arXiv subjects

Gentaro Watanabe

Publications and source records attributed to Gentaro Watanabe.

At least 73 records · Page 4Linked to original sources

arXiv2007

Dynamical Simulation of Nuclear "Pasta": Soft Condensed Matter in Dense Stars

More than twenty years ago, it was predicted that nuclei can adopt interesting shapes, such as rods or slabs, etc., in the cores of supernovae and the crusts of neutron stars. These non-spherical nuclei are referred to as nuclear "pasta". In recent years, we have been studying the dynamics of the pasta phases using a method called quantum molecular dynamics (QMD) and have opened up a new aspect of study for this system. Our findings include: dynamical formation of the pasta phases by cooling down the hot uniform nuclear matter; phase diagrams in the density versus temperature plane; structural transitions between the pasta phases induced by compression and elucidation of the mechanism by which they proceed. In the present article, we given an overview of the basic physics and astrophysics of the pasta phases and review our works for readers in other fields.

cond-mat.soft↗

arXiv2007

Impact of nuclear "pasta" on neutrino transport in collapsing stellar cores

Nuclear ``pasta'', nonspherical nuclei in dense matter, is predicted to occur in collapsing supernova cores. We show how pasta phases affect the neutrino transport cross section via weak neutral current using several nuclear models. This is the first calculation of the neutrino opacity of the phases with rod-like and slab-like nuclei taking account of finite temperature effects, which are well described by the quantum molecular dynamics. We also show that pasta phases can occupy 10--20% of the mass of supernova cores in the later stage of the collapse.

astro-ph↗

arXiv2007

Structure of vortices in rotating Bose-Einstein condensates

We calculate the structure of individual vortices in rotating Bose-Einstein condensates in a transverse harmonic trap. Making a Wigner-Seitz approximation for the unit cell of the vortex lattice, we derive the Gross-Pitaevskii equation for the condensate wave function in each cell of the lattice, including effects of varying coarse grained density. We calculate the Abrikosov parameter, the fractional core area, and the energy of individual cells.

cond-mat.other↗

arXiv2006

Understanding nuclear "pasta": current status and future prospects

In cores of supernovae and crusts of neutron stars, nuclei can adopt interesting shapes, such as rods or slabs, etc., which are referred to as nuclear "pasta." In recent years, we have studied the pasta phases focusing on their dynamical aspects with a quantum molecular dynamic (QMD) approach. In this article, we review these works. We also focus on the treatment of the Coulomb interaction.

nucl-th↗

arXiv2006

Non-uniform vortex lattices in inhomogeneous rotating Bose-Einstein condensates

We derive a general framework, in terms of elastic theory, for describing the distortion of the vortex lattice in a rotating Bose-Einstein condensate at arbitrary rotation speed and determining the dependence of the distortion on the density inhomogeneity of the system. In the rapidly rotating limit, we derive the energetics in terms of Landau levels, including excitation to higher levels; the distortion depends on the excitation of higher levels as well as on the density gradient. As we show, the dominant effect of higher Landau levels in a distorted lattice in equilibrium is simply to renormalize the frequency entering the lowest Landau level condensate wave function -- from the transverse trap frequency, $ω$, to the rotational frequency, $Ω$, of the system. Finally, we show how the equilibrium lattice distortion emerges from elastohydrodynamic theory for inhomogeneous systems.

cond-mat.stat-mech↗

arXiv2006

BCS-BEC Crossover in Atomic Fermi Gases with a Narrow Resonance

We determine the effects on the BCS-BEC crossover of the energy dependence of the effective two-body interaction, which at low energies is determined by the effective range. To describe interactions with an effective range of either sign, we consider a single-channel model with a two-body interaction having an attractive square well and a repulsive square barrier. We investigate the two-body scattering properties of the model, and then solve the Eagles-Leggett equations for the zero temperature crossover, determining the momentum dependent gap and the chemical potential self-consistently. From this we investigate the dependence of the crossover on the effective range of the interaction.

