arXiv subjects

Tomoya Isoshima

Publications and source records attributed to Tomoya Isoshima.

18 recordsLinked to original sources

arXiv2006

Effect of Quadratic Zeeman Energy on the Vortex of Spinor Bose-Einstein Condensates

The spinor Bose-Einstein condensate of atomic gases has been experimentally realized by a number of groups. Further, theoretical proposals of the possible vortex states have been sugessted. This paper studies the effects of the quadratic Zeeman energy on the vortex states. This energy was ignored in previous theoretical studies, although it exists in experimental systems. We present phase diagrams of various vortex states taking into account the quadratic Zeeman energy. The vortex states are calculated by the Gross-Pitaevskii equations. Several new kinds of vortex states are found. It is also found that the quadratic Zeeman energy affects the direction of total magnetization and causes a significant change in the phase diagrams.

cond-mat.other↗

arXiv2005

Vortex oscillations in confined Bose-Einstein condensate interacting with 1D optical lattice

We study Bose-Einstein condensate of atomic Boson gases trapped in a composite potential of a harmonic potential and an optical lattice potential. We found a series of collective excitations that induces localized vortex oscillations with a characteristic wavelength. The oscillations might be observed experimentally when radial confinement is tight. We present the excitation spectra of the vortex oscillation modes and propose a way to experimentally excite the modes.

cond-mat.other↗

arXiv2004

Quantum Numbers for Excitations of Bose-Einstein Condensates in 1D Optical Lattices

The excitation spectrum and the band structure of a Bose-Einstein condensate in a periodic potential are investigated. Analyses within full 3D systems, finite 1D systems, and ideal periodic 1D systems are compared. We find two branches of excitations in the spectra of the finite 1D model. The band structures for the first and (part of) the second band are compared between a finite 1D and the fully periodic 1D systems, utilizing a new definition of a effective wavenumber and a phase-slip number. The upper and lower edges of the first gap coincide well between the two cases. The remaining difference is explained by the existence of the two branches due to the finite-size effect.

cond-mat.soft↗

arXiv2004

Precessional motion of a vortex in a finite-temperature Bose-Einstein condensate

We study the precessing motion of a vortex in a Bose-Einstein condensate of atomic gases. In addition to the former zero-temperature studies, finite temperature systems are treated within the Popov and semiclassical approximations. Precessing vortices are discussed utilizing the rotating frame of reference. The relationship between the sign of the lowest excitation energy and the direction of precession is discussed in detail.

cond-mat.soft↗

arXiv2003

Instabilities of off-centered vortices in a Bose-Einstein condensate

We study numerically the excitations of off-centered vortices in a Bose-Einstein condensate. The displacement of a single vortex and the separation of a doubly quantized vortex are considered. We find that the core-localized excitations of the precessing vortices continue to feature the property which implies that the vortices are unstable. The core-localized, dipolar, and quadrupolar excitations are found to deform continuously as functions of vortex displacements and intervortex separation.

cond-mat.soft↗

arXiv2002

Axisymmetric Vortices in Spinor Bose-Einstein Condensates under Rotation

The relative stability of various axisymmetric vortices in a spinor Bose-Einstein condensate with F=1 is examined within extended Bogoliubov theory. This yields the phase diagram in the plane of external rotation frequency vs magnetization. We compare antiferromagnetic, nonmagnetic, and ferromagnetic cases. The excitation spectrum is evaluated under rotation to investigate the local stability of the possible vortices and the vortex nucleation frequency.

cond-mat.soft↗

arXiv2001

Stability of a Vortex in Spinor Bose-Einstein Condensate

We propose a method to create a vortex in BEC utilizing the spin degree of freedom. We consider the optical plug at the center of the vortex, which makes the vortex-creation process stable. We also investigate the instability of the halfway state to complete vortex state without the optical plug.

cond-mat.soft↗

arXiv2001

Vortex phase diagram in trapped Bose-Einstein condensation

The vortex phase diagram in the external rotation frequency versus temperature is calculated for dilute Bose-Einstein condensed gases. It is determined within the Bogoliubov-Popov theory for a finite temperature where the condensate and non-condensate fractions are treated in an equal footing. The temperature dependences of various thermodynamic instability lines for the vortex nucleation are computed to construct the phase diagram. Experiments are proposed to resolve a recent controversy on the vortex creation problem associated with the quantized vortex observation in $^{87}$Rb atom gases.

cond-mat.soft↗

arXiv2001

Quantum Vortex in a Vectorial Bose-Einstein Condensate

Quantum vortices in the multi-component Bose-Einstein condensation (BEC) are investigated theoretically. It is found that three kinds of the vortex configurations are possible and their physical properties are discussed in details, including the density distribution and the spin texture. By using the Bogoliubov theory extended to the three component BEC, the collective modes for these vortices are evaluated. The local vortex stability for these vortices are examined in light of the existence of the negative eigenvalue, yielding a narrow magnetization window for the local intrinsic stable region where the multi-components work together to stabilize a vortex in a self-organized way.

cond-mat.soft↗

arXiv2000

Double Phase Transitions in Magnetized Spinor Bose-Einstein Condensation

It is investigated theoretically that magnetized Bose-Einstein condensation (BEC) with the internal (spin) degrees of freedom exhibits a rich variety of phase transitions, depending on the sign of the interaction in the spin channel. In the antiferromagnetic interaction case there exist always double BEC transitions from single component BEC to multiple component BEC. In the ferromagnetic case BEC becomes always unstable at a lower temperature, leading to a phase separation. The detailed phase diagram for the temperature vs the polarization, the spatial spin structure, the distribution of non-condensates and the excitation spectrum are examined for the harmonically trapped systems.

