Superfluid Model Research Articles

Resonance suppression of the r -mode instability in superfluid neutron stars: Accounting for muons and entrainment

We calculate the finite-temperature r-mode spectrum of a superfluid neutron star accounting for both muons in the core and the entrainment between neutrons and protons. We show that the standard perturbation scheme, considering the rotation rate as an expansion parameter, breaks down in this case. We develop an original perturbation scheme which circumvents this problem by treating both the perturbations due to rotation and (weak) entrainment simultaneously. Applying this scheme, we propose a simple method for calculating the superfluid r-mode eigenfrequency in the limit of vanishing rotation rate. We also calculate the r-mode spectrum at finite rotation rate for realistic microphysics input (adopting, however, the Newtonian framework and Cowling approximation when considering perturbed oscillation equations) and show that the normal r-mode exhibits resonances with superfluid r-modes at certain values of temperatures and rotation frequencies in the parameter range relevant to neutron stars in low-mass X-ray binaries (LMXBs). This turns the recently suggested phenomenological model of resonance r-mode stabilization into a quantitative theory, capable of explaining observations. A strong dependence of resonance rotation rates and temperatures on the neutron superfluidity model allows us to constrain the latter by confronting our calculations with the observations of neutron stars in LMXBs.

Estimations of level density parameters by using artificial neural network for phenomenological level density models

The main aim of this study is to develop accurate artificial neural network (ANN) algorithms to estimate level density parameters. An efficient Bayesian-based algorithm is presented for classification algorithms. Unknown model parameters are estimated using the observed data, from which the Bayesian-based algorithm is predicted. This paper focuses on the Bayesian method for parameter estimations of Gilbert Cameron Model (GCM), Back Shifted Fermi Gas Model (BSFGM) and Generalised Super Fluid Model (GSM), which are known as the phonemological level density models. Obtained level density parameters have been compared with the Reference Input Parameter Library for Calculation of Nuclear Reactions and Nuclear Data Evaluations (RIPL) data. R values of the Bayesian method have been found as 0.9946, 0.9981 and 0.9824 for BSFGM, GCM and GSM, respectively. In order to validate our results, default level density parameters of TALYS 1.95 code have been changed with our newly obtained results and photo-neutron cross-section calculations of the 117Sn(γ,n)116Sn, 118Sn(γ,n)117Sn, 119Sn(γ,n)118Sn and 120Sn(γ,n)119Sn reactions have been calculated by using these newly obtained level density parameters.

The Two-Fluid Model of Superfluid in Quantum Turbulent State―Strange Phenomena Caused by the Interaction between Quantized Vortices and a Thermal Component

The Two-Fluid Model of Superfluid in Quantum Turbulent State―Strange Phenomena Caused by the Interaction between Quantized Vortices and a Thermal Component

Scaling Behavior for the Susceptibility of the Vacuum

Using the two-component superfluid model of Winterberg for space, two models for the susceptibility of the cosmic vacuum as a function of the cosmic scale parameter, a, are presented. We also consider the possibility that Newton’s constant can scale, i.e., G-1=G-1(a), to form the most general scaling laws for polarization of the vacuum. The positive and negative values for the Planckion mass, which form the basis of the Winterberg model, are inextricably linked to the value of G, and as such, both G and Planck mass are intrinsic properties of the vacuum. Scaling laws for the non-local, smeared, cosmic susceptibility, , the cosmic polarization, , the cosmic macroscopic gravitational field, , and the cosmic gravitational field mass density, , are worked out, with specific examples. At the end of recombination, i.e., the era of last scattering, using the polarization to explain dark matter, and the gravitational field mass density to explain dark energy, we find that, . While this is an unconventional assignment, differing from the ΛCDM model, we believe this is correct, as localized dark matter (LDM) contributions can be much higher in this epoch than cosmic smeared values for susceptibility. All density parameter assignments in Friedmanns’ equation are cosmic averages, valid for distance scales in excess of 100 Mpc in the current epoch. We also evaluate the transition from ordinary matter dominance, to dark matter dominance, for the cosmos as a whole. We obtain for the transition points, z=1.66, for susceptibility model I, and, z=2.53 , for susceptibility model II.

Global Regularity of the 2D HVBK Equations

The Hall-Vinen-Bekharevich-Khalatnikov (HVBK) equations are a macroscopic model of superfluidity at non-zero temperatures. For smooth, compactly supported data, we prove the global well-posedness of strong solutions to these equations in $\mathbb{R}^2$, in the incompressible and isothermal case. The proof utilises a contraction mapping argument to establish local well-posedness for high-regularity data, following which we demonstrate global regularity using an analogue of the Beale-Kato-Majda criterion in this context. In the appendix, we address the sufficient conditions on a 2D vorticity field, in order to have a finite kinetic energy.

