449 research outputs found
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
Ultrafast Momentum Imaging of Pseudospin-Flip Excitations in Graphene
The pseudospin of Dirac electrons in graphene manifests itself in a peculiar
momentum anisotropy for photo-excited electron-hole pairs. These interband
excitations are in fact forbidden along the direction of the light
polarization, and are maximum perpendicular to it. Here, we use time- and
angle-resolved photoemission spectroscopy to investigate the resulting
unconventional hot carrier dynamics, sampling carrier distributions as a
function of energy and in-plane momentum. We first show that the
rapidly-established quasi-thermal electron distribution initially exhibits an
azimuth-dependent temperature, consistent with relaxation through collinear
electron-electron scattering. Azimuthal thermalization is found to occur only
at longer time delays, at a rate that depends on the substrate and the static
doping level. Further, we observe pronounced differences in the electron and
hole dynamics in n-doped samples. By simulating the Coulomb- and
phonon-mediated carrier dynamics we are able to disentangle the influence of
excitation fluence, screening, and doping, and develop a microscopic picture of
the carrier dynamics in photo-excited graphene. Our results clarify new aspects
of hot carrier dynamics that are unique to Dirac materials, with relevance for
photo-control experiments and optoelectronic device applications.Comment: 23 pages, 12 figure
Analgesic action of acetaminophen in symptomatic osteoarthritis of the knee
Objectives. The study was designed to investigate the analgesic effects and mechanisms of acetaminophen (paracetamol) in symptomatic osteoarthritis (OA) of the knee. Methods. Twenty patients with symptomatic OA were randomly allocated to two groups treated with either acetaminophen or rofecoxib for 3 months. Visits and measurements were scheduled upon entry (T0), at month 1 (T1) and at month 3 (T3). The intensity of joint pain was evaluated with a 100-mm visual analogue scale (VAS). The physical function of the affected knee was evaluated with a questionnaire comparable to the Western Ontario McMaster Universities Osteoarthritis Index (WOMAC). Levels of serotonin, substance P (SP) and β-endorphin (BEND) were determined with commercial enzyme-linked immunoassay kits. The expression of κ opioid receptor (KOR) in peripheral mononuclear blood cells (PBMCs) was quantified by real-time PCR. Results. Both acetaminophen and rofecoxib relieved pain considerably but with different kinetics, and affected different biomarkers. Rofecoxib appeared to be more efficient, reducing pain intensity by 56% at T1 (P<0.01), whereas acetaminophen reduced it by only 29%. Physical function improved in both groups by T3. Correlated with the pain relief, acetaminophen significantly reduced plasma BEND levels, whereas rofecoxib did not do so. In both groups plasma SP levels were elevated compared with T0. A reduction in serum serotonin was detected in the rofecoxib group at T1 (P=0.004) but had recovered at T3. No changes in KOR mRNA in PBMCs were observed in either group. Conclusions. There is a correlation between reduction in circulating BEND and OA pain relief in patients treated with acetaminophe
Epitaxial film growth and magnetic properties of Co_2FeSi
We have grown thin films of the Heusler compound Co_2FeSi by RF magnetron
sputtering. On (100)-oriented MgO substrates we find fully epitaxial
(100)-oriented and L2_1 ordered growth. On Al_2O_3 (11-20) substrates, the film
growth is (110)-oriented, and several in-plane epitaxial domains are observed.
