688 research outputs found
Coupling of spin and vibrational degrees of freedom of adsorbates at metal surfaces probed by vibrational sum-frequency generation
Vibrational spectroscopy using sum-frequency generation has been used to
investigate the coupling between a ferromagnetic thin film and adsorbed
molecules, here CO on Ni/Cu(100). The CO stretching vibration exhibits a
strong magnetic contrast with a pronounced temperature dependence, underlining
the high sensitivity of this adsorbate-specific spectroscopy method. Our
results indicate that the strong temperature dependence is caused by dynamical
changes in the surface chemical bond when the CO stretch vibration is coupled
to thermally excited external vibrational modes
Ultra-cold atoms in an optical cavity: two-mode laser locking to the cavity avoiding radiation pressure
The combination of ultra-cold atomic clouds with the light fields of optical
cavities provides a powerful model system for the development of new types of
laser cooling and for studying cooperative phenomena. These experiments
critically depend on the precise tuning of an incident pump laser with respect
to a cavity resonance. Here, we present a simple and reliable experimental
tuning scheme based on a two-mode laser spectrometer. The scheme uses a first
laser for probing higher-order transversal modes of the cavity having an
intensity minimum near the cavity's optical axis, where the atoms are confined
by a magnetic trap. In this way the cavity resonance is observed without
exposing the atoms to unwanted radiation pressure. A second laser, which is
phase-locked to the first one and tuned close to a fundamental cavity mode
drives the coherent atom-field dynamics.Comment: 7 pages, 7 figure
Excited-state band mapping and momentum-resolved ultrafast population dynamics in In/Si(111) nanowires investigated with XUV-based time- and angle-resolved photoemission spectroscopy
We investigate the excited state electronic structure of the model phase transition system In/Si(111) using femtosecond time- and angle-resolved photoemission spectroscopy (trARPES). An extreme ultraviolet 500 kHz laser source at 21.7 eV is utilized both to map the energy of excited states above the Fermi level and follow the momentum-resolved population dynamics on a femtosecond timescale. Excited-state band mapping is used to characterize the normally unoccupied electronic structure above the Fermi level in both structural phases of In/Si(111): the metallic (4 x 1) and the gapped (8 x 2) phases. The extracted band positions are compared withband- structure calculations utilizing density functional theory within both the local density approximation and GW approximations (single-particle Green's function (G) + screened Coulomb interaction (W)). While good overall agreement is found between the GW-calculated band structure and experiment, deviations in specific momentum regions may indicate the importance of excitonic effects not accounted for at this level of approximation. To probe the dynamics of these excited states, their momentum- resolved transient population dynamics are extracted with trARPES. The transient intensities are compared to a simulated spectral function modeled by a state population employing a transient elevated electronic temperature as determined experimentally. This allows the momentum-resolved population dynamics to be quantitatively reproduced, revealing important insights into the transfer of energy from the electronic system to the lattice. In particular, a comparison between the magnitude and relaxation time of the transient electronic temperature observed by trARPES with those of the lattice as probed in previous ultrafast electron diffraction studies implies a highly nonthermal phonon distribution at the surface following photo-excitation. This suggests that the energy from the initially excited electronic system is initially transferred to high-energy optical phonon modes followed by cooling and thermalization of the photo-excited system by much slower phonon-phonon coupling
Reference Values of the QOLIBRI from General Population Samples in the United Kingdom and The Netherlands
The Quality of Life after Traumatic Brain Injury (QOLIBRI) instrument is an internationally validated patient-reported outcome measure for assessing disease-specific health-related quality of life (HRQoL) in individuals after traumatic brain injury (TBI). However, no reference values for general populations are available yet for use in clinical practice and research in the field of TBI. The aim of the present study was, therefore, to establish these reference values for the United Kingdom (UK) and the Netherlands (NL). For this purpose, an online survey with a reworded version of the QOLIBRI for general populations was used to collect data on 4403 individuals in the UK and 3399 in the NL. This QOLIBRI version was validated by inspecting descriptive statistics, psychometric criteria, and comparability of the translations to the original version. In particular, measurement