The holographic p + ip solution failed to win the competition in dRGT massive gravity

In this paper, the holographic p-wave superfluid model with charged complex vector field is studied in dRGT massive gravity beyond the probe limit. The stability of p-wave and p + ip solutions are compared in the grand canonical ensemble. The p-wave solution always get lower value of grand potential than the p + ip solution, showing that the holographic system still favors an anisotropic (p-wave) solution even with considering a massive gravity theory in bulk. In the holographic superconductor models with dRGT massive gravity in bulk, a key scaling symmetry is found to be violated by fixing the reference metric parameter c_0. Therefore, in order to get the dependence of condensate and grand potential on temperature, different values of horizon radius should be considered in numerical work. With a special choice of model parameters, we further study the dependence of critical back-reaction strength on the graviton mass parameter, beyond which the superfluid phase transition become first order. We also give the dependence of critical temperature on the back reaction strength b and graviton mass parameter m^2.

Constraining Neutron Superfluidity with R-Mode Physics.

We constrain the parameters of neutron superfluidity in the cores of neutron stars making use of the recently proposed effect of resonance stabilization of r modes. To this end, we, for the first time, calculate the finite-temperature r-mode spectra for realistic models of rotating superfluid neutron stars, accounting for both muons and neutron-proton entrainment in their interiors. We find that the ordinary (normal) r mode exhibits avoided crossings with superfluid r modes at certain stellar temperatures and spin frequencies. Near the avoided crossings, the normal r mode dissipates strongly, which leads to substantial suppression of the r-mode instability there. The extreme sensitivity of the positions of avoided crossings to the neutron superfluidity model allows us to constrain the latter by confronting the calculated spectra with observations of rapidly rotating neutron stars in low-mass x-ray binaries.

Bose–Einstein condensation of dilute alpha clusters above the four-α threshold in 16O in the field theoretical superfluid cluster model

Abstract Observed well-developed $\alpha$ cluster states in $^{16}$O located above the four-$\alpha$ threshold are investigated from the viewpoint of Bose–Einstein condensation of $\alpha$ clusters by using a field-theoretical superfluid cluster model in which the order parameter is defined. The experimental energy levels are reproduced well for the first time by calculation. In particular, the observed 16.7 MeV $0_7^+$ and 18.8 MeV $0_8^+$ states with low-excitation energies from the threshold are found to be understood as a manifestation of the states of the Nambu–Goldstone zero-mode operators, associated with the spontaneous symmetry-breaking of the global phase, which is caused by the Bose–Einstein condensation of the vacuum 15.1 MeV $0^+_6$ state with a dilute well-developed $\alpha$ cluster structure just above the threshold. This gives evidence of the existence of the Bose–Einstein condensate of $\alpha$ clusters in $^{16}$O. It is found that the emergence of the energy level structure with a well-developed $\alpha$ cluster structure above the threshold is robust, almost independently of the condensation rate of $\alpha$ clusters under significant condensation rate. The finding of the mechanism that causes the level structure that is similar to $^{12}$C to emerge above the four-$\alpha$ threshold in $^{16}$O reinforces the concept of Bose–Einstein condensation of $\alpha$ clusters in addition to $^{12}$C.

Holographic boiling and generalized thermodynamic description beyond local equilibrium

Tuning a very simple two-component holographic superfluid model, we can have a first order phase transition between two superfluid phases in the probe limit. In- spired by the potential landscape discussion, an intuitive physical picture for systems with first order phase transitions is provided. We stress that holography perfectly offers a generalized thermodynamic description of certain strongly coupled systems even out of local equilibrium, which enables us to carefully study domain wall structures of the system under first order phase transitions, either static or in real time dynamics. We numerically construct the 1D domain wall configuration and compute the surface tension of the domain wall from its generalized grand potential. We also numerically simulate the real time dynamics of a 2D bubble nucleation process (holographic boiling). The surface tension of the 1D domain wall nicely matches the final state of the 2D bubble nucleation process when the bubble radius is large enough.

Is there a spectral turnover in the spin noise of millisecond pulsars?

