Quartetting Wave Function Approach to 20^{20}Ne: Shell Model and Local Density Approximation

We investigate $\alpha$-like correlations in $^{20}$Ne. A quartet of nucleons (different spin/isospin) is moving in a mean field produced by the $^{16}$O core nucleus. Improving the Thomas-Fermi model (local density approach), a shell model is considered for the core nucleus. The effective potential of the $\alpha$-like quartet and the wave function for the center-of-mass (c.o.m.) motion are calculated and compared with other approaches.Comment: 10 pages, 7 figures. arXiv admin note: substantial text overlap with arXiv:1707.0451

Correlations and Clustering in Dilute Matter

Nuclear systems are treated within a quantum statistical approach. Correlations and cluster formation are relevant for the properties of warm dense matter, but the description is challenging and different approximations are discussed. The equation of state, the composition, Bose condensation of bound fermions, the disappearance of bound states at increasing density because of Pauli blocking are of relevance for different applications in astrophysics, heavy ion collisions, and nuclear structure.Comment: 22 pages, 7 figures, contribution to the special-topics volume on nuclear correlations and cluster physics, edited by W. U. Schr\"ode

Entropy production in open quantum systems: exactly solvable qubit models

We present analytical results for the time-dependent information entropy in exactly solvable two-state (qubit) models. The first model describes dephasing (decoherence) in a qubit coupled to a bath of harmonic oscillators. The entropy production for this model in the regimes of "complete" and "incomplete" decoherence is discussed. As another example, we consider the damped Jaynes-Cummings model describing a spontaneous decay of a two-level system into the field vacuum. It is shown that, for all strengths of coupling, the open system passes through the mixed state with the maximum information entropy.Comment: 9 pages, 3 figure

Quasiparticle light elements and quantum condensates in nuclear matter

Nuclei in dense matter are influenced by the medium. In the cluster mean field approximation, an effective Schr\"odinger equation for the $A$-particle cluster is obtained accounting for the effects of the surrounding medium, such as self-energy and Pauli blocking. Similar to the single-baryon states (free neutrons and protons), the light elements ($2 \le A \le 4$, internal quantum state $\nu$) are treated as quasiparticles with energies $E_{A,\nu}(P; T, n_n,n_p)$ that depend on the center of mass momentum $\vec P$, the temperature $T$, and the total densities $n_n,n_p$ of neutrons and protons, respectively. We consider the composition and thermodynamic properties of nuclear matter at low densities. At low temperatures, quartetting is expected to occur. Consequences for different physical properties of nuclear matter and finite nuclei are discussed.Comment: 5 pages, 1 figure, 2 table

Light clusters in nuclear matter: Excluded volume versus quantum many-body approaches

The formation of clusters in nuclear matter is investigated, which occurs e.g. in low energy heavy ion collisions or core-collapse supernovae. In astrophysical applications, the excluded volume concept is commonly used for the description of light clusters. Here we compare a phenomenological excluded volume approach to two quantum many-body models, the quantum statistical model and the generalized relativistic mean field model. All three models contain bound states of nuclei with mass number A <= 4. It is explored to which extent the complex medium effects can be mimicked by the simpler excluded volume model, regarding the chemical composition and thermodynamic variables. Furthermore, the role of heavy nuclei and excited states is investigated by use of the excluded volume model. At temperatures of a few MeV the excluded volume model gives a poor description of the medium effects on the light clusters, but there the composition is actually dominated by heavy nuclei. At larger temperatures there is a rather good agreement, whereas some smaller differences and model dependencies remain.Comment: 12 pages, 6 figures, published version, minor change

Light nuclei quasiparticle energy shift in hot and dense nuclear matter

Nuclei in dense matter are influenced by the medium. In the cluster mean field approximation, an effective Schr\"odinger equation for the $A$-particle cluster is obtained accounting for the effects of the correlated medium such as self-energy, Pauli blocking and Bose enhancement. Similar to the single-baryon states (free neutrons and protons), the light elements ($2 \le A \le 4$, internal quantum state $\nu$) are treated as quasiparticles with energies $E_{A,\nu}(\vec P; T, n_n,n_p)$. These energies depend on the center of mass momentum $\vec P$, as well as temperature $T$ and the total densities $n_n,n_p$ of neutrons and protons, respectively. No $\beta$ equilibrium is considered so that $n_n, n_p$ (or the corresponding chemical potentials $\mu_n, \mu_p$) are fixed independently. For the single nucleon quasiparticle energy shift, different approximate expressions such as Skyrme or relativistic mean field approaches are well known. Treating the $A$-particle problem in appropriate approximations, results for the cluster quasiparticle shifts are given. Properties of dense nuclear matter at moderate temperatures in the subsaturation density region considered here are influenced by the composition. This in turn is determined by the cluster quasiparticle energies, in particular the formation of clusters at low densities when the temperature decreases, and their dissolution due to Pauli blocking as the density increases. Our finite-temperature Green function approach covers different limiting cases: The low-density region where the model of nuclear statistical equilibrium and virial expansions can be applied, and the saturation density region where a mean field approach is possible

Many Body Theory for Quartets, Trions, and Pairs in Low Density Multi-Component Fermi-Systems

A selfconsistent many body approach for the description of gases with quartets, trions, and pairs is presented. Applications to 3D Fermi systems at low density are discussed