Influence of induced interactions on the superfluid transition in dilute fermi gases

We calculate the effects of induced interactions on the transition temperature to the BCS state in dilute Fermi gases. For a pure Fermi system with two species having equal densities, the transition temperature is suppressed by a factor (4e)(1/3) approximately 2.2, and for nu fermion species, the transition temperature is increased by a factor (4e)(nu/3-1) approximately 2.2(nu-3). For mixtures of fermions and bosons the exchange of boson density fluctuations gives rise to an attractive interaction, and we estimate the increase of the transition temperature due to this effect.     

Nuclear interface energy at finite temperatures

The thermodynamic properties at finite temperatures of the plane interface between two phases of nuclear matter in equilibrium are examined theoretically, and explored numerically. The microscopic hamiltonian, the Skyrme I′ interaction, is used in the Thomas-Fermi approximation to obtain the finite-temperature extensions of earlier zero-temperature results which used the Hartree-Fock and Thomas-Fermi methods. Approximate analytic fits are given to the χ_i (proton fraction on the dense-matter side) dependence of the critical temperature, and to the T and χ_i dependences of the surface thermodynamic potentials, the density of surface neutrons, the surface entropy and the neutron and proton chemical potentials at phase equilibrium. These fits are an ingredient in a compressible liquid-drop nuclear model, the basis of an equation of state for hot, dense matter needed in certain astrophysical applications.The liquid-drop model is used here to construct an isolated, low-T nucleus, whose properties are compared with the original zero-T Hartree-Fock calculations which lead to the Skyrme I interaction, and with other mass formulae. The low-temperature expansion of the surface energy is compared with that obtained in other calculations. The nuclear level density at the Fermi surface, related to the low-T expansion of the entropy of the whole nucleus, is also discussed.     

Phase transitions in cold and warm dense matter

We consider the phase transitions, in dense matter, from nuclei to bubbles and from bubbles to uniform matter. A simplified version of the compressible liquid-drop model allows us to discuss analytically the densities at which the free energies of the different phases are equal, and the density discontinuities of the phases in equilibrium. A reasonable agreement with detailed numerical calculations is obtained only if the compressibility of the matter inside nuclei, and particularly outside bubbles, is taken into account. The dependence of the bubbles-uniform matter transition on the various elements of the Coulomb energy is discussed in detail: the transition is actually a first-order one, but it becomes of second order if the lattice Coulomb energy is turned off.The insight into the effect on the transition of the ratio of surface-plus-Coulomb energy to compression modulus allows us to understand the dependence of the transition densities on temperature and on the microscopic model employed.     

The effect of impurity scattering on the thermally induced charge imbalance in a clean superconductor

We calculate the charge imbalance generated in a current-carrying clean superconductor by a temperature gradient, assuming that non-magnetic or magnetic impurities are the only source of scattering. The charge imbalance is obtained from exact solutions of the quasiparticle Boltzmann equation, valid at any temperature and superfluid velocity.     

Constraints on neutron star radii based on chiral effective field theory interactions

We show that microscopic calculations based on chiral effective field theory interactions constrain the properties of neutron-rich matter below nuclear densities to a much higher degree than is reflected in commonly used equations of state. Combined with observed neutron star masses, our results lead to a radius R=9.7-13.9 km for a 1.4M⊙ star, where the theoretical range is due, in about equal amounts, to uncertainties in many-body forces and to the extrapolation to high densities.