The temperature dependence of the electrical resistivity shows a power law with
an exponent of 7/2 at low temperatures. Investigation of the bulk magnetic
properties reveals an extrapolated saturation magnetization of 5.0 mu_B/fu at 0
K. The films on Al_2O_3 show an in-plane uniaxial anisotropy, while the
epitaxial films are magnetically isotropic in the plane. Measurements of the
X-ray magnetic circular dichroism of the films allowed us to determine element
specific magnetic moments. Finally we have measured the spin polarization at
the surface region by spin-resolved near-threshold photoemission and found it
strongly reduced in contrast to the expected bulk value of 100%. Possible
reasons for the reduced magnetization are discussed.Comment: 9 pages, 12 figure
Analgesic action of acetaminophen in symptomatic osteoarthritis of the knee
Objectives. The study was designed to investigate the analgesic effects and mechanisms of acetaminophen (paracetamol) in symptomatic osteoarthritis (OA) of the knee. Methods. Twenty patients with symptomatic OA were randomly allocated to two groups treated with either acetaminophen or rofecoxib for 3 months. Visits and measurements were scheduled upon entry (T0), at month 1 (T1) and at month 3 (T3). The intensity of joint pain was evaluated with a 100-mm visual analogue scale (VAS). The physical function of the affected knee was evaluated with a questionnaire comparable to the Western Ontario McMaster Universities Osteoarthritis Index (WOMAC). Levels of serotonin, substance P (SP) and β-endorphin (BEND) were determined with commercial enzyme-linked immunoassay kits. The expression of κ opioid receptor (KOR) in peripheral mononuclear blood cells (PBMCs) was quantified by real-time PCR. Results. Both acetaminophen and rofecoxib relieved pain considerably but with different kinetics, and affected different biomarkers. Rofecoxib appeared to be more efficient, reducing pain intensity by 56% at T1 (P<0.01), whereas acetaminophen reduced it by only 29%. Physical function improved in both groups by T3. Correlated with the pain relief, acetaminophen significantly reduced plasma BEND levels, whereas rofecoxib did not do so. In both groups plasma SP levels were elevated compared with T0. A reduction in serum serotonin was detected in the rofecoxib group at T1 (P=0.004) but had recovered at T3. No changes in KOR mRNA in PBMCs were observed in either group. Conclusions. There is a correlation between reduction in circulating BEND and OA pain relief in patients treated with acetaminophe
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
Floquet states in dissipative open quantum systems
We theoretically investigate basic properties of nonequilibrium steady states
of periodically-driven open quantum systems based on the full solution of the
Maxwell-Bloch equation. In a resonantly driving condition, we find that the
transverse relaxation, also known as decoherence, significantly destructs the
formation of Floquet states while the longitudinal relaxation does not directly
affect it. Furthermore, by evaluating the quasienergy spectrum of the
nonequilibrium steady states, we demonstrate that the Rabi splitting can be
observed as long as the decoherence time is as short as one third of the
Rabi-cycle. Moreover, we find that Floquet states can be formed even under
significant dissipation when the decoherence time is substantially shorter than
the cycle of driving, once the driving field strength becomes strong enough. In
an off-resonant condition, we demonstrate that the Floquet states can be
realized even in weak field regimes because the system is not excited and the
decoherence mechanism is not activated. Once the field strength becomes strong
enough, the system can be excited by nonlinear processes and the decoherence
process becomes active. As a result, the Floquet states are significantly
disturbed by the environment even in the off-resonant condition. Thus, we show
here that the suppression of heating is a key condition for the realization of
Floquet states in both on and off-resonant conditions not only because it
prevents material damage but also because it contributes to preserving
coherence
Floquet states in dissipative open quantum systems
We theoretically investigate basic properties of nonequilibrium steady states of periodically-driven open quantum systems based on the full solution of the Maxwell–Bloch equation. In a resonant driving condition, we find that the transverse relaxation, also known as decoherence, significantly destructs the formation of Floquet states while the longitudinal relaxation does not directly affect it. Furthermore, by evaluating the quasienergy spectrum of the nonequilibrium steady states, we demonstrate that Rabi splitting can be observed as long as the decoherence time is as short as one third of the Rabi-cycle. Moreover, we find that Floquet states can be formed even under significant dissipation when the decoherence time is substantially shorter than the cycle of driving, once the driving field strength becomes strong enough. In an off-resonant condition, we demonstrate that the Floquet states can be realized even in weak field regimes because the system is not excited and the decoherence mechanism is not activated. Once the field strength becomes strong enough, the system can be excited by multi-photon absorption and the decoherence process becomes active. As a result, the Floquet states are significantly disturbed by the environment even in the off-resonant condition. Thus, we show here that the suppression of energy transfer from light to matter is a key condition for the realization of Floquet states in both on- and off-resonant conditions not only because it prevents material damage but also because it contributes to preserving coherence
- …