Collective Particle-hole Excitations Research Articles

Bosonization of Fermionic Many-Body Dynamics

We consider the quantum many-body evolution of a homogeneous Fermi gas in three dimensions in the coupled semiclassical and mean-field scaling regime. We study a class of initial data describing collective particle-hole pair excitations on the Fermi ball. Using a rigorous version of approximate bosonization, we prove that the many-body evolution can be approximated in Fock space norm by a quasifree bosonic evolution of the collective particle-hole excitations.

Beyond 132Sn: Examples of new data on exotic neutron-rich Te isotopes from fission and β-decay

Exotic nuclei beyond the 132Sn double shell-closure have both single-particle and collective particlehole excitations and are expected to have competing excitation patterns from both type of excitations together with possible structural changes. We are, therefore, studying the region in the close vicinity beyond 132Sn and further with the neutron increase with experimental methods such as induced fission and β decay. A short overview of this knowledge will be given together with examples of newly obtained data at preliminary stage.

New nuclear structure data beyond 136Sn

Exotic nuclei beyond the 132Sn double shell-closure are influenced by both the Sn superfluity and the evolving collectivity only few nucleons away. Toward even more neutron-rich nuclei, especially at intermediate mass number, the interplay between single-particle and collective particle-hole excitations competes. In some cases with the extreme addition of neutrons also other effects as the formation of neutron skin, stabilization as sub-shell gaps or orbital crossings may be expected. The knowledge of nuclear ingredients is especially interesting beyond 132Sn and little is known on how the excitation modes develop with the addition of both protons and neutrons and for example systematic prompt and decay studies can be such very sensitive probe. Recently, we have approached this region of nuclei in several experimental measurements following 238U projectile fission on 9Be and n-induced fission on 241Pu and 235U. Consistent data analysis allows to access various spins and excitation energies and provide new input to theory. Examples from these studies on several nuclei in the A~140 region were presented during the conference together with the possible interpretation of the new data. Here, we will illustrate one example on 136I using two complementary data sets.

Polarons and molecules in a Fermi gas with orbital Feshbach resonance

We study the impurity problem in a gas of $^{173}$Yb atoms near the recently discovered orbital Feshbach resonance. In an orbital Feshbach resonance, atoms in the electronic ground state $^1S_0$ interact with those in the long-lived excited $^3P_0$ state with magnetically tunable interactions. We consider an impurity atom with a given hyperfine spin in the $^3P_0$ state interacting with a single-component Fermi sea of atoms in the ground $^1S_0$ manifold. Close to the orbital Feshbach resonance, the impurity can induce collective particle-hole excitations out of the Fermi sea, which can be regarded as the polaron state. While as tuning toward the BEC regime of the resonance, a molecular state becomes the ground state of the system. We show that a polaron to molecule transition exists in $^{173}$Yb atoms close to the orbital Feshbach resonance. Furthermore, due to the spin-exchange nature of the orbital Feshbach resonance, the formation of both the polaron and the molecule involve spin-flipping processes with interesting density distributions among the relevant hyperfine spin states. We show that the polaron to molecule transition can be detected using Raman spectroscopy.

Universal trimers emerging from a spin-orbit-coupled Fermi sea

We report the existence of a universal trimer state induced by an impurity interacting with a two-component spin-orbit-coupled Fermi gas in two dimensions. In the zero-density limit with a vanishing Fermi sea, the trimer is stabilized by the symmetry of the single-particle spectrum under spin-orbit coupling, and is therefore universal against the short-range details of the interaction potential. When the Fermi energy increases, we show that the trimer is further stabilized by particle-hole fluctuations over a considerable parameter region. We map out the phase diagram consisting of trimers, dimers, and polarons, and discuss the detection of these states using radio-frequency spectroscopy. The universal trimer revealed in our work is a direct manifestation of intriguing three-body correlations emerging from a many-body environment, which, in our case, is cooperatively supported by the single-particle spectral symmetry and the collective particle-hole excitations.

