Anisotropic pressure in dense neutron matter under the presence of a strong magnetic field

Dense neutron matter with recently developed BSk19 and BSk21 Skyrme effective forces is considered in magnetic fields up to $10^{20}$ G at zero temperature. The breaking of the rotational symmetry by the magnetic field leads to the differentiation between the pressures along and perpendicular to the field direction which becomes significant in the fields $H>H_{th}\sim10^{18}$ G. The longitudinal pressure vanishes in the critical field $10^{18}<H_c\lesssim10^{19}$ G, resulting in the longitudinal instability of neutron matter. For the Skyrme force fitted to the stiffer underlying equation of state (BSk21 vs. BSk19) the threshold $H_{th}$ and critical $H_c$ magnetic fields become larger. The longitudinal and transverse pressures as well as the anisotropic equation of state of neutron matter are determined under the conditions relevant for the cores of magnetars.Comment: 7 pages, 3 figures; published online 7 December 201

Correlation functions in BCS-BEC transition region of nuclear matter

The density, spin and isospin correlation functions in nuclear matter with a neutron-proton (np) condensate are calculated to study the possible signatures of the BEC-BCS crossover in the low-density region. It is shown that the criterion of the crossover (Phys. Rev. Lett. 2005, v. 95, 090402), consisting in the change of the sign of the density correlation function at low momentum transfer, fails to describe correctly the density-driven BEC-BCS transition at finite isospin asymmetry or finite temperature. As an unambiguous signature of the BEC-BCS transition, there can be used the presence (BCS regime) or absence (BEC regime) of the singularity in the momentum distribution of the quasiparticle density of states.Знайдені кореляційні функції за щільністю, спіном та ізоспіном в ядерній матерії з нейтрон-протонним конденсатом з метою вивчення можливих ознак БЕК-БКШ переходу в області низьких густин. Показано, що критерій переходу (Phys. Rev. Lett. 2005, v. 95, 090402), який полягає у зміні знака кореляційної функції за щільністю при малих переданих імпульсах, є неадекватним для опису БЕК-БКШ-переходу при скінченої асиметрії або температурі. Відповідним однозначним критерієм є наявність (БКШ) або відсутність (БЕК) особливості в імпульсному розподілі щільності станів квазічастинок.Получены корреляционные функции по плотности, спину и изоспину в ядерной материи с нейтрон-протонным конденсатом с целью изучения возможных признаков БКШ-БЭК перехода в области малых плотностей. Показано, что критерий перехода (Phys. Rev. Lett. 2005, v. 95, 090402), состоящий в изменении знака корреляционной функции по плотности при малых переданных импульсах, является неадекватным для описания БКШ-БЭК-перехода при конечной асимметрии или температуре. Соответствующим однозначным критерием является наличие (БКШ) или отсутствие (БЭК) особенности в импульсном распределении плотности состояний квазичастиц

Pairing effects in low density domain of nuclear matter

Using equations, governing np pairing correlations in S=1, T=0 pairing channel (PRC 63 (2001) 021304(R)), it is shown that at low densities equations for the energy gap in the spectrum of quasiparticles and chemical potentials of protons and neutrons allow solutions with negative chemical potential. This corresponds to appearance of Bose--Einstein condensate (BEC) of deuterons in low density region of nuclear matter.Comment: 4 pages, 3 figure

Density, spin and isospin correlations in low-density two-component Fermi superfluid

Finding the distinct features of the crossover from the regime of large overlapping Cooper pairs to the limit of non-overlapping pairs of fermions (Shafroth pairs) in multi-component Fermi systems remains a topical problem in a quantum many-body theory. Here this transition is studied by calculating the two-body density, spin and isospin correlation functions in dilute two-component Fermi superfluid, taking as an example an infinite system of protons and neutrons (nuclear matter). It is shown that criterion of the crossover (Phys. Rev. Lett. 95, 090402 (2005)), formulated for ultracold fermionic atomic gases and consisting in the change of the sign of the density correlation function at low momentum transfer, fails to describe correctly the density-driven BEC–BCS transition at finite isospin asymmetry or finite temperature. As an unambiguous signature of the BEC–BCS transition, one can use the presence (BCS regime) or absence (BEC regime) of the singularity in the momentum distribution of the quasiparticle density of states

