Computing nuclear response functions with time-dependent coupled-cluster theory

TL;DR

This paper introduces a method to compute nuclear response functions using time-dependent coupled-cluster theory, capturing correlations and collective excitations in nuclei, validated on helium and oxygen isotopes, and exploring responses under strong electric fields.

Contribution

The paper develops a novel time-dependent coupled-cluster approach to calculate nuclear response functions, incorporating correlations beyond mean-field and analyzing nuclear dynamics under external fields.

Findings

Accurate computation of electric dipole responses in $^4$He and $^{16}$O.

Observation of collective proton-neutron oscillations in $^{16}$O and $^{24}$O.

Spectral behavior under strong electric fields aligns with previous mean-field results.

Abstract

We compute nuclear response functions by solving the time-dependent A-body Schr\"odinger equation, recording the time-dependent transition moment and extracting spectral information via Fourier transforms. The solution of the time-dependent many-body problem accounts for correlations on top of the mean field by taking advantage of a time-dependent formulation of coupled-cluster theory. As a validation, we focus on electric dipole transitions in $^4$He and $^{16}$O and compare moments of the response function distribution to the results of an equivalent static framework, finding negligible discrepancies. We investigate how proton and neutron densities evolve in time, and we see the traditional picture of soft and giant dipole resonances as collective oscillations of protons and neutrons emerging from our calculations in $^{16}$O and $^{24}$O. This method also allows us to investigate the behavior of the nucleus in the presence of a strong electric field. In that regime, the behavior of the system becomes chaotic. Qualitatively, the spectral information obtained in this limit is in line with previous time-dependent mean-field results.