Spontaneous symmetry breaking and response functions

Abstract We study the quantum phase transition occurring in an infinite homogeneous system of spin 1/2 fermions in a non-relativistic context. As an example we consider neutrons interacting through a simple spin–spin Heisenberg force. The two critical values of the coupling strength—signaling the onset into the system of a finite magnetization and of the total magnetization, respectively—are found and their dependence upon the range of the interaction is explored. The spin response function of the system in the region where the spin-rotational symmetry is spontaneously broken is also studied. For a ferromagnetic interaction the spin response along the direction of the spontaneous magnetization occurs in the particle-hole continuum and displays, for not too large momentum transfers, two distinct peaks. The response along the direction orthogonal to the spontaneous magnetization displays instead, beyond a softened and depleted particle-hole continuum, a collective mode to be identified with a Goldstone boson of type II. Notably, the random phase approximation on a Hartree–Fock basis accounts for it, in particular for its quadratic—close to the origin—dispersion relation. It is shown that the Goldstone boson contributes to the saturation of the energy-weighted sum rule for ≈25% when the system becomes fully magnetized (that is in correspondence of the upper critical value of the interaction strength) and continues to grow as the interaction strength increases.