Magnetization of Dense Neutron Matter in a Strong Magnetic Field

Spin polarized states in neutron matter at a strong magnetic field up to ¹⁸ G are considered in the model with the Skyrme effective interaction. Analyzing the self-consistent equations at zero temperature, it is shown that a thermodynamically stable branch of solutions for the spin polarization parameter as a function of the density corresponds to the negative spin polarization when the majority of neutron spins are oriented oppositely to the direction of the magnetic field. In addition, beginning from some threshold density dependent on the magnetic field strength, the self-consistent equations have also two other branches of solutions for the spin polarization parameter with the positive spin polarization. The free energy corresponding to one of these branches turns out to be very close to the free energy corresponding to the thermodynamically preferable branch with the negative spin polarization. As a consequence, at a strong magnetic field, the state with the positive spin polarization can be realized as a metastable state at the high density region in neutron matter which changes into a thermodynamically stable state with the negative spin polarization with decrease in the density at some threshold value. The calculations of the neutron spin polarization parameter, energy per neutron, and chemical potentials of spin-up and spin-down neutrons as functions of the magnetic field strength show that the influence of the magnetic field remains small at the field strengths up to 10¹⁷ G.Розглянуто спiновi поляризованi стани в нейтроннiй матерiї у сильних магнiтних полях до 10¹⁸ Гс у моделi з ефективною взаємодiєю Скiрма. На пiдставi аналiзу рiвнянь самоузгодження при нульовiй температурi показано, що термодинамiчно стiйка гiлка розв’язкiв для параметра спiнової поляризацiї як функцiї густини вiдповiдає вiд’ємнiй спiновiй поляризацiї, коли бiльшiсть нейтронних спiнiв орiєнтується протилежно магнiтному полю. Крiм цього, починаючи з деякої граничної густини, що залежить вiд напруженостi магнiтного поля, рiвняння самоузгодження мають також двi iнших гiлки розв’язкiв для параметра спiнової поляризацiї з додатною спiновою поляризацiєю. Вiльна енергiя, що вiдповiдає однiй iз цих гiлок, виявляється дуже близькою до вiльної енергiї, що вiдповiдає термодинамiчно стiйкiй гiлцi з вiд’ємною спiновою поляризацiєю. Як наслiдок, у сильному магнiтному полi стан з додатною спiновою поляризацiєю може реалiзовуватися як метастабiльний стан за високими густинами нейтронної матерiї, який змiнюється на термодинамiчний стiйкий стан з вiд’ємною спiновою поляризацiєю при зменшеннi густини, починаючи з деякої граничної густини

Superfluidity of a condensate with np pairing correlations in asymmetric nuclear matter

Influence of asymmetry on superfluidity of nuclear matter with triplet-singlet pairing of nucleons (in spin and isospin spaces) is considered within the framework of a Fermi-liquid theory. Solutions of self-consistent equations for the energy gap at T=0 are obtained. It is shown, that if the chemical potentials of protons and neutrons are determined in the zero gap width approximation, then the energy gap for some values of density and asymmetry parameter of nuclear matter demonstrates double-valued behavior. However, with account for the feedback of pairing correlations through the normal distribution functions of nucleons two-valued behavior of the energy gap turns into universal one-valued behavior. At T=0 the energy gap has a discontinuities as a function of density in a narrow layer model. These discontinuities depend on the asymmetry parameter

Inter-Modular Linkers play a crucial role in governing the biosynthesis of non-ribosomal peptides

Motivation: Non-ribosomal peptide synthetases (NRPSs) are modular enzymatic machines that catalyze the ribosome-independent production of structurally complex small peptides, many of which have important clinical applications as antibiotics, antifungals and anti-cancer agents. Several groups have tried to expand natural product diversity by intermixing different NRPS modules to create synthetic peptides. This approach has not been as successful as anticipated, suggesting that these modules are not fully interchangeable. Results: We explored whether Inter-Modular Linkers (IMLs) impact the ability of NRPS modules to communicate during the synthesis of NRPs. We developed a parser to extract 39 804 IMLs from both well annotated and putative NRPS biosynthetic gene clusters from 39 232 bacterial genomes and established the first IMLs database. We analyzed these IMLs and identified a striking relationship between IMLs and the amino acid substrates of their adjacent modules. More than 92% of the identified IMLs connect modules that activate a particular pair of substrates, suggesting that significant specificity is embedded within these sequences. We therefore propose that incorporating the correct IML is critical when attempting combinatorial biosynthesis of novel NRPS. Availability and implementation: The IMLs database as well as the NRPS-Parser have been made available on the web at https://nrps-linker.unc.edu. The entire source code of the project is hosted in GitHub repository (https://github.com/SWFarag/nrps-linker). Supplementary information: Supplementary data are available at Bioinformatics online

BCS and BEC p-wave pairing in Bose-Fermi gases

The pairing of fermionic atoms in a mixture of atomic fermion and boson gases at zero temperature is investigated. The attractive interaction between fermions, that can be induced by density fluctuations of the bosonic background, can give rise to a superfluid phase in the Fermi component of the mixture. The atoms of both species are assumed to be in only one internal state, so that the pairing of fermions is effective only in odd-l channels. No assumption about the value of the ratio between the Fermi velocity and the sound velocity in the Bose gas is made in the derivation of the energy gap equation. The gap equation is solved without any particular "ansatz" for the pairing field or the effective interaction. The p-wave superfluidity is studied in detail. By increasing the strength and/or decreasing the range of the effective interaction a transition of the fermion pairing regime, from the Bardeen-Cooper-Schrieffer state to a system of tightly bound couples can be realized. These composite bosons behave as a weakly-interacting Bose-Einstein condensate.Comment: 14 pages, 6 eps-figures. To be published in European Physical Journal

Superfluid states with finite momentum of Cooper pairs in nuclear matter

Superfluid states of symmetric nuclear matter with finite total momentum of Cooper pairs (nuclear LOFF phase) are studied with the use of Fermi–liquid theory in the model with Skyrme effective forces. It is considered the case of four–fold splitting of the excitation spectrum due to finite superfluid momentum and coupling of T = 0 and T = 1 pairing channels. It has been shown that at zero temperature the energy gap in triplet–singlet (TS) pairing channel (in spin and isospin spaces) for the SkM∗ force demonstrates double–valued behavior as a function of superfluid momentum. As a consequence, the phase transition at the critical superfluid momentum from the LOFF phase to the normal state will be of a first order. Behavior of the energy gap as a function of density for TS pairing channel under increase of superfluid momentum changes from one–valued to universal two–valued. It is shown that two–gap solutions, describing superposition of states with singlet–triplet (ST) and TS pairing of nucleons appear as a result of branching from one–gap ST solution. Comparison of the free energies shows that the state with TS pairing of nucleons is thermodynamically most preferable