2-particle-2-hole excitations in strongly interacting Fermi systems

Full text: Proceeding from a least-action principle, the equations of motion for time-dependent two-particle-two-hole correlations are developed. These equations of motion are a generalization of the random-phase approximation (RPA). Thus, they not only describe of elementary excitations in strongly interacting Fermi systems like liquid 3He, nuclear matter, neutron star matter, but they contain new physics beyond the RPA: they contain mode-coupling effects and finite-lifetime of excitations and are, therefore, suitable for understanding, for example, the phonon-spectrum on 3He and 3He-4He mixtures. The equations of the theory involve matrix elements of the Hamiltonian and identity operators in a nonorthogonal basis of states incorporating short-range correlations. The linear response of the dynamically correlated system due to a weak external perturbation is also investigated. (author)