1,542 research outputs found
Dynamics of few-body states in a medium
Strongly interacting matter such as nuclear or quark matter leads to few-body
bound states and correlations of the constituents. As a consequence quantum
chromodynamics has a rich phase structure with spontaneous symmetry breaking,
superconductivity, condensates of different kinds. All this appears in many
astrophysical scenarios. Among them is the formation of hadrns during the early
stage of the Universe, the structure of a neutron star, the formation of nuclei
during a supernova explosion. Some of these extreme conditions can be simulated
in heavy ion colliders. To treat such a hot and dense system we use the Green
function formalism of many-body theory. It turns out that a systematic Dyson
expansion of the Green functions leads to modified few-body equations that are
capable to describe phase transitions, condensates, cluster formation and more.
These equations include self energy corrections and Pauli blocking. We apply
this method to nonrelativistic and relativistic matter. The latter one is
treated on the light front. Because of the medium and the inevitable truncation
of space, the few-body dynamics and states depend on the thermodynamic
parameters of the medium.Comment: 3 pages, 2 figures, talk presented at the 19th European Conference on
Few-Body System
Boson-Fermion pairing in a Boson-Fermion environment
Propagation of a Boson-Fermion (B-F) pair in a B-F environment is considered.
The possibility of formation of stable strongly correlated B-F pairs, embedded
in the continuum, is pointed out. The new Fermi gas of correlated B-F pairs
shows a strongly modified Fermi surface. The interaction between like particles
is neglected in this exploratory study. Various physical situations where our
new pairing mechanism could be of importance are invoked.Comment: 8 pages, 8 figers, to be published in Phys. Rev.
ESCRT machinery mediates selective microautophagy of endoplasmic reticulum in yeast
ER-phagy, the selective autophagy of endoplasmic reticulum (ER), safeguards organelle homeostasis by eliminating misfolded proteins and regulating ER size. ER-phagy can occur by macroautophagic and microautophagic mechanisms. While dedicated machinery for macro-ER-phagy has been discovered, the molecules and mechanisms mediating micro-ER-phagy remain unknown. Here, we first show that micro-ER-phagy in yeast involves the conversion of stacked cisternal ER into multilamellar ER whorls during microautophagic uptake into lysosomes. Second, we identify the conserved Nem1-Spo7 phosphatase complex and the ESCRT machinery as key components for micro-ER-phagy. Third, we demonstrate that macro- and micro-ER-phagy are parallel pathways with distinct molecular requirements. Finally, we provide evidence that the ESCRT machinery directly functions in scission of the lysosomal membrane to complete the microautophagic uptake of ER. These findings establish a framework for a mechanistic understanding of micro-ER-phagy and, thus, a comprehensive appreciation of the role of autophagy in ER homeostasis
Few-Body States in Fermi-Systems and Condensation Phenomena
Residual interactions in many particle systems lead to strong correlations. A
multitude of spectacular phenomenae in many particle systems are connected to
correlation effects in such systems, e.g. pairing, superconductivity,
superfluidity, Bose-Einstein condensation etc. Here we focus on few-body bound
states in a many-body surrounding.Comment: 10 pages, proceedings 1st Asian-Pacific Few-Body Conference, needs
fbssuppl.sty of Few-Body System
Estimating Electric Fields from Vector Magnetogram Sequences
Determining the electric field (E-field) distribution on the Sun's
photosphere is essential for quantitative studies of how energy flows from the
Sun's photosphere, through the corona, and into the heliosphere. This E-field
also provides valuable input for data-driven models of the solar atmosphere and
the Sun-Earth system. We show how Faraday's Law can be used with observed
vector magnetogram time series to estimate the photospheric E-field, an
ill-posed inversion problem. Our method uses a "poloidal-toroidal
decomposition" (PTD) of the time derivative of the vector magnetic field. The
PTD solutions are not unique; the gradient of a scalar potential can be added
to the PTD E-field without affecting consistency with Faraday's Law. We present
an iterative technique to determine a potential function consistent with ideal
MHD evolution; but this E-field is also not a unique solution to Faraday's Law.