ABSTRACT Pulsar timing arrays provide a unique means to detect nanohertz gravitational waves through long-term measurements of pulse arrival times from an ensemble of millisecond pulsars. After years of observations, some timing array pulsars have been shown to be dominated by low-frequency red noise, including spin noise that might be associated with pulsar rotational irregularities. The power spectral density of pulsar timing red noise is usually modelled with a power law or a power law with a turnover frequency below which the noise power spectrum plateaus. If there is a turnover in the spin noise of millisecond pulsars, residing within the observation band of current and/or future pulsar timing measurements, it may be easier than projected to resolve the gravitational-wave background from supermassive binary black holes. Additionally, the spectral turnover can provide valuable insights on neutron star physics. In the recent study by Melatos and Link, the authors provided a derivation of the model for power spectral density of spin noise from superfluid turbulence in the core of a neutron star, from first principles. The model features a spectral turnover, which depends on the dynamical response time of the superfluid and the steady-state angular velocity lag between the crust and the core of the star. In this work, we search for a spectral turnover in spin noise using the first data release of the International Pulsar Timing Array. Through Bayesian model selection, we find no evidence of a spectral turnover. Our analysis also shows that data from PSRs J1939+2134, J1024–0719, and J1713+0747 prefers the power-law model to the superfluid turbulence model.

Collapse of spherical overdensities in superfluid models of dark matter

Aims. We intend to understand cosmological structure formation within the framework of superfluid models of dark matter with finite temperatures. Of particular interest is the evolution of small-scale structures where the pressure and superfluid properties of the dark matter fluid are prominent. We compare the growth of structures in these models with the standard cold dark matter paradigm and non-superfluid dark matter. Methods. The equations for superfluid hydrodynamics were computed numerically in an expanding ΛCDM background with spherical symmetry; the effect of various superfluid fractions, temperatures, interactions, and masses on the collapse of structures was taken into consideration. We derived the linear perturbation of the superfluid equations, giving further insights into the dynamics of the superfluid collapse. Results. We found that while a conventional dark matter fluid with self-interactions and finite temperatures experiences a suppression in the growth of structures on smaller scales, as expected due to the presence of pressure terms, a superfluid can collapse much more efficiently than was naively expected due to its ability to suppress the growth of entropy perturbations and thus gradients in the thermal pressure. We also found that the cores of the dark matter halos initially become more superfluid during the collapse, but eventually reach a point where the superfluid fraction falls sharply. The formation of superfluid dark matter halos surrounded by a normal fluid dark matter background is therefore disfavored by the present work.

Resolving cosmological singularity problem in logarithmic superfluid theory of physical vacuum

A paradigm of the physical vacuum as a non-trivial quantum object, such as superfluid, opens an entirely new prospective upon origins and interpretations of Lorentz symmetry and spacetime, black holes, cosmological evolution and singularities. Using the logarithmic superfluid model, one can formulate a post-relativistic theory of superfluid vacuum, which is not only essentially quantum but also successfully recovers special and general relativity in the “phononic” (low-momenta) limit. Thus, it represents spacetime as an induced observer-dependent phenomenon. We focus on the cosmological aspects of the logarithmic superfluid vacuum theory and show how can the related singularity problem be resolved in this approach.

Normal Charge Densities in Quantum Critical Superfluids.

The normal density of a translation-invariant superfluid often vanishes at zero temperature, as is observed in superfluid Helium and conventional superconductors described by BCS theory. Here we show that this need not be the case. We investigate the normal density in models of quantum critical superfluids using gauge-gravity duality. Models with an emergent infrared Lorentz symmetry lead to a vanishing normal density. On the other hand, models which break the isotropy between time and space may enjoy a nonvanishing normal density, depending on the spectrum of irrelevant deformations around the underlying quantum critical ground state. Our results may shed light on recent measurements of the superfluid density and low energy spectral weight in superconducting overdoped cuprates.

Enhancement of α -particle formation near Sn100

The superfluid tunneling model is applied to the calculation of ground-state--to--ground-state \ensuremath{\alpha} decay in the even-even neutron-deficient Te-Ba nuclei. We show that there is a larger \ensuremath{\alpha}-particle formation probability in nuclei of this region above $^{100}\mathrm{Sn}$ when compared to analogous nuclei above $^{208}\mathrm{Pb}$. This is consistent with the expected systematic variation of the pair gap $\mathrm{\ensuremath{\Delta}}$ as a function of mass number. The recent experimental data on the \ensuremath{\alpha} decay of the $N=Z$ nuclei $^{104}\mathrm{Te}$ and $^{108}\mathrm{Xe}$ are shown to leave open the possibility of enhanced \ensuremath{\alpha}-particle formation involving nucleon correlations beyond the standard treatment of like-nucleon pairing, which is the mechanism suggested as underlying ``superallowed'' \ensuremath{\alpha} decay.

