1,323 research outputs found
Local states of free bose fields
These notes contain an extended version of lectures given at the ``Summer
School on Large Coulomb Systems'' in Nordfjordeid, Norway, in august 2003. They
furnish a short introduction to the theory of quantum harmonic systems, or free
bose fields. The main issue addressed is the one of local states. I will adopt
the definition of Knight of ``strictly local excitation of the vacuum'' and
will then state and prove a generalization of Knight's Theorem which asserts
that finite particle states cannot be perfectly localized. It will furthermore
be explained how Knight's a priori counterintuitive result can be readily
understood if one remembers the analogy between finite and infinite dimensional
harmonic systems alluded to above. I will also discuss the link between the
above result and the so-called Newton-Wigner position operator thereby
illuminating, I believe, the difficulties associated with the latter. I will in
particular argue that those difficulties do not find their origin in special
relativity or in any form of causality violation, as is usually claimed
Switching of +/-360deg domain wall states in a nanoring by an azimuthal Oersted field
We demonstrate magnetic switching between two domain wall vortex
states in cobalt nanorings, which are candidate magnetic states for robust and
low power MRAM devices. These domain wall (DW) or "twisted onion"
states can have clockwise or counterclockwise circulation, the two states for
data storage. Reliable switching between the states is necessary for any
realistic device. We accomplish this switching by applying a circular Oersted
field created by passing current through a metal atomic force microscope tip
placed at the center of the ring. After initializing in an onion state, we
rotate the DWs to one side of the ring by passing a current through the center,
and can switch between the two twisted states by reversing the current, causing
the DWs to split and meet again on the opposite side of the ring. A larger
current will annihilate the DWs and create a perfect vortex state in the rings.Comment: 5 pages, 5 figure
Rapid Evolution of Testis Size Relative to Sperm Morphology Suggests that Post-Copulatory Selection Targets Sperm Number in \u3cem\u3eAnolis\u3c/em\u3e Lizards
Post-copulatory sexual selection is thought to be responsible for much of the extraordinary diversity in sperm morphology across metazoans. However, the extent to which post-copulatory selection targets sperm morphology versus sperm production is generally unknown. To address this issue, we simultaneously characterized the evolution of sperm morphology (length of the sperm head, midpiece and flagellum) and testis size (a proxy for sperm production) across 26 species of Anolis lizards, a group in which sperm competition is likely. We found that the length of the sperm midpiece has evolved 2â3 times faster than that of the sperm head or flagellum, suggesting that midpiece size may be the most important aspect of sperm morphology with respect to post-copulatory sexual selection. However, testis size has evolved faster than any aspect of sperm morphology or body size, supporting the hypothesis that post-copulatory sexual selection acts more strongly upon sperm production than upon sperm morphology. Likewise, evolutionary increases in testis size, which typically indicate increased sperm competition, are not associated with predictable changes in sperm morphology, suggesting that any effects of post-copulatory selection on sperm morphology are either weak or variable in direction across anoles. Collectively, our results suggest that sperm production is the primary target of post-copulatory sexual selection in this lineage
Evidence for Extending Anomalous Miocene Volcanism at the Edge of the East Antarctic Craton
Using field observations followed by petrological, geochemical, geochronological, and geophysical data, we infer the presence of a previously unknown Miocene subglacial volcanic center ~230 km from the South Pole. Evidence of volcanism is from boulders of olivineâbearing amygdaloidal/vesicular basalt and hyaloclastite deposited in a moraine in the southern Transantarctic Mountains. 40Ar/39Ar ages from five specimens plus UâPb ages of detrital zircon from glacial till indicate igneous activity 25â17 Ma. The likely source of the volcanism is a circular â735 nT magnetic anomaly 60 km upflow from the sampling site. Subaqueous textures of the volcanics indicate eruption beneath ice or into water at the margin of an ice mass during the early Miocene. These rocks record the southernmost Cenozoic volcanism in Antarctica and expand the known extent of the oldest lavas associated with West Antarctic Rift system. They may be an expression of lithospheric foundering beneath the southern Transantarctic Mountains
Outcome of depression in later life in primary care: longitudinal cohort study with three yearsâ follow-up
