## Abstract

The dynamical density correlation function of liquid helium II at zero temperature is expressed in terms of a static restoring force Ω0 and a polarization operator M within Mori's theory. M is approximated in terms of two-mode decay integrals and Ω0 is related self-consistently to the liquid structure factor. The nonlinear integral equations for M and Ω0 are solved by an interation procedure and the dynamical structure factor S(q,ω) obtained is compared with the experimental results of Cowley and Woods. The elementary excitation spectrum calculated has a roton minimum Δ of 11°K and shows Pitaevskji bending for large momenta. The existence of a resonance of S(q,ω) is found to be the explanation for the measured excitation peak to exceed 2Δ. The variation of the single excitation strength as a function of momentum is analyzed. The multiphonon contribution to S(q,ω) is discussed and the existence of a double-peak structure therein is found for intermediate wave numbers.

Original language | English |
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Pages (from-to) | 3825-3842 |

Number of pages | 18 |

Journal | Physical Review B |

Volume | 13 |

Issue number | 9 |

DOIs | |

State | Published - 1976 |