Thermo-fluid analyses for UCN cryogenic system

The TUCAN collaboration is now developing the new cryogenic system for ultra-cold neutrons experiments (UCN). This cryogenic system requires superfluid helium (He II) around 0.8 K to 1.15 K because cold neutrons are down-scattered by isopure He II with such temperature range to become UCN. Dynamic heat load applied to He II during UCN production will be as high as approximately 10 W. In order to satisfy such requirements, not only 4He but also 3He has to be circulated through the cryogenic system. There are sixteen heat exchangers for 4He and 3He. In order to design these heat exchangers properly, thermo-fluid simulations are carried out. Some simulations of them are nontrivial because they include film boiling phenomena in superfluid, molecular gas dynamics effect, transition from single phase flow to two phase flow and discrimination of such kinds of the complicated flow behavior. In addition, Time dependent thermo-fluid behaviors of He-II are also simulated using two fluid model and superfluid turbulent model to clarify the He II behavior. In this presentation, the simulation schemes and some results will be discussed.

Analytical study of holographic p-wave superfluid models in Gauss\u2013Bonnet gravity

In Gauss–Bonnet gravity, we analytically investigate the p-wave superfluid models in five dimensional AdS soliton and AdS black hole in order to explore the influences of the higher curvature correction on the holographic superfluid phase transition. We observe that the analytical findings are in good agreement with the numerical computations. Our results show that the critical chemical potential of the system increases with the increase of the Gauss–Bonnet parameter in AdS soliton background, while the critical temperature decreases as the Gauss–Bonnet factor grows if the phase transition of the system is of the second order in AdS black hole background, both of which indicate that the higher curvature correction hinders the formation of the condensation of the vector operator. Moreover, the critical exponent of the system takes the mean-field value 1/2, which is independent of the Gauss–Bonnet parameter and the spatial component of the gauge field.

Nodal line topological superfluid and multiply protected Majorana Fermi arc in a three-dimensional time-reversal-invariant superfluid model

We theoretically study a time-reversal-invariant three-dimensional superfluid model by stacking in z direction identical bilayer models with intralayer spin–orbit coupling and contrary Zeeman energy splitting for different layers, which has been suggested recently to realize two-dimensional time-reversal-invariant topological superfluid. We find that this model shows two kinds of topologically nontrivial phases: gapless phases with nodal lines in pairs protected by chiral symmetry and a gapped phase, both of which support a time-reversal-invariant Majorana Fermi arc (MFA) on the yz and xz side surface. These MFA abide by time-reversal and particle-hole symmetries and are topologically protected by the winding numbers in mirror subspaces and the Z2 numbers of two-dimensional DIII class topological superfluid, which are different from MFA in the time-reversal broken Weyl superfluid protected by nonzero Chern numbers. This important observation means that MFA in our model represents a new type of topological state not explored previously. The Zeeman field configuration in our model is relevant to the antiferromagnetic topological insulator MnBi2Te4, thus our work stimulates the further studies on superconducting effects in the realistic antiferromagnetic topological insulator.

Generation of vortices and stabilization of vortex lattices in holographic superfluids

Within the simplest holographic superfluid model and without any ingredient put by hand, it is shown that vortices can be generated when the angular velocity of rotating superfluids exceeds certain critical values, which can be precisely determined by linear perturbation analyses (quasi-normal modes of the bulk AdS black brane). These vortices appear at the edge of the superfluid system first, and then automatically move into the bulk of the system, where they are eventually stabilized into certain vortex lattices. For the case of 18 vortices generated, we find (at least) five different patterns of the final lattices formed due to different initial perturbations, which can be compared to the known result for such lattices in weakly coupled Bose-Einstein condensates from free energy analyses.

Superfluid vacuum theory and deformed dispersion relations

Using the logarithmic superfluid model of physical vacuum, one can formulate an essentially quantum post-relativistic theory, which successfully recovers Einstein’s theory of relativity in low-momenta limit, but otherwise has different foundations and predictions. We present an analytical example of the dispersion relation and show that it should have a Landau form which ensures the suppression of dissipative fluctuations. We show that in the low-momentum sector of the theory, a dispersion relation becomes relativistic with small deformations, such that a photon acquires effective mass, but a much more complex picture arises at large momenta.

Holographic p-wave superfluid in the AdS soliton background with RF2 corrections

We investigate the holographic p-wave superfluid in the background metric of the AdS soliton with RF2 corrections. Two models, namely, the Maxwell complex vector field model and Yang-Mills theory, are studied in the above context by employing the Sturm-Liouville approach as well as the shooting method. When turning on the spatial components of the gauge field, one observes that, in the probe limit, the inclusion of RF2 corrections hinders the superfluid phase transition. On the other hand, however, in the absence of the superfluid velocity, it is found that the RF2 corrections lead to distinct effects for the two models. Regardless of either the RF2 correction or the spatial component of the gauge field, the phase transition of the system is observed to be always of the second order. Moreover, a linear relationship between the charge density and chemical potential is largely established near the critical point in both holographic superfluid models.