagnetic transition metals Fe, Co, and
Ni are presented. A model for the dynamics of excited electrons is developed,
which is based on the Boltzmann equation and includes effects of
photoexcitation, electron-electron scattering, secondary electrons (cascade and
Auger electrons), and transport of excited carriers out of the detection
region. From this we determine the time-resolved two-photon photoemission
(TR-2PPE). Thus a direct comparison of calculated relaxation times with
experimental results by means of TR-2PPE becomes possible. The comparison
indicates that the magnitudes of the spin-averaged relaxation time \tau and of
the ratio \tau_\uparrow/\tau_\downarrow of majority and minority relaxation
times for the different ferromagnetic transition metals result not only from
density-of-states effects, but also from different Coulomb matrix elements M.
Taking M_Fe > M_Cu > M_Ni = M_Co we get reasonable agreement with experiments.Comment: 23 pages, 11 figures, added a figure and an appendix, updated
reference
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
Spin Motion in Electron Transmission through Ultrathin Ferromagnetic Films Accessed by Photoelectron Spectroscopy
Ab initio and model calculations demonstrate that the spin motion of
electrons transmitted through ferromagnetic films can be analyzed in detail by
means of angle- and spin-resolved core-level photoelectron spectroscopy. The
spin motion appears as precession of the photoelectron spin polarization around
and as relaxation towards the magnetization direction. In a systematic study
for ultrathin Fe films on Pd(001) we elucidate its dependence on the Fe film
thickness and on the Fe electronic structure. In addition to elastic and
inelastic scattering, the effect of band gaps on the spin motion is addressed
in particular.Comment: 4 pages, 5 figure
Energy-resolved electron-spin dynamics at surfaces of p-doped GaAs
Electron-spin relaxation at different surfaces of p-doped GaAs is
investigated by means of spin, time and energy resolved 2-photon photoemission.
These results are contrasted with bulk results obtained by time-resolved
Faraday rotation measurements as well as calculations of the Bir-Aronov-Pikus
spin-flip mechanism. Due to the reduced hole density in the band bending region
at the (100) surface the spin-relaxation time increases over two orders of
magnitude towards lower energies. At the flat-band (011) surface a constant
spin relaxation time in agreement with our measurements and calculations for
bulk GaAs is obtained.Comment: 6 pages, 4 figure
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