arXiv subjects

Gerd Roepke

Publications and source records attributed to Gerd Roepke.

8 recordsLinked to original sources

arXiv2026

Stellar heavy-element slope index

The distribution of heavy elements in stars is described using a phenomenological approach, in which Lagrange parameters related to temperature and the chemical potentials of protons and neutrons are introduced within a freeze-out concept. Slope parameters are considered which describe the gross behavior of the distribution of the heavy elements. Universality and deviations from universality are discussed, and various examples are provided. These slope parameters may be of interest for characterizing the conditions under which heavy elements form, but the astrophysical sites where heavy elements are produced remain to be determined.

astro-ph.SR↗

arXiv2019

Alpha Decay to Doubly Magic Core in Quartetting Wave Function Approach

We present a microscopic calculation of $α$-cluster formation in heavy nuclei $^{104}$Te ($α$+$^{100}$Sn), $^{212}$Po ($α$+$^{208}$Pb) and their neighbors $^{102}$Sn, $^{102}$Te, $^{210}$Pb and $^{210}$Po by using the quartetting wave function approach. Improving the local density approximation, the shell structure of the core nucleus is considered, and the center-of-mass (c.o.m.) effective potential for the quartet is obtained self-consistently from the shell model wavefunctions. The $α$-cluster formation and decay probabilities are obtained by solving the bound-state of the c.o.m. motion of the quartet and the scattering state of the formed $α$-cluster in the Gurvitz approach. Striking shell effects on the $α$-cluster formation probabilities are analyzed for magic numbers 50, 82 and 126. The computed $α$-decay half-lives of these special nuclei are compared with the newest experimental data.

nucl-th↗

arXiv2018

The BCS-BEC crossover: From ultra-cold Fermi gases to nuclear systems

This report adresses topics and questions of common interest in the fields of ultra-cold gases and nuclear physics in the context of the BCS-BEC crossover. The BCS-BEC crossover has recently been realized experimentally, and essentially in all of its aspects, with ultra-cold Fermi gases. This realization, in turn, has raised the interest of the nuclear physics community in the crossover problem, since it represents an unprecedented tool to test fundamental and unanswered questions of nuclear many-body theory. Here, we focus on the several aspects of the BCS-BEC crossover, which are of broad joint interest to both ultra-cold Fermi gases and nuclear matter, and which will likely help to solve in the future some open problems in nuclear physics (concerning, for instance, neutron stars). Similarities and differences occurring in ultra-cold Fermi gases and nuclear matter will then be emphasized, not only about the relative phenomenologies but also about the theoretical approaches to be used in the two contexts. After an introduction to present the key concepts of the BCS-BEC crossover, this report discusses the mean-field treatment of the superfluid phase, both for homogeneous and inhomogeneous systems, as well as for symmetric (spin- or isospin-balanced) and asymmetric (spin- or isospin-imbalanced) matter. Pairing fluctuations in the normal phase are then considered, with their manifestations in thermodynamic and dynamic quantities. The last two Sections provide a more specialized discussion of the BCS-BEC crossover in ultra-cold Fermi gases and nuclear matter, respectively. The separate discussion in the two contexts aims at cross communicating to both communities topics and aspects which, albeit arising in one of the two fields, share a strong common interest.

cond-mat.quant-gas↗

arXiv2017

Fermi liquid, clustering, and structure factor in dilute warm nuclear matter

Properties of nuclear systems at subsaturation densities can be obtained from different approaches. We demonstrate the use of the density autocorrelation function which is related to the isothermal compressibility and, after integration, to the equation of state. This way we connect the Landau Fermi liquid theory well elaborated in nuclear physics with the approaches to dilute nuclear matter describing cluster formation. A quantum statistical approach is presented, based on the cluster decomposition of the polarization function. The fundamental quantity to be calculated is the dynamic structure factor. Comparing with the Landau Fermi liquid theory which is reproduced in lowest approximation, the account of bound state formation and continuum correlations gives the correct low-density result as described by the second virial coefficient and by the mass action law (nuclear statistical equilibrium). Going to higher densities, the inclusion of medium effects is more involved compared with other quantum statistical approaches, but the relation to the Landau Fermi liquid theory gives a promising approach to describe not only thermodynamic but also collective excitations and non-equilibrium properties of nuclear systems in a wide region of the phase diagram.

nucl-th↗

arXiv2017

Status of Alpha-Particle Condensate Structure of the Hoyle State

The present understanding of the structure of the Hoyle state in 12C is reviewed. Most of the theoretical approaches to the Hoyle state are shortly summarized. The corresponding results are analyzed with respect to whether they give evidence to the alpha particle condensation structure of the Hoyle state (and other Hoyle-like states in heavier self-conjugate nuclei) or not.

nucl-th↗

arXiv2017

Alpha particle clusters and their condensation in nuclear systems

In this article we review the present status of alpha clustering in nuclear systems. First of all, an important aspect is condensation in nuclear matter. Second, the alpha container model, recently been proposed by Tohsaki-Horiuchi-Schuck-Roepke (THSR), will be outlined and the ensuing condensate aspect of the Hoyle state at 7.65 MeV in 12C will be investigated in some detail. After 15 years since the proposal of the alpha condensation concept a critical assessment of this idea will be given. Alpha gas states in other nuclei like 16O and 13C will be considered. The THSR wave function can also describe configurations of one alpha particle on top of a doubly magic core. The cases of 20Ne and 212Po will be investigated.

nucl-th↗

arXiv2008

Appearance of Light Clusters in Post-bounce Evolution of Core-Collapse Supernovae

We explore the abundance of light clusters in core-collapse supernovae at post-bounce stage in a quantum statistical approach. Adopting the profile of a supernova core from detailed numerical simulations, we study the distribution of light bound clusters up to alpha particles (A=2-4) as well as heavy nuclei (A > 4) in dense matter at finite temperature. Within the frame of a cluster-mean field approach, the abundances of light clusters are evaluated accounting for self-energy, Pauli blocking and effects of continuum correlations. We find that deuterons and tritons, in addition to 3He and 4He, appear abundantly in a wide region from the surface of the proto-neutron star to the position of the shock wave. The appearance of light clusters may modify the neutrino emission in the cooling region and the neutrino absorption in the heating region, and, thereby, influence the supernova mechanism.

astro-ph↗

arXiv2006

Excitonic versus electron-hole liquid phases in Tm[Se,Te] compounds

We discuss, from a theoretical point of view, excitonic phases at the pressure induced semiconductor-semimetal transition in $\rm TmSe_{0.45}Te_{0.55}$,focusing, in particular, on the stability against an electron-hole liquid. The electron-hole pair density parameter $r_s(E_g,T)$ is calculated within the quasi-static plasmon pole approximation as a function of temperature $T$ and energy gap $E_g$ and converted into $-E_g(r_s,T)$. A comparison of this quantity, which is the electron-hole pair chemical potential, with the exciton binding energy reveals that excitons should be suppressed in contrast to experimental evidence for excitonic phases. We suspect therefore inter-valley exciton scattering and exciton-phonon scattering to substantially stabilise excitons in $\rm TmSe_{0.45}Te_{0.55}$.

cond-mat.str-el↗