Static Response Function for Longitudinal and Transverse Excitations in Superfluid Helium

摘要

The sum rule formalism is used to evaluate rigorous bounds for the densityand current static response functions in superfluid helium at zero temperature.Both lower and upper bounds are considered. The bounds are expressed in termsof ground state properties (density and current correlation funtions) and ofthe interatomic potential. The results for the density static responsesignificantly improve the Feynman approximation and turn out to be close to theexperimental (neutron scattering) data. A quantitative prediction for thetransverse current response is given. The role of one-phonon and multi-particleexcitations in the longitudinal and transverse channels is discussed.(Phys.Rev.B, in press)