arXiv subjects

Herbert Müther

Publications and source records attributed to Herbert Müther.

11 recordsLinked to original sources

arXiv2018

Pairing in Nuclear Matter and Finite Nuclei

Effects of pairing with isospin $T=0$ and $T=1$ are systematically studied in a model, which is based on a realistic nucleon-nucleon interaction and allows to describe the transition from infinite nuclear matter to finite nuclei. Special attention is paid to the development of the spin-orbit term in the mean field of nucleons in finite nuclei. The spin-orbit term yields a drastic suppression of $T=0$ proton-neutron pairing but does not lead to a complete disappearance in finite nuclei. Arguments are presented, why no clear evidence of $T=0$ pairing can be observed in the binding energies of finite nuclei.

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arXiv2016

Relativistic Effects and Three-Nucleon Forces in Nuclear Matter and Nuclei

We review a large body of predictions obtained within the framework of relativistic meson theory together with the Dirac-Brueckner-Hartree-Fock approach to nuclear matter and finite nuclei. The success of this method has been largely related to its ability to take into account important three-body effects. Therefore, the overarching theme of this article is the interpretation of the so-called "Dirac effects" as an effective three-nucleon force. We address the equation of state of isospin symmetric and asymmetric nucleonic matter and related issues, ranging from proton and neutron density distributions to momentum distributions and short-range correlations. A central part of the discussion is devoted to the optical model potential for nucleon-nucleus scattering. We also take the opportunity to explore similarities and differences with predictions based on the increasingly popular chiral effective field theory.

nucl-th↗

arXiv2007

Vertex renormalization in weak decays of Cooper pairs and cooling compact stars

At temperatures below the critical temperature of superfluid phase transition baryonic matter emits neutrinos by breaking and recombination of Cooper pairs formed in the condensate. The strong interactions in the nuclear medium modify the weak interaction vertices and the associated neutrino loss rates. We study these modifications non-perturbatively by summing infinite series of particle-hole loops in the S-wave superfluid neutron matter. The pairing and particle-hole interactions in neutron matter are described in the framework of the BCS and Fermi-liquid theories derived from microscopic interactions. Consistent with the f-sum rule, the leading order contribution to the polarization tensor arises at O(q^2) in the small momentum transfer, q, expansion. The associated neutrino emission rate is parametrically suppressed compared to its one-loop counterpart by a factor of the order of 5\times 10^{-3}, the parameter being the baryon recoil in units of temperature.

astro-ph↗

arXiv2006

The entropy of a correlated system of nucleons

Realistic nucleon-nucleon interaction induce correlations to the nuclear many-body system which lead to a fragmentation of the single-particle strength over a wide range of energies and momenta. We address the question of how this fragmentation affects the thermodynamical properties of nuclear matter. In particular, we show that the entropy can be computed with the help of a spectral function which can be evaluated in terms of the self-energy obtained in the Self-Consistent Green's Function approach. Results for the density and temperature dependences of the entropy per particle for symmetric nuclear matter are presented and compared to the results of lowest order finite temperature Brueckner--Hartree--Fock calculations. The effects of correlations on the calculated entropy are small, if the appropriate quasi-particle approximation is used. The results demonstrate the thermodynamical consistency of the self-consistent T-matrix approximation for the evaluation of the Green's functions.

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arXiv2006

Anomalous specific heat jump in a two-component ultracold Fermi gas

The thermodynamic functions of a Fermi gas with spin population imbalance are studied in the temperature-asymmetry plane in the BCS limit. The low temperature domain is characterized by anomalous enhancement of the entropy and the specific heat above their values in the unpaired state, decrease of the gap and eventual unpairing phase transition as the temperature is lowered. The unpairing phase transition induces a second jump in the specific heat, which can be measured in calorimetric experiments. While the superfluid is unstable against a supercurrent carrying state, it may sustain a metastable state if cooled adiabatically down from the stable high-temperature domain. In the latter domain the temperature dependence of the gap and related functions is analogous to the predictions of the BCS theory.

cond-mat.supr-con↗

arXiv2006

Deformed Fermi surfaces in ultracold Fermi gases

The superfluid transition in an ultracold two-component atomic Fermi gas is analyzed in the case where the two components have different densities. We describe a superfluid state which spontaneously breaks the rotational-symmetry by deforming the Fermi surfaces of both species into ellipsoidal form. At relatively large hyperfine-spin asymmetries, this deformation is shown to help the appearance of pairing, which in the rotationally-symmetric (BCS) case would be forbidden by Pauli blocking. The prospects for experimental detection of such a deformed Fermi surface phase are discussed.