Objectives To study the duration of depression, recovery over time, and predictors of prognosis in an older cohort (â„55 years) in primary care
The Kaon-Photoproduction Of Nucleons In The Quark Model
In this paper, we develop a general framework to study the
meson-photoproductions of nucleons in the chiral quark model. The S and U
channel resonance contributions are expressed in terms of the
Chew-Goldberger-Low-Nambu (CGLN) amplitudes. The kaon-photoproduction
processes, , , and , are calculated. The initial results show that the quark
model provides a much improved description of the reaction mechanism for the
kaon-photoproductions of the nucleon with less parameters than the traditional
phenomenological approaches.Comment: 25 pages, 9 postscript figures can be obtained from the author
Electromagnetic Form Factors of the Nucleon in an Improved Quark Model
Nucleon electromagnetic form factors are studied in the cloudy bag model
(CBM) with center-of-mass and recoil corrections. This is the first
presentation of a full set of nucleon form factors using the CBM. The center of
mass motion is eliminated via several different momentum projection techniques
and the results are compared. It is found that the shapes of these form factors
are significantly improved with respect to the experimental data if the Lorentz
contraction of the internal structure of the baryon is also appropriately taken
into account.Comment: revtex, 28 pages, 8 ps figs include
Peptide bond formation in the protonated serine dimer following vacuum UV photon-induced excitation
Possible routes for intra-cluster bond formation (ICBF) in protonated serine dimers have been studied. We found no evidence of ICBF following low energy collision-induced dissociation (in correspondence with previous works), however, we do observe clear evidence for ICBF following photon absorption in the 4.6â14 eV range. Moreover, the comparison of photon-induced dissociation measurements of the protonated serine dimer to those of a protonated serine dipeptide provides evidence that ICBF, in this case, involves peptide bond formation (PBF). The experimental results are supported by ab initio molecular dynamics and exploration of several excited state potential energy surfaces, unraveling a pathway for PBF following photon absorption. The combination of experiments and theory provides insight into the PBF mechanisms in clusters of amino acids, and reveals the importance of electronic excited states reached upon UV/VUV light excitatio
GeantV: Results from the prototype of concurrent vector particle transport simulation in HEP
Full detector simulation was among the largest CPU consumer in all CERN
experiment software stacks for the first two runs of the Large Hadron Collider
(LHC). In the early 2010's, the projections were that simulation demands would
scale linearly with luminosity increase, compensated only partially by an
increase of computing resources. The extension of fast simulation approaches to
more use cases, covering a larger fraction of the simulation budget, is only
part of the solution due to intrinsic precision limitations. The remainder
corresponds to speeding-up the simulation software by several factors, which is
out of reach using simple optimizations on the current code base. In this
context, the GeantV R&D project was launched, aiming to redesign the legacy
particle transport codes in order to make them benefit from fine-grained
parallelism features such as vectorization, but also from increased code and
data locality. This paper presents extensively the results and achievements of
this R&D, as well as the conclusions and lessons learnt from the beta
prototype.Comment: 34 pages, 26 figures, 24 table
Versatile Coordination of Cyclopentadienyl-Arene Ligands and Its Role in Titanium-Catalyzed Ethylene Trimerization
Cationic titanium(IV) complexes with ansa-(η5-cyclopentadienyl,η6-arene) ligands were synthesized and characterized by X-ray crystallography. The strength of the metal-arene interaction in these systems was studied by variable-temperature NMR spectroscopy. Complexes with a C1 bridge between the cyclopentadienyl and arene moieties feature hemilabile coordination behavior of the ligand and consequently are active ethylene trimerization catalysts. Reaction of the titanium(IV) dimethyl cations with CO results in conversion to the analogous cationic titanium(II) dicarbonyl species. Metal-to-ligand backdonation in these formally low-valent complexes gives rise to a strongly bonded, partially reduced arene moiety. In contrast to the η6-arene coordination mode observed for titanium, the more electron-rich vanadium(V) cations [cyclopentadienyl-arene]V(NiPr2)(NC6H4-4-Me)+ feature η1-arene binding, as determined by a crystallographic study. The three different metal-arene coordination modes that we experimentally observed model intermediates in the cycle for titanium-catalyzed ethylene trimerization. The nature of the metal-arene interaction in these systems was studied by DFT calculations.
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