Finally, we explore a variational approach that minimizes an energy functional
to determine a unique E-field, similar to Longcope's "Minimum Energy Fit". The
PTD technique, the iterative technique, and the variational technique are used
to estimate E-fields from a pair of synthetic vector magnetograms taken from an
MHD simulation; and these E-fields are compared with the simulation's known
electric fields. These three techniques are then applied to a pair of vector
magnetograms of solar active region NOAA AR8210, to demonstrate the methods
with real data.Comment: 41 pages, 10 figure
Action-Specific Effects Underwater
Action-specific effects on perception are apparent in terrestrial environments. For example, targets that require more effort to walk, jump, or throw to look farther away than when the targets require less effort. Here, we examined whether action-specific effects would generalize to an underwater environment. Instead, perception might be geometrically precise, rather than action-specific, in an environment that is novel from an evolutionary perspective. We manipulated ease to swim by giving participants swimming flippers or taking them away. Those who estimated distance while wearing the flippers judged underwater targets to be closer than did participants who had taken them off. In addition, participants with better swimming ability judged the targets to be closer than did those with worse swimming ability. These results suggest perceived distance underwater is a function of the perceiver’s ability to swim to the targets
Chemical Raman Enhancement of Organic Adsorbates on Metal Surfaces
Using a combination of first-principles theory and experiments, we provide a
quantitative explanation for chemical contributions to surface-enhanced Raman
spectroscopy for a well-studied organic molecule, benzene thiol, chemisorbed on
planar Au(111) surfaces. With density functional theory calculations of the
static Raman tensor, we demonstrate and quantify a strong mode-dependent
modification of benzene thiol Raman spectra by Au substrates. Raman active
modes with the largest enhancements result from stronger contributions from Au
to their electron-vibron coupling, as quantified through a deformation
potential, a well-defined property of each vibrational mode. A straightforward
and general analysis is introduced that allows extraction of chemical
enhancement from experiments for specific vibrational modes; measured values
are in excellent agreement with our calculations.Comment: 5 pages, 4 figures and Supplementary material included as ancillary
fil
Cooper pair sizes in 11Li and in superfluid nuclei: a puzzle?
We point out a strong influence of the pairing force on the size of the two
neutron Cooper pair in Li, and to a lesser extent also in He. It
seems that these are quite unique situations, since Cooper pair sizes of stable
superfluid nuclei are very little influenced by the intensity of pairing, as
recently reported. We explore the difference between Li and heavier
superfulid nuclei, and discuss reasons for the exceptional situation in
Li.Comment: 9 pages. To be published in J. of Phys. G special issue on Open
Problems in Nuclear Structure (OPeNST
Absorption spectrum of a weakly n-doped semiconductor quantum well
We calculate, as a function of temperature and conduction band electron
density, the optical absorption of a weakly n-doped, idealized semiconductor
quantum well. In particular, we focus on the absorption band due to the
formation of a charged exciton. We conceptualize the charged exciton as an
itinerant excitation intimately linked to the dynamical response of itinerant
conduction band electrons to the appearance of the photo-generated valence band
hole. Numerical results for the absorption in the vicinity of the exciton line
are presented and the spectral weights associated with, respectively, the
charged exciton band and the exciton line are analyzed in detail. We find, in
qualitative agreement with experimental data, that the spectral weight of the
charged exciton grows with increasing conduction band electron density and/or
decreasing temperature at the expense of the exciton.Comment: 5 pages, 4 figure
The stable isotopic composition of molecular hydrogen in the tropopause region probed by the CARIBIC aircraft
More than 450 air samples that were collected in the upper troposphere – lower stratosphere (UTLS) region by the CARIBIC aircraft (Civil Aircraft for the Regular Investigation of the atmosphere Based on an Instrument Container) have been analyzed for molecular hydrogen (H2) mixing ratios (χ¬(H2)) and H2 isotopic composition (deuterium content, δD).
More than 120 of the analyzed samples contained air from the lowermost stratosphere (LMS). These show that ¬ χ(H2) does not vary appreciably with O3-derived height above the thermal tropopause (TP), whereas δD does increase with height. The isotope enrichment is caused by H2 production and destruction processes that enrich the stratospheric H2 reservoir in deuterium (D); the exact shapes of the profiles are mainly determined by mixing of stratospheric with tropospheric air. Tight negative correlations are found between δD and the mixing ratios of methane (¬ χ(CH4)) and nitrous oxide (χ-(N2O)), as a result of the relatively long lifetimes of these three species. The correlations are described by δD[‰] = −0.35 · ¬χ(CH4)[ppb]+768 and δD[‰] = −1.90 · χ(N2O)[ppb]+745. These correlations are similar to previously published results and likely hold globally for the LMS.
Samples that were collected from the Indian subcontinentup to 40° N before, during and after the summer monsoon season show no significant seasonal change in χ¬(H2), but δD is up to 12.3‰ lower in the July, August and September monsoon samples. This δD decrease is correlated with the χ¬(CH4) increase in these samples. The significant correlation with χ¬(CH4) and the absence of a perceptible χ-(H2) increase that accompanies the δD decrease indicates that microbial production of very D-depleted H2 in the wet season may contribute to this phenomenon.
Some of the samples have very high χ¬(H2) and very low δD values, which indicates a pollution effect. Aircraft engine exhaust plumes are a suspected cause, since the effect mostly occurs in samples collected close to airports, but no similar signals are found in other chemical tracers to support this. The isotopic source signature of the H2 pollution seems to be on the low end of the signature for fossil fuel burning
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