Static Response Function for Longitudinal and Transverse Excitations in Superfluid Helium

TL;DR

This paper uses the sum rule formalism to establish bounds for the static response functions in superfluid helium, improving upon previous models and aligning closely with experimental data.

Contribution

It introduces rigorous bounds for density and current static response functions in superfluid helium, incorporating ground state properties and interatomic potentials.

Findings

Bounds for density static response closely match experimental data.

Quantitative predictions for transverse current response are provided.

The role of multi-particle excitations is discussed.

Abstract

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