441 research outputs found
An experimental setup for high resolution 10.5 eV laser-based angle-resolved photoelectron spectroscopy using a time-of-flight electron analyzer
We present an experimental setup for laser-based angle-resolved
time-of-flight (LARTOF) photoemission. Using a picosecond pulsed laser, photons
of energy 10.5 eV are generated through higher harmonic generation in xenon.
The high repetition rate of the light source, variable between 0.2-8 MHz,
enables high photoelectron count rates and short acquisition times. By using a
Time-of-Flight (ToF) analyzer with angle-resolving capabilities electrons
emitted from the sample within a circular cone of up to \pm15 degrees can be
collected. Hence, simultaneous acquisition of photoemission data for a complete
area of the Brillouin zone is possible. The current photon energy enables bulk
sensitive measurements, high angular resolution and the resulting covered
momentum space is large enough to enclose the entire Brillouin zone in cuprate
high-Tc superconductors. Fermi edge measurements on polycrystalline Au shows an
energy resolution better than 5 meV. Data from a test measurement of the
Au(111) surface state is presented along with measurements of the Fermi surface
of the high-Tc superconductor Bi2212.Comment: 9 pages, 7 figure
A spin- and angle-resolving photoelectron spectrometer
A new type of hemispherical electron energy analyzer that permits angle and
spin resolved photoelectron spectroscopy has been developed. The analyzer
permits standard angle resolved spectra to be recorded with a two-dimensional
detector in parallel with spin detection using a mini-Mott polarimeter. General
design considerations as well as technical solutions are discussed and test
results from the Au(111) surface state are presented
The J_{eff}=1/2 insulator Sr3Ir2O7 studied by means of angle-resolved photoemission spectroscopy
The low-energy electronic structure of the J_{eff}=1/2 spin-orbit insulator
Sr3Ir2O7 has been studied by means of angle-resolved photoemission
spectroscopy. A comparison of the results for bilayer Sr3Ir2O7 with available
literature data for the related single-layer compound Sr2IrO4 reveals
qualitative similarities and similar J_{eff}=1/2 bandwidths for the two
materials, but also pronounced differences in the distribution of the spectral
weight. In particuar, photoemission from the J_{eff}=1/2 states appears to be
suppressed. Yet, it is found that the Sr3Ir2O7 data are in overall better
agreement with band-structure calculations than the data for Sr2IrO4.Comment: 5 pages, 3 figure
Direct observation of decoupled Dirac states at the interface between topological and normal insulators
Several proposed applications and exotic effects in topological insulators
rely on the presence of helical Dirac states at the interface between a
topological and a normal insulator. In the present work, we have used
low-energy angle-resolved photoelectron spectroscopy to uncover and
characterize the interface states of BiSe thin films and
BiTe/BiSe heterostuctures grown on Si(111). The results
establish that Dirac fermions are indeed present at the
topological-normal-insulator boundary and absent at the
topological-topological-insulator interface. Moreover, it is demonstrated that
band bending present within the topological-insulator films leads to a
substantial separation of the interface and surface states in energy. These
results pave the way for further studies and the realization of
interface-related phenomena in topological-insulator thin-film
heterostructures.Comment: 9 pages, 5 figure
BVOC-aerosol-climate feedbacks investigated using NorESM
Both higher temperatures and increased CO2 concentrations are (separately) expected to increase the emissions of biogenic volatile organic compounds (BVOCs). This has been proposed to initiate negative climate feedback mechanisms through increased formation of secondary organic aerosol (SOA). More SOA can make the clouds more reflective, which can provide a cooling. Furthermore, the increase in SOA formation has also been proposed to lead to increased aerosol scattering, resulting in an increase in diffuse radiation. This could boost gross primary production (GPP) and further increase BVOC emissions. In this study, we have used the Norwegian Earth System Model (NorESM) to investigate both these feedback mechanisms. Three sets of experiments were set up to quantify the feedback with respect to (1) doubling the CO2, (2) increasing temperatures corresponding to a doubling of CO2 and (3) the combined effect of both doubling CO2 and a warmer climate. For each of these experiments, we ran two simulations, with identical setups, except for the BVOC emissions. One simulation was run with interactive BVOC emissions, allowing the BVOC emissions to respond to changes in CO2 and/or climate. In the other simulation, the BVOC emissions were fixed at present-day conditions, essentially turning the feedback off. The comparison of these two simulations enables us to investigate each step along the feedback as well as estimate their overall relevance for the future climate. We find that the BVOC feedback can have a significant impact on the climate. The annual global BVOC emissions are up to 63 % higher when the feedback is turned on compared to when the feedback is turned off, with the largest response when both CO2 and climate are changed. The higher BVOC levels lead to the formation of more SOA mass (max 53 %) and result in more particles through increased new particle formation as well as larger particles through increased condensation. The corresponding changes in the cloud properties lead to a -0.43 W m(-2) stronger net cloud forcing. This effect becomes about 50 % stronger when the model is run with reduced anthropogenic aerosol emissions, indicating that the feedback will become even more important as we decrease aerosol and precursor emissions. We do not find a boost in GPP due to increased aerosol scattering on a global scale. Instead, the fate of the GPP seems to be controlled by the BVOC effects on the clouds. However, the higher aerosol scattering associated with the higher BVOC emissions is found to also contribute with a potentially important enhanced negative direct forcing (-0.06 W m(-2)). The global total aerosol forcing associated with the feedback is -0.49 W m(-2), indicating that it has the potential to offset about 13 % of the forcing associated with a doubling of CO2.Peer reviewe
Casimir Effect for the Piecewise Uniform String
The Casimir energy for the transverse oscillations of a piecewise uniform
closed string is calculated. In its simplest version the string consists of two