cond-mat.soft↗

arXiv1999

Creation of persistent current and vortex in a Bose-Einstein condensate of alkali-metal atoms

It is shown theoretically that a persistent current can be continuously created in a Bose-Einstein condensate (BEC) of alkali atoms confined in a multiply connected region by making use of a spin-degree of freedom of the order parameter of a BEC. We demonstrate that this persistent current is easily transformed into a vortex. Relaxation processes of these BEC after the confining field is turned off are also studied so that our analyses are compared with time of flight experiments. The results are shown to clearly reflect the existence of a persistent current.

cond-mat.soft↗

arXiv1999

A simple method to create a vortex in Bose-Einstein condensate of alkali atoms

Bose-Einstein condensation in alkali atoms has materialized quite an interesting system, namely a condensate with a spin degree of freedom. In analogy with the A-phase of the superfluid $^3$He, numerous textures with nonvanishing vorticity have been proposed. In the present paper, interesting properties of such spin textures are analyzed. We propose a remarkably simple method to create a vortex state of a BEC in alkali atoms.

cond-mat↗

arXiv1999

Spin domain formation in spinor Bose-Einstein condensation

The spatial structure of the spinor Bose-Einstein condensates with the spin degrees of freedom is analyzed based on the generalized Gross-Pitaevskii equation (GP) in the light of the present spin domain experiment on m_F=\pm 1, and 0 of the hyperfine state F=1 of ^{23}Na atom gases. The GP solutions in three- and one-spatial dimensional cases reproduce the observed spin domain structures, revealing the length scale associated with the existence of the weak interaction of the spin-spin channel, other than the ordinary coherence length related to the density-density channel. The obtained domain structure in GP is compared with the result in Thomas-Fermi approximation. The former solution is found to better describe the observed features than the latter.

cond-mat.soft↗

arXiv1999

Instability of Non-vortex State toward a Quantized Vortex in Bose-Einstein Condensate under External Rotation

The instability condition of the non-vortex state toward vortex formation is exa mined within the Bogoliubov theory when a Bose-Einstein condensate is under exte rnally forced rotation. The obtained critical angular velocity combined with the previous stability cond itions for a votex yields a detailed phase diagram in the critical velocity vs t he system parameter. This facilitates vortex formation experiments for alkali atom gases confined in a harmonic potential.

cond-mat.soft↗

arXiv1998

Vortex Stabilization in Dilute Bose-Einstein Condensate Under Rotation

The stability of a quantized vortex state in Bose-Einstein condensation is examined within Bogoliubov theory for alkali atom gases confined in a harmonic potential under forced rotation. By solving the non-linear Bogoliubov equations coupled with the Gross-Pitavskii equation, the elementary excitations and the total energy of the systems are calculated as a function of rotation velocity. There are two distinct criteria of vortex stability; The position of the excitation energy levels relative to the condensate energy level yields the local stability criterion, and the total energy relative to that of the non-vortex state yields the global stability criterion. The vortex stability phase diagram in the rotation velocity vs the particle density of the system is obtained, allowing one to locate the appropriate region to observe the singly quantized vortex.

cond-mat.soft↗

arXiv1998

Vortex stabilization in Bose-Einstein condensate of alkali atom gas

A quantized vortex in the Bose-Einstein condensation (BEC), which is known to be unstable intrinsically, is demonstrated theoretically to be stabilized by the finite temperature effect. The mean-field calculation of Popov approximation within the Bogoliubov theory is employed, giving rise to a self-consistent solution for BEC confined by a harmonic potential. Physical origin of this vortex stabilization is investigated. An equivalent effect is also proved to be induced by an additional pinning potential at the vortex center produced by a focused laser beam even at the lowest temperature. The self-consistent solutions give detailed properties of a stable vortex, such as the spatial profiles of the condensate and non-condensate, the particle current density around the core, the whole excitation spectrum, and their temperature dependences.

cond-mat↗

arXiv1998

Bose-Einstein Condensation in a Confined Geometry with and without a Vortex

Various widely-used mean-field type theories for a dilute Bose gas are critically examined in the light of the recent discovery of Bose-Einstein condensation of atomic gases in a confined geometry. By numerically solving the mean-field equations within the framework of the Bogoliubov approximation both stationary non-uniform case and the vortex case under rotation in a cylindrically symmetric vessel are investigated. We obtain spatial structures of condensate, non-condensate, anomalous correlation. The low lying excitation spectra, the local density of states and the circulating current density in a vortex corresponding to various levels of mean-field theories are predicted.

cond-mat↗

arXiv1997

Excitation spectrum in a cylindrical Bose-Einstein gas

Whole excitation spectrum is calculated within the Popov approximation of the Bogoliubov theory for a cylindrical symmetric Bose-Einstein gas trapped radially by a harmonic potential. The full dispersion relation and its temperature dependence of the zero sound mode propagating along the axial direction are evaluated in a self-consistent manner. The sound velocity is shown to depend not only on the peak density, but also on the axial area density. Recent sound velocity experiment on Na atom gas is discussed in light of the present theory.

cond-mat↗