156,752 research outputs found
Terminal velocities of luminous, early-type SMC stars
Ultraviolet spectra from the Space Telescope Imaging Spectrograph (STIS) are
used to determine terminal velocities for 11 O and B-type giants and
supergiants in the Small Magellanic Cloud (SMC) from the Si IV and C IV
resonance lines. Using archival data from observations with the Goddard
High-Resolution Spectrograph and the International Ultraviolet Explorer
telescope, terminal velocities are obtained for a further five B-type
supergiants. We discuss the metallicity dependence of stellar terminal
velocities, finding no evidence for a significant scaling between Galactic and
SMC metallicities for Teff < 30,000 K, consistent with the predictions of
radiation driven wind theory for supergiant stars. A comparison of the
ratio between the SMC and Galactic samples, while
consistent with the above statement, emphasizes that the uncertainties in the
distances to galactic O-stars are a serious obstacle to a detailed comparison
with theory. For the SMC sample there is considerable scatter in this ratio at
a given effective temperature, perhaps indicative of uncertainties in stellar
masses.Comment: 28 pages, 8 figures, accepted by ApJ; minor revisions prior to
acceptanc
Teaching Theoretical Physics: the cases of Enrico Fermi and Ettore Majorana
We report on theoretical courses by Fermi and Majorana, giving evidence of
the first appearance and further development of Quantum Mechanics teaching in
Italy. On the basis of original documents, we make a comparison between Fermi's
and Majorana's approaches. A detailed analysis is carried out of Fermi's course
on Theoretical Physics attended by Majorana in 1927-28. Three (previously
unknown) programs on advanced Physics courses submitted by Majorana to the
University of Rome between 1933 and 1936 and the course he held in Naples in
1938 complete our analysis: Fermi's phenomenological approach resounded in
Majorana, who however combined it with a deeper theoretical approach, closer to
the modern way of presenting Quantum Mechanics.Comment: latex, 21 pages; a contribution in the centenary of the birth of
Ettore Majoran
The local structure of OH species on the V2O3(0 0 0 1) surface: a scanned-energy mode photoelectron diffraction study
Scanned-energy mode photoelectron diffraction (PhD), using O 1s photoemission, together with multiple-scattering simulations, have been used to investigate the structure of the hydroxyl species, OH, adsorbed on a V2O3(0 0 0 1) surface. Surface OH species were obtained by two alternative methods; reaction with molecular water and exposure to atomic H resulted in closely similar PhD spectra. Both qualitative assessment and the results of multiple-scattering calculations are consistent with a model in which only the O atoms of outermost layer of the oxide surface are hydroxylated. These results specifically exclude significant coverage of OH species atop the outermost V atoms, i.e. in vanadyl O atom sites. Ab initio density-functional theory cluster calculations provide partial rationalisation of this result, which is discussed the context of the general understanding of this system
Early diagenetic vivianite [Fe-3(PO4)(2) center dot 8H(2)O] in a contaminated freshwater sediment and insights into zinc uptake: a mu-EXAFS, mu-XANES and Raman study
The sediments in the Salford Quays, a heavily-modified urban water body, contain high levels of organic matter, Fe, Zn and nutrients as a result of past contaminant inputs. Vivianite [Fe3(PO4)2 · 8H2O] has been observed to have precipitated within these sediments during early diagenesis as a result of the release of Fe and P to porewaters. These mineral grains are small (<100 ÎŒm) and micron-scale analysis techniques (SEM, electron microprobe, ÎŒ-EXAFS, ÎŒ-XANES and Raman) have been applied in this study to obtain information upon the structure of this vivianite and the nature of Zn uptake in the mineral. Petrographic observations, and elemental, X-ray diffraction and Raman spectroscopic analysis confirms the presence of vivianite. EXAFS model fitting of the FeK-edge spectra for individual vivianite grains produces FeâO and FeâP co-ordination numbers and bond lengths consistent with previous structural studies of vivianite (4O atoms at 1.99â2.05 Ă
; 2P atoms at 3.17â3.25 Ă
). One analysed grain displays evidence of a significant Fe3+ component, which is interpreted to have resulted from oxidation during sample handling and/or analysis. EXAFS modelling of the Zn K-edge data, together with linear combination XANES fitting of model compounds, indicates that Zn may be incorporated into the crystal structure of vivianite (4O atoms at 1.97 Ă
; 2P atoms at 3.17 Ă
). Low levels of Zn sulphate or Zn-sorbed goethite are also indicated from linear combination XANES fitting and to a limited extent, the EXAFS fitting, the origin of which may either be an oxidation artifact or the inclusion of Zn sulphate into the vivianite grains during precipitation. This study confirms that early diagenetic vivianite may act as a sink for Zn, and potentially other contaminants (e.g. As) during its formation and, therefore, forms an important component of metal cycling in contaminated sediments and waters. Furthermore, for the case of Zn, the EXAFS fits for Zn phosphate suggest this uptake is structural and not via surface adsorption
Experimental evidence of replica symmetry breaking in random lasers
Spin-glass theory is one of the leading paradigms of complex physics and
describes condensed matter, neural networks and biological systems, ultracold
atoms, random photonics, and many other research fields. According to this
theory, identical systems under identical conditions may reach different states
and provide different values for observable quantities. This effect is known as
Replica Symmetry Breaking and is revealed by the shape of the probability
distribution function of an order parameter named the Parisi overlap. However,
a direct experimental evidence in any field of research is still missing. Here
we investigate pulse-to-pulse fluctuations in random lasers, we introduce and
measure the analogue of the Parisi overlap in independent experimental
realizations of the same disordered sample, and we find that the distribution
function yields evidence of a transition to a glassy light phase compatible
with a replica symmetry breaking.Comment: 10 pages, 5 figure
Visualizing plasmon-exciton polaritons at the nanoscale using electron microscopy
Polaritons are compositional light-matter quasiparticles that have recently
enabled remarkable breakthroughs in quantum and nonlinear optics, as well as in
material science. Despite the enormous progress, however, a direct
nanometer-scale visualization of polaritons has remained an open challenge.
Here, we demonstrate that plasmon-exciton polaritons, or plexcitons, generated
by a hybrid system composed of an individual silver nanoparticle and a
few-layer transition metal dichalcogenide can be spectroscopically mapped with
nanometer spatial resolution using electron energy loss spectroscopy in a
scanning transmission electron microscope. Our experiments reveal important
insights about the coupling process, which have not been reported so far. These
include nanoscale variation of Rabi splitting and plasmon-exciton detuning, as
well as absorption-dominated extinction signals, which in turn provide the
ultimate evidence for the plasmon-exciton hybridization in the strong coupling
regime. These findings pioneer new possibilities for in-depth studies of
polariton-related phenomena with nanometer spatial resolution
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