791 research outputs found
The Nucleon Spectral Function at Finite Temperature and the Onset of Superfluidity in Nuclear Matter
Nucleon selfenergies and spectral functions are calculated at the saturation
density of symmetric nuclear matter at finite temperatures. In particular, the
behaviour of these quantities at temperatures above and close to the critical
temperature for the superfluid phase transition in nuclear matter is discussed.
It is shown how the singularity in the thermodynamic T-matrix at the critical
temperature for superfluidity (Thouless criterion) reflects in the selfenergy
and correspondingly in the spectral function. The real part of the on-shell
selfenergy (optical potential) shows an anomalous behaviour for momenta near
the Fermi momentum and temperatures close to the critical temperature related
to the pairing singularity in the imaginary part. For comparison the selfenergy
derived from the K-matrix of Brueckner theory is also calculated. It is found,
that there is no pairing singularity in the imaginary part of the selfenergy in
this case, which is due to the neglect of hole-hole scattering in the K-matrix.
From the selfenergy the spectral function and the occupation numbers for finite
temperatures are calculated.Comment: LaTex, 23 pages, 21 PostScript figures included (uuencoded), uses
prc.sty, aps.sty, revtex.sty, psfig.sty (last included
Critical Enhancement of the In-medium Nucleon-Nucleon Cross Section at low Temperatures
The in-medium nucleon-nucleon cross section is calculated starting from the
thermodynamic T-matrix at finite temperatures. The corresponding
Bethe-Salpeter-equation is solved using a separable representation of the Paris
nucleon-nucleon-potential. The energy-dependent in-medium N-N cross section at
a given density shows a strong temperature dependence. Especially at low
temperatures and low total momenta, the in-medium cross section is strongly
modified by in-medium effects. In particular, with decreasing temperature an
enhancement near the Fermi energy is observed. This enhancement can be
discussed as a precursor of the superfluid phase transition in nuclear matter.Comment: 10 pages with 4 figures (available on request from the authors),
MPG-VT-UR 34/94 accepted for publication in Phys. Rev.
Exact Solution of the Munoz-Eaton Model for Protein Folding
A transfer-matrix formalism is introduced to evaluate exactly the partition
function of the Munoz-Eaton model, relating the folding kinetics of proteins of
known structure to their thermodynamics and topology. This technique can be
used for a generic protein, for any choice of the energy and entropy
parameters, and in principle allows the model to be used as a first tool to
characterize the dynamics of a protein of known native state and equilibrium
population. Applications to a -hairpin and to protein CI-2, with
comparisons to previous results, are also shown.Comment: 4 pages, 5 figures, RevTeX 4. To be published in Phys. Rev. Let
Thermodynamics of - condensate in asymmetric nuclear matter
We study the neutron-proton pairing in nuclear matter as a function of
isospin asymmetry at finite temperatures and the saturation density using
realistic nuclear forces and Brueckner-renormalized single particle spectra.
Our computation of the thermodynamic quantities shows that while the difference
of the entropies of the superconducting and normal phases anomalously changes
its sign as a function of temperature for arbitrary asymmetry, the grand
canonical potential does not; the superconducting state is found to be stable
in the whole temperature-asymmetry plane. The pairing gap completely disappears
for density-asymmetries exceeding .Comment: 7 pages, including 3 figures, uses revte
A Self-Consistent Solution to the Nuclear Many-Body Problem at Finite Temperature
The properties of symmetric nuclear matter are investigated within the
Green's functions approach. We have implemented an iterative procedure allowing
for a self-consistent evaluation of the single-particle and two-particle
propagators. The in-medium scattering equation is solved for a realistic
(non-separable) nucleon-nucleon interaction including both particle-particle
and hole-hole propagation. The corresponding two-particle propagator is
constructed explicitely from the single-particle spectral functions. Results
are obtained for finite temperatures and an extrapolation to T=0 is presented.Comment: 11 pages 5 figure
Deuteron life-time in hot and dense nuclear matter near equilibrium
We consider deuteron formation in hot and dense nuclear matter close to
equilibrium and evaluate the life-time of the deuteron fluctuations within the
linear response theory. To this end we derive a generalized linear Boltzmann
equation where the collision integral is related to equilibrium correlation
functions. In this framework we then utilize finite temperature Green functions
to evaluate the collision integrals. The elementary reaction cross section is
evaluated within the Faddeev approach that is suitably modified to reflect the
properties of the surrounding hot and dense matter.Comment: 15 pages, 5 figure
Conformations of Proteins in Equilibrium
We introduce a simple theoretical approach for an equilibrium study of
proteins with known native state structures. We test our approach with results
on well-studied globular proteins, Chymotrypsin Inhibitor (2ci2), Barnase and
the alpha spectrin SH3 domain and present evidence for a hierarchical onset of
order on lowering the temperature with significant organization at the local
level even at high temperatures. A further application to the folding process
of HIV-1 protease shows that the model can be reliably used to identify key
folding sites that are responsible for the development of drug resistance .Comment: 6 pages, 3 eps figure
17 ways to say yes:Toward nuanced tone of voice in AAC and speech technology
People with complex communication needs who use speech-generating devices have very little expressive control over their tone of voice. Despite its importance in human interaction, the issue of tone of voice remains all but absent from AAC research and development however. In this paper, we describe three interdisciplinary projects, past, present and future: The critical design collection Six Speaking Chairs has provoked deeper discussion and inspired a social model of tone of voice; the speculative concept Speech Hedge illustrates challenges and opportunities in designing more expressive user interfaces; the pilot project Tonetable could enable participatory research and seed a research network around tone of voice. We speculate that more radical interactions might expand frontiers of AAC and disrupt speech technology as a whole
Phases of asymmetric nuclear matter with broken space symmetries
Isoscalar Cooper pairing in isospin asymmetric nuclear matter occurs between
states populating two distinct Fermi surfaces, each for neutrons and protons.
The transition from a BCS-like to the normal (unpaired) state, as the isospin
asymmetry is increased, is intervened by superconducting phases which
spontaneously break translational and rotational symmetries. One possibility is
the formation of a condensate with a periodic crystallinelike structure where
Cooper pairs carry net momentum (the nuclear
Larkin-Ovchinnikov-Fulde-Ferrell-phase). Alternatively, perturbations of the
Fermi surfaces away from spherical symmetry allow for minima in the condensate
free energy which correspond to a states with quadrupole deformations of Fermi
surfaces and zero momentum of the Cooper pairs. In a combined treatment of
these phases we show that, although the Cooper pairing with finite momentum
might arise as a local minimum, the lowest energy state features are deformed
Fermi surfaces and Cooper pairs with vanishing total momentum.Comment: 22 pages, 6 figures, RevTex; v2: matches published version; v3:
changes in the frontmatter, content unchange
Electron Heating by Debye-Scale Turbulence in Guide-Field Reconnection
We report electrostatic Debye-scale turbulence developing within the diffusion region of asymmetric magnetopause reconnection with amoderate guide field using observations by the Magnetospheric Multiscale mission. We show that Buneman waves and beam modes cause efficient and fast thermalization of the reconnection electron jet by irreversible phase mixing, during which the jet kinetic energy is transferred into thermal energy. Our results show that the reconnection diffusion region in the presence of a moderate guide field is highly turbulent, and that electrostatic turbulence plays an important role in electron heating.Peer reviewe
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