parts I and II having in general different tension and mass density, but is
always obeying the condition that the velocity of sound is equal to the
velocity of light. The model, first introduced by Brevik and Nielsen in 1990,
possesses attractive formal properties implying that it becomes easily
regularizable by several methods, the most powerful one being the contour
integration method. We also consider the case where the string is divided into
2N pieces, of alternating type-I and type-II material. The free energy at
finite temperature, as well as the Hagedorn temperature, are found. Finally, we
make some remarks on the relationship between this kind of theory and the
theory of quantum star graphs, recently considered by Fulling et al.Comment: 10 pages, 1 figure, Submitted to the volume "Cosmology, Quantum
Vacuum, and Zeta Functions", in honour of Professor Emilio Elizalde on the
occasion of his 60th birthda
BVOC–aerosol–climate feedbacks investigated using NorESM
Both higher temperatures and increased CO2 concentrations are
(separately) expected to increase the emissions of biogenic volatile organic
compounds (BVOCs). This has been proposed to initiate negative climate
feedback mechanisms through increased formation of secondary organic aerosol
(SOA). More SOA can make the clouds more reflective, which can provide a
cooling. Furthermore, the increase in SOA formation has also been proposed to
lead to increased aerosol scattering, resulting in an increase in diffuse
radiation. This could boost gross primary production (GPP) and further
increase BVOC emissions. In this study, we have used the Norwegian Earth
System Model (NorESM) to investigate both these feedback mechanisms. Three
sets of experiments were set up to quantify the feedback with respect to (1)Â doubling
the CO2, (2)Â increasing temperatures corresponding to a doubling of
CO2 and (3)Â the combined effect of both doubling CO2 and a
warmer climate. For each of these experiments, we ran two simulations, with
identical setups, except for the BVOC emissions. One simulation was run with
interactive BVOC emissions, allowing the BVOC emissions to respond to changes
in CO2 and/or climate. In the other simulation, the BVOC emissions
were fixed at present-day conditions, essentially turning the feedback off.
The comparison of these two simulations enables us to investigate each step
along the feedback as well as estimate their overall relevance for the future
climate.
We find that the BVOC feedback can have a significant impact on the climate.
The annual global BVOC emissions are up to 63 % higher when the feedback
is turned on compared to when the feedback is turned off, with the largest
response when both CO2 and climate are changed. The higher BVOC
levels lead to the formation of more SOA mass (max 53 %) and result in
more particles through increased new particle formation as well as larger
particles through increased condensation. The corresponding changes in the
cloud properties lead to a −0.43 W m−2 stronger net cloud forcing.
This effect becomes about 50 % stronger when the model is run with
reduced anthropogenic aerosol emissions, indicating that the feedback will
become even more important as we decrease aerosol and precursor emissions. We
do not find a boost in GPP due to increased aerosol scattering on a global
scale. Instead, the fate of the GPP seems to be controlled by the BVOC effects
on the clouds. However, the higher aerosol scattering associated with the
higher BVOC emissions is found to also contribute with a potentially
important enhanced negative direct forcing (−0.06 W m−2). The global
total aerosol forcing associated with the feedback is −0.49 W m−2,
indicating that it has the potential to offset about 13 % of the forcing
associated with a doubling of CO2.</p
Radiative forcing in the 21st century due to ozone changes in the troposphere and the lower stratosphere
Radiative forcing due to changes in ozone is expected for the 21st century. An assessment on changes in the tropospheric oxidative state through a model intercomparison ("OxComp'') was conducted for the IPCC Third Assessment Report (IPCC-TAR). OxComp estimated tropospheric changes in ozone and other oxidants during the 21st century based on the "SRES'' A2p emission scenario. In this study we analyze the results of 11 chemical transport models (CTMs) that participated in OxComp and use them as input for detailed radiative forcing calculations. We also address future ozone recovery in the lower stratosphere and its impact on radiative forcing by applying two models that calculate both tropospheric and stratospheric changes. The results of OxComp suggest an increase in global-mean tropospheric ozone between 11.4 and 20.5 DU for the 21st century, representing the model uncertainty range for the A2p scenario. As the A2p scenario constitutes the worst case proposed in IPCC-TAR we consider these results as an upper estimate. The radiative transfer model yields a positive radiative forcing ranging from 0.40 to 0.78 W m(-2) on a global and annual average. The lower stratosphere contributes an additional 7.5-9.3 DU to the calculated increase in the ozone column, increasing radiative forcing by 0.15-0.17 W m(-2). The modeled radiative forcing depends on the height distribution and geographical pattern of predicted ozone changes and shows a distinct seasonal variation. Despite the large variations between the 11 participating models, the calculated range for normalized radiative forcing is within 25%, indicating the ability to scale radiative forcing to global-mean ozone column change
Lipidic cubic phase serial millisecond crystallography using synchrotron radiation.
Lipidic cubic phases (LCPs) have emerged as successful matrixes for the crystallization of membrane proteins.Moreover, the viscous LCP also provides a highly effective delivery medium for serial femtosecond crystallography (SFX) at X-ray free-electron lasers (XFELs). Here, the adaptation of this technology to perform serial millisecond crystallography (SMX) at more widely available synchrotron microfocus beamlines is described. Compared with conventional microcrystallography, LCP-SMX eliminates the need for difficult handling of individual crystals and allows for data collection at room temperature. The technology is demonstrated by solving a structure of the light-driven protonpump bacteriorhodopsin (bR) at a resolution of 2.4 A ° . The room-temperature structure of bR is very similar to previous cryogenic structures but shows small yet distinct differences in the retinal ligand and proton-transfer pathway
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