invariance (MI) was tested to examine whether the items of the instrument were understood in the same way by different individuals in the general population samples and in the TBI sample across the two countries, which is necessary in order to establish reference values. In the general population samples, the reworded QOLIBRI displayed good psychometric properties, including MI across countries and in the non-TBI and TBI samples. Therefore, differences in the QOLIBRI scores can be attributed to real differences in HRQoL. Individuals with and without a chronic health condition did differ significantly, with the latter reporting lower HRQoL. In conclusion, we provided reference values for healthy individuals and individuals with at least one chronic condition from general population samples in the UK and the NL. These can be used in the interpretation of disease-specific HRQoL assessments after TBI applying the QOLIBRI on the individual level in clinical as well as research contexts
Search for Charginos with a Small Mass Difference with the Lightest Supersymmetric Particle at \sqrt{s} = 189 GeV
A search for charginos nearly mass-degenerate with the lightest
supersymmetric particle is performed using the 176 pb^-1 of data collected at
189 GeV in 1998 with the L3 detector. Mass differences between the chargino and
the lightest supersymmetric particle below 4 GeV are considered. The presence
of a high transverse momentum photon is required to single out the signal from
the photon-photon interaction background. No evidence for charginos is found
and upper limits on the cross section for chargino pair production are set. For
the first time, in the case of heavy scalar leptons, chargino mass limits are
obtained for any \tilde{\chi}^{+-}_1 - \tilde{\chi}^0_1 mass difference
Formation of the in Two-Photon Collisions at LEP
The two-photon width of the meson has been
measured with the L3 detector at LEP. The is studied in the decay
modes , KK, KK,
KK, , , and
using an integrated luminosity of 140 pb at GeV and
of 52 pb at GeV. The result is
(BR) keV. The dependence of the cross section is studied for
GeV. It is found to be better described by a Vector Meson
Dominance model form factor with a J-pole than with a -pole. In addition,
a signal of events is observed at the mass. Upper limits
for the two-photon widths of the , , and are also
given
Search for Scalar Leptons in e+e- collisions at \sqrt{s}=189 GeV
We report the result of a search for scalar leptons in e+e- collisions at 189
GeV centre-of-mass energy at LEP. No evidence for such particles is found in a
data sample of 176 pb^{-1}. Improved upper limits are set on the production
cross sections for these new particles. New exclusion contours in the parameter
space of the Minimal Supersymmetric Standard Model are derived, as well as new
lower limits on the masses of these supersymmetric particles. Under the
assumptions of common gaugino and scalar masses at the GUT scale, we set an
absolute lower limit on the mass of the lightest scalar electron of 65.5 Ge
K0s K0s Final State in Two-Photon Collisions and Implications for Glueballs
The K0s K0s final state in two-photon collisions is studied with the L3
detector at LEP. The mass spectrum is dominated by the formation of the
f_2'(1525) tensor meson in the helicity-two state with a two-photon width times
the branching ratio into K Kbar of 76 +- 6 +- 11 eV. A clear signal for the
formation of the f_J(1710) is observed and it is found to be dominated by the
spin-two helicity-two state. No resonance is observed in the mass region around
2.2 GeV and an upper limit of 1.4 eV at 95% C.L. is derived for the two-photon
width times the branching ratio into K0s K0s for the glueball candidate
xi(2230)
Bose-Einstein Correlations of Neutral and Charged Pions in Hadronic Z Decays
Bose-Einstein correlations of both neutral and like-sign charged pion pairs
are measured in a sample of 2 million hadronic Z decays collected with the L3
detector at LEP. The analysis is performed in the four-momentum difference
range 300 MeV < Q < 2 GeV. The radius of the neutral pion source is found to be
smaller than that of charged pions. This result is in qualitative agreement
with the string fragmentation model
Direct Observation of Longitudinally Polarised W Bosons
The three different helicity states of W bosons, produced in the reaction
e+e- -> W+W- -> l nu q q~ are studied using leptonic and hadronic W decays at
sqrt{s}=183GeV and 189GeV. The W polarisation is also measured as a function of
the scattering angle between the W- and the direction of the e- beam. The
analysis demonstrates that W bosons are produced with all three helicities, the
longitudinal and the two transverse states. Combining the results from the two
center-of-mass energies and with leptonic and hadronic W decays, the fraction
of longitudinally polarised W bosons is measured to be 0.261 +/- 0.051(stat.)
+/- 0.016(syst.) in agreement with the expectation from the Standard Model
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