Quantum fluctuations induce collective multiphonons in finite Fermi liquids

TL;DR

This paper demonstrates that quantum fluctuations in finite Fermi liquids lead to the emergence of collective multiphonon states, with a nearly harmonic spectrum matching experimental observations in atomic nuclei.

Contribution

It introduces an extended nuclear time-dependent density-functional theory that accounts for quantum fluctuations and trajectory interference, revealing multiphonon states beyond the independent-particle approximation.

Findings

Collective multiphonon states emerge at high excitation energies.

The spectrum of these states is nearly harmonic and matches experimental data.

Quantum fluctuations are essential for the formation of multiphonon states.

Abstract

We show that collective multiphonon states in atomic nuclei emerge at high excitation energies when quantum fluctuations in the collective space are included beyond the independent-particle approximation. The quadrupole response of a nucleus is studied using an extension of the nuclear time-dependent density-functional theory that mixes several many-body trajectories. While a single trajectory can account for the excitation of the first collective quantum, the second and the third quanta emerge due to the interference between trajectories. The collective spectrum, found as nearly harmonic, is in excellent agreement with the experimentally observed three quanta of the isoscalar giant quadrupole resonance in $^{40}$Ca. This study offers guidance for multiphonon searches in other self-bound systems and demonstrates the resistance to internal excitation of finite Fermi liquids.