cond-mat.supr-con↗

arXiv2006

Breathing mode of rapidly rotating Bose-Einstein condensates

We show that the breathing mode of a rapidly-rotating, harmonically-trapped Bose-Einstein condensate may be described by a generalized lowest Landau level (LLL) wave function, in which the oscillator length is treated as a variable. Using this wave function in a variational Lagrangian formalism, we show that the frequency of the breathing mode for a two-dimensional cloud is $2ω_{\perp}$, where $ω_{\perp}$ is the trap frequency. We also study large-amplitude oscillations and confirm that the above result is not limited to linear oscillations. The resulting mode frequency can be understood in terms of orbits of a single particle in a harmonic trap. The mode frequency is also calculated for a cloud in three dimensions and the result for the axial breathing mode frequency agrees with recent experimental data in the rapid rotation regime.

cond-mat.other↗

arXiv2005

Recent progress on understanding "pasta" phases in dense stars

In cores of supernovae and crusts of neutron stars, nuclei can adopt interesting shapes, such as rods or slabs, etc., which are referred to as nuclear "pasta." Recently, we have been studying the pasta phases focusing on their dynamical aspects with quantum molecular dynamic (QMD) approach. We review our findings on the following topics: dynamical formation of the pasta phases by cooling down the hot uniform nuclear matter; a phase diagram on the density versus temperature plane; structural transitions between the pasta phases induced by compression and their mechanism. Properties of the nuclear interaction used in our works are also discussed.

nucl-th↗

arXiv2005

Simulation of Transitions between "Pasta" Phases in Dense Matter

Calculations of equilibrium properties of dense matter predict that at subnuclear densities nuclei can be rodlike or slablike. To investigate whether transitions between phases with non-spherical nuclei can occur during the collapse of a star, we perform quantum molecular dynamic simulations of the compression of dense matter. We have succeeded in simulating the transitions between rodlike and slablike nuclei and between slablike nuclei and cylindrical bubbles. Our results strongly suggest that non-spherical nuclei can be formed in the inner cores of collapsing stars.

nucl-th↗

arXiv2004

Landau levels and the Thomas-Fermi structure of rapidly rotating Bose-Einstein condensates

We show that, within mean-field theory, the density profile of a rapidly rotating harmonically trapped Bose-Einstein condensate is of the Thomas-Fermi form as long as the number of vortices is much larger than unity. Two forms of the condensate wave function are explored: i) the lowest Landau level (LLL) wave function with a regular lattice of vortices multiplied by a slowly varying envelope function, which gives rise to components in higher Landau levels; ii) the LLL wave function with a nonuniform vortex lattice. From variational calculations we find it most favorable energetically to retain the LLL form of the wave function but to allow the vortices to deviate slightly from a regular lattice. The predicted distortions of the lattice are small, but in accord with recent measurements at lower rates of rotation.

cond-mat.soft↗

arXiv2004

Global structure of Bose-Einstein condensates at high rotation: Beyond the lowest Landau level description

We study the global density profile of a rapidly rotating Bose-Einstein condensate in a harmonic trap with transverse frequency $ω_{\perp}$. By introducing an additional variational degree of freedom to the lowest Landau level wave function, we demonstrate that with increasing strength of the interparticle interaction, the global density profile changes from a Gaussian to the inverted parabolic one characteristic of Thomas-Fermi theory. The criterion for the lowest Landau level wave function to be a good approximation for the global structure is that the mean field energy be small compared with $\hbar ω_{\perp}/N_{\rm v}$, where $N_{\rm v}$ is the number of vortices in the cloud. This condition is more stringent than the requirement that the mean field energy be small compared with $\hbar ω_{\perp}$ which is necessary for the lowest Landau level wave function to be a good approximation to the local structure. Our results show that the lowest Landau level wave function is inappropriate for the global structure of the system realized in recent experiments even though this wave function can describe the local structure well.

cond-mat.soft↗

arXiv2003

Nuclear pasta structure in hot neutron stars

Structure of cold and hot dense matter at subnuclear densities is investigated by quantum molecular dynamics (QMD) simulations. Obtained phase diagrams show that the density of the phase boundaries between the different nuclear structures decreases with increasing temperature due to the thermal expansion of nuclear matter region.

astro-ph↗

arXiv2003

Electron screening in the liquid-gas mixed phases of nuclear matter

Screening effects of electrons on inhomogeneous nuclear matter, which includes spherical, slablike, and rodlike nuclei as well as spherical and rodlike nuclear bubbles, are investigated in view of possible application to cold neutron star matter and supernova matter at subnuclear densities. Using a compressible liquid-drop model incorporating uncertainties in the surface tension, we find that the energy change due to the screening effects broadens the density region in which bubbles and nonspherical nuclei appear in the phase diagram delineating the energetically favorable shape of inhomogeneous nuclear matter. This conclusion is considered to be general since it stems from a model-independent feature that the electron screening acts to decrease the density at which spherical nuclei become unstable against fission and to increase the density at which uniform matter becomes unstable against proton clustering.

