A QUANTUM-FIELD THEORY APPROACH TO SIMPLE LIQUIDS - THE ELEMENTARY EXCITATIONS OF HELIUM-4

In the framework of Quantum Field Theory we propose a novel approach t o the theory of bosonic simple liquids, formulated in terms of a scala r, complex quantum field, the wvave-field Psi (x, t). The problem is t reated in a mean-held approximation, diagonalizing the quadratic part of the Hamiltonian a la Bogoliubov and calculating in a perturbative s cheme the one-loop correction to the self-energy function. We derive t he dispersion relation and the dynamic and static structure factors of the liquid He-4, obtaining results that are successfully compared wit h experimental data.