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

Isolation of Candidate Genes Involved in Cold Temperatures Response in \u3cem\u3eFestuca Pratensis\u3c/em\u3e Huds., Using Suppression Subtractive Hybridisation and Microarray Approaches

The objective of this work was to isolate candidate genes which are differentially expressed following cold-acclimation and develop SNPs to test for associations between candidate genes and frost tolerance. The ability to develop sufficient levels of tolerance against freezing temperatures through cold-acclimation (hardening) is crucial for survival of grasses and winter cereals in temperate climate. Meadow fescue (Festauca pratensis Huds.) is one of the most important forage grass species in Northern Europe. The preference of Festuca instead of Lolium in Norway is due to its superior combination of winter hardiness and forage quality

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.

Calculated and experimental Schmid factors for chip flow deformation of textured CVD α-alumina coatings

A thorough analysis of Schmid factors (m) for three different (basal and two prismatic) slip systems and three different coating textures, (0001), (011\uaf2) and (112\uaf0), was done in order to understand the influence of CVD α-alumina coating textures on the ability of the coatings to deform plastically at different locations on the rake face of a cutting tool insert during a metal machining operation. Schmid factor diagrams were constructed using MATLAB/MTEX in order to visualize the angular dependence of an external force relative to the α-Al O crystals (grains) on the Schmid factor. The diagrams were also used to extract m-value frequency distributions for different slip systems and textures. In addition, lateral m-value distribution maps were obtained from experimental textured coatings using electron backscatter diffraction. These maps show the ability for neighboring grains to deform plastically in the coatings. Cutting tool inserts with differently textured α-Al2O3 coatings were subjected to dry machining of a quench-tempered steel. Using scanning electron microscopy, the microstructure and surface topography of the worn alumina layers were investigated and it was found that a flatter surface morphology and higher probability of discrete plastic deformation are connected to less wear. This was observed in the (0001)-textured sample, which also exhibited the highest m-values in the wear zone with highest temperatures and external forces. It was observed that basal slip is most easily activated, followed by prismatic slip systems 1 and 2 in this case. For (011\uaf2) and (112\uaf0) textured coatings the differences in m-values for the three slip systems are not that big, and the distributions are relatively wide. It is clear that the Schmid factor analysis forms a basis that is important for understanding crater wear, especially when it is connected to local plastic deformation, of textured CVD α-Al2O3 coatings. The methodology of this work can be expanded to other coating systems and also more generally to applications where it is of interest to analyze the deformation behavior and local plastic anisotropy of textured materials

Spontaneous breaking of rotational symmetry in superconductors

We show that homogeneous superconductors with broken spin/isospin symmetry lower their energy via a transition to a novel superconducting state where the Fermi-surfaces are deformed to a quasi-ellipsoidal form at zero total momentum of Cooper pairs. In this state, the gain in the condensation energy of the pairs dominates over the loss in the kinetic energy caused by the lowest order (quadrupole) deformation of Fermi-surfaces from the spherically symmetric form. There are two energy minima in general, corresponding to the deformations of the Fermi-spheres into either prolate or oblate forms. The phase transition from spherically symmetric state to the superconducting state with broken rotational symmetry is of the first order.Comment: 5 pages, including 3 figures, published versio

Thermodynamics of nn-pp 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 $\alpha_c= (n_n-n_p)/n \simeq 0.11$.Comment: 7 pages, including 3 figures, uses revte

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 $\beta$-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

Four-particle condensate in strongly coupled fermion systems

Four-particle correlations in fermion systems at finite temperatures are investigated with special attention to the formation of a condensate. Instead of the instability of the normal state with respect to the onset of pairing described by the Gorkov equation, a new equation is obtained which describes the onset of quartetting. Within a model calculation for symmetric nuclear matter, we find that below a critical density, the four-particle condensation (alpha-like quartetting) is favored over deuteron condensation (triplet pairing). This pairing-quartetting competition is expected to be a general feature of interacting fermion systems, such as the excition-biexciton system in excited semiconductors. Possible experimental consequences are pointed out.Comment: LaTeX, 11 pages, 2 figures, uses psfig.sty (included), to be published in Phys. Rev. Lett., tentatively scheduled for 13 April 1998 (Volume 80, Number 15

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