Overview of Neutron-Proton Pairing

The role of neutron-proton pairing correlations on the structure of nuclei along the $N=Z$ line is reviewed. Particular emphasis is placed on the competition between isovector ($T=1$) and isoscalar $(T=0$) pair fields. The expected properties of these systems, in terms of pairing collective motion, are assessed by different theoretical frameworks including schematic models, realistic Shell Model and mean field approaches. The results are contrasted with experimental data with the goal of establishing clear signals for the existence of neutron-proton ($np$) condensates. We will show that there is clear evidence for an isovector $np$ condensate as expected from isospin invariance. However, and contrary to early expectations, a condensate of deuteron-like pairs appears quite elusive and pairing collectivity in the $T=0$ channel may only show in the form of a phonon. Arguments are presented for the use of direct reactions, adding or removing an $np$ pair, as the most promising tool to provide a definite answer to this intriguing question.Comment: 89 pages, 59 figures. Accepted for publication in Progress in Particle and Nuclear Physics (ELSEVIER

Partial-wave contributions to pairing in nuclei

We present a detailed study of partial-wave contributions of nuclear forces to pairing in nuclei. For T=1, J=0 pairing, partial waves beyond the standard 1S0 channel play an interesting role for the pair formation in nuclei. The additional contributions are dominated by the repulsive 3P1 partial wave. Their effects, and generally spin-triplet nuclear forces between paired nucleons, are influenced by the interplay of spin-orbit partners. We explore the impact of including partial waves beyond the 1S0 channel on neutron-neutron pairing gaps in semi-magic isotopic chains. In addition, we show that nuclear forces favor T=1, J=0 over T=0, J=1 pairing, except in low-j orbitals. This is in contrast to the free-space motivation that suggests the formation of deuteron-like T=0 pairs in N=Z nuclei. The suppression of T=0 pairing is because the 3S1 strength is distributed on spin-orbit partners and because of the effects of the repulsive 1P1 channel and of D waves.Comment: 10 pages, 16 figure

Geometry of the shears mechanism in nuclei

5 pages, 3 figures, accepted for publication in Physical Review C, Rapid CommunicationThe geometry of the shears mechanism in nuclei is derived from the nuclear shell model. This is achieved by taking the limit of large angular momenta (classical limit) of shell-model matrix elements

Properties of isocalar-pair condensates

It is pointed out that the ground state of n neutrons and n protons in a single-j shell, interacting through an isoscalar (T=0) pairing force, is not paired, J=0, but rather spin-aligned, J=n. This observation is explained in the context of a model of isoscalar P (J=1) pairs, which is mapped onto a system of p bosons, leading to an approximate analytic solution of the isoscalar-pairing limit in jj coupling.Comment: 7 pages, 3 figures, 1 tabl

Plate kinematics of the central Atlantic during the Oligocene and early Miocene

A new plate motions model for the northwest Africa–North America Plate pair during the Oligocene and early Miocene is presented. The model is accompanied by a high-resolution isochron map for the central Atlantic region, resulting from a re-examination of 423 ship tracks from the NGDC data base for the area between the 15°20′ FZ and the Azores triple junction. A new digital model of fracture zones for this region and a set of 309 magnetic profiles crossing the Oligocene to recent oceanic crust within the study area allowed to determine accurate finite reconstruction poles for the North America–northwest Africa conjugate plate pair between the early Miocene (Chron 6) and the early Oligocene (Chron 13). For times older than Chron 7 (∼25 Ma), the finite reconstruction poles were calculated using a reliable data set coming exclusively from the region south of the Canary Islands FZ (∼32°N), which allowed to test the rigidity of the northwest African oceanic lithosphere during the Oligocene–early Miocene phase of Atlas orogeny. A comparison of theoretical magnetic isochrons with observed magnetic lineations systematically shows that anomalously high spreading rates occurred in the area north of the Canary Islands FZ before Chron 7, thereby suggesting that the formation of the Atlas mountain, rather than being a localized intracontinental process, was logically linked to the central Atlantic spreading history. Thus, an independent Moroccan Plate could have existed during the Oligocene–early Miocene time interval, which included both the oceanic lithosphere north of the Canary Islands FZ and the northern Maghrebian areas of Morocco, Algeria and Tunisia. In this eventuality, the Atlas mountain belt should be reinterpreted as a giant flower structure associated with dextral transpression

Recent kinematics of the tectonic plates surrounding the Red Sea and Gulf of Aden

The Red Sea and Gulf of Aden represent two young basins that formed between Africa and Arabia since the early Oligocene, floored by oceanic crust or by transitional and thinned continental crust. While in the easternmost Gulf of Aden the rift–drift transition can be dated chron C6 (~20.1 Ma), here we show that in the Red Sea the first pulse of sea floor spreading occurred during chron C3n.2n (~4.6 Ma) around ~17.1°N (present–day coordinates) and propagated southwards from this location, separating the Danakil microplate from Arabia. It is also shown that sea floor spreading between Arabia and Nubia started later, around chron 2A (~2.58 Ma), and propagated northwards. At present, there is no magnetic evidence for the existence of a linear spreading center in the northern Red Sea at latitudes higher than ~24°N and in the southern Red Sea below ~14.8°N. The present–day plate kinematics of this region can be described with high accuracy by a network of five interacting plates (Nubia, Arabia, Somalia, Sinai, and Danakil) and six triple junctions. For times older than anomaly 2A (~2.58 Ma) and up to anomaly 3, the absence of marine magnetic anomalies between Arabia and Nubia prevents a rigorous kinematic description of the five–plates system. However, there is strong evidence that the unique changes in plate motions during the last five Myrs were a dramatic slowdown at chron C2 (~1.77 Ma) in the spreading or extension rates along the ridge and rift axes, thereby a good representation of the real plate motions can be obtained anyway by backward extension of the oldest Arabia – Nubia and Arabia – Danakil stage rotations determined on the basis of marine magnetic anomalies, respectively C2 – C2A and C2A – C3. The proposed kinematic reconstructions are accompanied by a geodynamic explanation for the genesis of large continent–continent fracture zones at the rift–drift transition and by an analysis of the strain associated with plate motions in Afar, northeastern Egypt, and Sinai

On the linear term in the nuclear symmetry energy

The nuclear symmetry energy calculated in the RPA from the pairing plus symmetry force Hamiltonian with equidistant single-nucleon levels is for mass number A=48 approximately proportional to T(T+1.025), where T is the isospin quantum number. An isovector character of the pair field assumed to produce the observed odd-even mass staggering is essential for this result. The RPA contribution to the symmetry energy cannot be simulated by adding to the Hartree-Fock-Bogolyubov energy a term proportional to the isospin variance in the Bogolyubov quasiparticle vacuum, and there are significant corrections to the approximation which consist in adding half the isocranking angular velocity. The present calculation employs a smaller single-nucleon level spacing than used in a previous investigation of the model.Comment: 8 pages. Minor inaccuracies and misspellings correcte