cond-mat.supr-con↗

arXiv2005

Alpha matter on a lattice

We obtain the equation of state of interacting alpha matter and the critical temperature of Bose-Einstein condensation of alpha particles within an effective scalar field theory. We start from a non-relativistic model of uniform alpha matter interacting with attractive two-body and repulsive three-body potentials and reformulate this model as a O(2) symmetric scalar ϕ^6 field theory with negative quartic and positive sextic interactions. Upon restricting the Matsubara sums, near the temperature of Bose-Einstein condensation, to the zeroth order modes we further obtain an effective classical theory in three spatial dimensions. The phase diagram of the alpha matter is obtained from simulations of this effective field theory on a lattice using local Monte-Carlo algorithms.

nucl-th↗

arXiv2005

Sum rules of single-particle spectral functions in hot asymmetric nuclear matter

The neutron and proton single-particle spectral functions in asymmetric nuclear matter fulfill energy weighted sum rules. The validity of these sum rules within the self-consistent Green's function approach is investigated. The various contributions to these sum rules and their convergence as a function of energy provide information about correlations induced by the realistic interaction between the nucleons. These features are studied as a function of the asymmetry of nuclear matter.

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arXiv2005

Pairing in a two-component ultracold Fermi gas: phases with broken space symmetries

We explore the phase diagram of a two-component ultracold atomic Fermi gas interacting with zero-range forces in the limit of weak-coupling. We focus on the dependence of the pairing gap and the free energy on the variations in the number densities of the two species while the total density of the system is held fixed. As the density asymmetry is increased, the system exhibits a transition from a homogenous Bardeen-Cooper-Schrieffer (BCS) phase to phases with spontaneously broken global space symmetries. One such realization is the deformed Fermi surface superfluidity (DFS) which exploits the possibility of deforming the Fermi-surfaces of the species into ellipsoidal form at zero total momentum of Cooper pairs. The critical asymmetries at which the transition from the DFS to the unpaired state occurs are larger than those for the BCS phase. In this precritical region the DFS phase lowers the condensation energy of the asymmetric BCS state. We compare quantitatively the DFS phase to another realization of superconducting phases with broken translational symmetry - the single plane wave Larkin-Ovchinnikov-Fulde-Ferrell (LOFF)phase, which is characterized by a non-vanishing center-of-mass momentum of the Cooper pairs. The possibility of the detection of the DFS phase in the time-of-flight experiments is discussed and quantified for the case of $^6$Li atoms trapped in two different hyperfine states.

cond-mat.supr-con↗

arXiv2004

Phases of asymmetric nuclear matter with broken space symmetries

Isoscalar Cooper pairing in isospin asymmetric nuclear matter occurs between states populating two distinct Fermi surfaces, each for neutrons and protons. The transition from a BCS-like to the normal (unpaired) state, as the isospin asymmetry is increased, is intervened by superconducting phases which spontaneously break translational and rotational symmetries. One possibility is the formation of a condensate with a periodic crystallinelike structure where Cooper pairs carry net momentum (the nuclear Larkin-Ovchinnikov-Fulde-Ferrell-phase). Alternatively, perturbations of the Fermi surfaces away from spherical symmetry allow for minima in the condensate free energy which correspond to a states with quadrupole deformations of Fermi surfaces and zero momentum of the Cooper pairs. In a combined treatment of these phases we show that, although the Cooper pairing with finite momentum might arise as a local minimum, the lowest energy state features are deformed Fermi surfaces and Cooper pairs with vanishing total momentum.

nucl-th↗

arXiv2004

Pairing in nuclear systems with effective Gogny and V_low-k interactions

The pairing properties of nuclear systems are a sensitive probe of the effective nucleon-nucleon interactions. We compare the 1S0 pairing gaps in nuclear and neutron matter derived from the phenomenological Gogny interaction and a renormalization group motivated low-momentum V_low-k interaction extracted from realistic interactions. We find that the pairing gaps predicted by these interactions are in an excellent agreement in a wide range of sub-nuclear densities. The close agreement between the predictions of the effective forces remains intact in the case where the single particle spectra in neutron and nuclear matter are renormalized within the Hartree-Fock theory.

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