Are the nuclei beyond 132Sn very exotic?

The term exotic nucleus is used for nuclei that have different from normal behavior. However, it turns out that the term normal is valid only for nuclei close to stability and more particularly for regions close to double-shell closures. As long as one drives away in the neutron-rich nuclei, especially at intermediate mass number, interplay between normal single-particle and many collective particle-hole excitations compete. In some cases with the addition of neutrons, these may turn to evolve as a skin, acting against the core nucleus that may also influence its shell evolution. Knowledge of these nuclear ingredients is especially interesting beyond the doubly-magic 132Sn, however a little is known on how the excitations modes develop with the addition of both protons and neutrons. Especially for the Sb nuclei, where one gradually increases these valence particles, the orbital evolution and its impact on exoticness is very intriguing. Experimental studies were conducted on several such isotopes using isomer and, β-decay spectroscopy at RIBF within EURICA. In particular, new data on 140Sb and 136Sb are examined and investigated in the framework of shell model calculations.

Alpha-particle scattering and isoscalar giant resonances in the SU(3) model.

Giant isoscalar monopole and quadrupole resonances are introduced into the SU(3) limit of the interacting boson model by means of S and D bosons simulating collective particle-hole excitations, in addition to the usual s and d bosons, representing either collective particle or hole pairs in the valence shell. The evaluated giant resonance energies, as well as the matrix elements of E0 and E2 transitions from the giant resonance levels to the ${0}_{1}^{+\mathrm{\ensuremath{-}}}$${2}_{1}^{+\mathrm{\ensuremath{-}}}$${4}_{1}^{+}$ members of the ground-state band, are utilized in coupled-channel calculations of giant resonance excitation through inelastic alpha scattering by the transitional isotopes $^{148}\mathrm{Sm}$ and $^{154}\mathrm{Sm}$: the calculated differential cross sections are then compared with experimental data.

Effects of spreading widths upon the direct nuclear reaction continuum

Simple concepts which can be directly applied to measurements of resonances and the accompanying continua have been studied. Pickup reaction spectra leading to hole states in the continuum were found to give an understanding of the excitation energy dependence of resonance damping and the concurrent rise of "background" continuum states. When these hole resonances are parametrized in terms of the shell model, they can be used to predict the energy dependence of the damping of more complex resonances, such as collective particle-hole excitations in the inelastic scattering spectra. Furthermore, it is proposed that part of the background continuum in pickup spectra is the semidirect excitation of the 2h-1p states, the density of which determines both the nuclear damping width and the continuum cross section.NUCLEAR REACTIONS $^{208}\mathrm{Pb}(^{3}\mathrm{He}, \ensuremath{\alpha})$, $E=75$ MeV, $^{209}\mathrm{Bi}(p, d)$, $E=62$ MeV; calculated continuum cross sections in a direct reaction.

Investigation of single-particle core-excitation coupling in the strontium and lead regions

The nuclei neighbouring 88Sr and 208Pb are treated as a core plus an additional particle or hole interacting with the core vibrations. With the help of the Green function technique the core vibrations are described as collective particle-hole excitations thus depending on the single-particle propagators. An iterative calculation has been performed for 88Sr, showing that the coupling of single-particle and collective modes is very important. Because of angular momentum coupling and parity selection rules the coupling possibilities are strongly reduced in the lead region.

Higher-order corrections to collective particle-hole excitations using green function techniques

Using Green function techniques, we determine higher-order corrections to the Tamm-Dancoff and Random Phase Approximation equations. In particular, we find that the second-order corrections to the Tamm-Dancoff equations consist of a t-matrix term and a core-polarization term. If we truncate our configuration space, we obtain an additional second-order correction from the particle-hole ladder term. We carry out numerical calculations of these second-order corrections for the negative-parity excited states of 4He using the Tabakin separable potential.