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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.(MPG-VT-UR 65/95, submitted to Phys. Rev. C) 1

Abstract. The influence of correlations on the critical temperature and density for the onset of superfluidity in nuclear matter is investigated within the scheme of Nozi~res and Schmitt-Rink [1]. For symmetric nuclear matter a smooth transition from Bose-Einstein condensation (BEC) of deuteronlike bound states at low densities and low temperatures to Bardeen-Cooper-Schrieffer (BCS) pairing at higher densities is described. Compared with the mean field approach a lowering of the critical temperature is obtained for symmetric nuclear matter as well as for pure neutron matter. The Mott transition in symmetric nuclear matter is discussed. Regions in the temperature-density plane are identified where correlated pairs give the main contribution to the composition of the system, so that approximations beyond the quasi-particle picture are requested. 05.30.Fk, 21.65.+f, 74.25.Bt Superfluidity and superconductivity are macroscopic quantum phenomena occuring for fermion systems with attractive interaction. A usual framework for its microscopic description is the BCS theory [2]. The BCS theory is a meanfield approach and describes the occurence of pairing correlations forming the condensate. This pairing becomes at the critical temperature identical with the special case of zero momentum bound states in the low density limit [1]. However, in the normal phase a meanfield theory is in general not capable to describe two-particle correlations, in particular bound states with finite momentum. This general problem applies also to nuclear matter. Nuclear matter has been treated within the BCS approach by several authors, see e.g., [3][4][5]. Of special interest with regard to two-particle correlations are the pairing in the 3Si channel (see, e.g., [6,7]) and the deuteron formation (see, e.g., [8]). In the normal phase there are works on the improvement of the meanfield approach including bound states and on the composition of nuclear matter (see, e.g., [9,10]). PACS:An attempt to include the influence of correlations on the critical temperature was proposed by Nozieres and SchmittRink for electron-hole systems [1]. The authors calculate the onset of superfluidity as a function of the coupling strength in the framework of BCS theory. The results are combined with a simple extension of the virial expansion to obtain a density formula including correlations in the normal phase. A similar density formula for nuclear matter is given by Schmidt et al. [10]. In the weak coupling limit Nozihres and Schmitt-Rink find the ordinary BCS critical temperature. The other limit, the strong coupling, is characterized by the formation of non-interacting bosonic bound states which can undergo a Bose-Einstein condensation with the corresponding critical temperature. A smooth transition from strong to weak coupling is obtained. The open question how to treat correlations (fluctuations) and pairing (superfluidity) consistently has recently been addressed in a number of publications [11][12][13].Within this paper we want to study the relevance of t...

The in-medium nucleon-nucleon cross section is calculated starting from the thermodynamic Tmatrix 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.