nucl-th↗

arXiv2003

Structure of cold nuclear matter at subnuclear densities by Quantum Molecular Dynamics

Structure of cold nuclear matter at subnuclear densities for the proton fraction $x=0.5$, 0.3 and 0.1 is investigated by quantum molecular dynamics (QMD) simulations. We demonstrate that the phases with slablike and rodlike nuclei, etc. can be formed dynamically from hot uniform nuclear matter without any assumptions on nuclear shape, and also systematically analyze the structure of cold matter using two-point correlation functions and Minkowski functionals. In our simulations, we also observe intermediate phases, which has complicated nuclear shapes. It is found out that these phases can be characterized as those with negative Euler characteristic. Our result implies the existence of these kinds of phases in addition to the simple ``pasta'' phases in neutron star crusts and supernova inner cores. In addition, we investigate the properties of the effective QMD interaction used in the present work to examine the validity of our results. The resultant energy per nucleon $ε_{n}$ of the pure neutron matter, the proton chemical $μ_{p}^{(0)}$ in pure neutron matter and the nuclear surface tension $E_{\rm surf}$ are generally reasonable in comparison with other nuclear interactions.

nucl-th↗

arXiv2002

Microscopic Study of Slablike and Rodlike Nuclei: Quantum Molecular Dynamics Approach

Structure of cold dense matter at subnuclear densities is investigated by quantum molecular dynamics (QMD) simulations. We succeeded in showing that the phases with slab-like and rod-like nuclei etc. can be formed dynamically from hot uniform nuclear matter without any assumptions on nuclear shape. We also observe intermediate phases, which has complicated nuclear shapes. Geometrical structures of matter are analyzed with Minkowski functionals, and it is found out that intermediate phases can be characterized as ones with negative Euler characteristic. Our result suggests the existence of these kinds of phases in addition to the simple ``pasta'' phases in neutron star crusts.

nucl-th↗

arXiv2001

Microscopic Study of Nuclear "Pasta" by Quantum Molecular Dynamics

Structure of cold dense stellar matter at subnuclear densities is investigated with quantum molecular dynamics (QMD). We succeeded in showing that the phases with slab-like and rod-like nuclei etc. can be formed dynamically without any assumptions on the nuclear shape. Our result suggests the existence of these kinds of phases in neutron star crusts.

nucl-th↗

arXiv2001

Formation of Nuclear "Pasta" in Cold Neutron Star Matter

Formation and decay of rod-like and slab-like nuclei, as may be encountered in the outer parts of neutron stars, are examined within a zero-temperature stability analysis with respect to perturbations inducing fission and proton clustering. The nonspherical nuclei are found to be stable against these perturbations, implying the possibility that the size of the stellar region containing such nuclei exceeds the equilibrium prediction.

nucl-th↗

arXiv2000

Effects of Neutrino Trapping on Thermodynamic Properties of Nuclear ``Pasta''

Geometrical structure of matter at subnuclear densities is investigated in the presence of a degenerate gas of neutrinos as encountered in stellar collapse. The crystalline phases with spherical, cylindrical and planar nuclei as well as with spherical and cylindrical nuclear bubbles are considered by using a compressible liquid-drop model. This model allows for uncertainties in the lepton fraction $Y_{\rm L}$ in addition to those in the nuclear surface tension $E_{\rm surf}$ and in the proton chemical potential in bulk neutron matter $μ_{\rm p}^{(0)}$. The phase diagrams obtained at zero temperature show that only the phases with rod-like and slab-like nuclei occur at typical values of $Y_{\rm L}$, $E_{\rm surf}$ and $μ_{\rm p}^{(0)}$, whereas the bubble phases, especially with spherical bubbles, are at best expected at hypothetically low values of $Y_{\rm L}$ and/or $E_{\rm surf}$. For the rod-like and slab-like nuclei, thermally induced displacements are calculated from their respective elastic constants. It is found that at temperatures appropriate to supernova cores, thermal fluctuations would destroy the layered lattice of slab-like nuclei almost independently of the nuclear models and of the degree of the neutrino degeneracy.

astro-ph↗