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Hiroki Saito

Publications and source records attributed to Hiroki Saito.

At least 109 records · Page 6Linked to original sources
arXiv2006

Symmetry Breaking in Bose-Einstein Condensates

A gaseous Bose-Einstein condensate (BEC) offers an ideal testing ground for studying symmetry breaking, because a trapped BEC system is in a mesoscopic regime, and situations exist under which symmetry breaking may or may not occur. Investigating this problem can explain why mean-field theories have been so successful in elucidating gaseous BEC systems and when many-body effects play a significant role. We substantiate these ideas in four distinct situations: namely, soliton formation in attractive BECs, vortex nucleation in rotating BECs, spontaneous magnetization in spinor BECs, and spin texture formation in dipolar BECs.

cond-mat.other↗
arXiv2006

Broken axisymmetry phase of a spin-1 ferromagnetic Bose-Einstein condensate

A spin-1 ferromagnetic Bose-Einstein condensate subject to a certain magnetic field exhibits a broken-axisymmetry phase in which the magnetization tilts against the applied magnetic field due to the competition between ferromagnetism and linear and quadratic Zeeman effects. The Bogoliubov analysis shows that in this phase two Goldstone modes associated with U(1) and SO(2) symmetry breakings exist, in which phonons and magnons are coupled to restore the two broken symmetries.

cond-mat.other↗
arXiv2006

Critical fluctuations in a soliton formation of attractive Bose-Einstein condensates

We employ mean-field, Bogoliubov, and many-body theories to study critical fluctuations in position and momentum of a Bose-Einstein condensate whose translation symmetry is spontaneously broken due to attractive interactions. In a homogeneous system, the many-body ground state of the symmetry-preserving Hamiltonian is very fragile against superposition of low-lying states, while mean-field theory predicts a stable bright soliton which spontaneously breaks translation symmetry. We show that weak symmetry-breaking perturbations cause the translation-symmetric many-body ground state to cross over to a many-body bright soliton. We argue that the center-of-mass fluctuations in the soliton state arise primarily from the depletion of the condensate to translation modes. We develop an extended mean-field theory to analytically reproduce these results obtained by the exact diagonalization method.

cond-mat.other↗
arXiv2006

Einstein--de Haas Effect in Dipolar Bose-Einstein Condensates

The general properties of the order parameter for a dipolar spinor Bose-Einstein condensate are discussed based on symmetries of interactions. An initially spin-polarized dipolar condensate is shown to dynamically generate a non-singular vortex via spin-orbit interactions -- a phenomenon reminiscent of the Einstein--de Haas effect in ferromagnets.

cond-mat.other↗
arXiv2005

Diagnostics for the ground state phase of a spin-2 Bose-Einstein condensate

We propose a method to determine the singlet-pair energy of a spin-2 Bose-Einstein condensate (BEC). By preparing the initial populations in the magnetic sublevels 0, 2, -2 with appropriate relative phases, we can obtain the coefficient of the spin singlet-pair term from the spin exchange dynamics. This method is suitable for hyperfine states with short lifetimes, since only the initial change in the population of each magnetic sublevel is needed. This method therefore enables the determination of the ground state phase of a spin-2 87Rb BEC at zero magnetic field, which is considered to lie in the immediate vicinity of the boundary between the antiferromagnetic and cyclic phases. We also show that the initial state in which relative phases are controlled can be prepared by Raman processes.

cond-mat.other↗
arXiv2005

Spontaneous magnetization and structure formation in a spin-1 ferromagnetic Bose-Einstein condensate

Motivated by recent experiments involving the non-destructive imaging of magnetization of a spin-1 87Rb Bose gas (Higbie et al., cond-mat/0502517), we address the question of how the spontaneous magnetization of a ferromagnetic BEC occurs in a spin-conserving system. Due to competition between the ferromagnetic interaction and the total spin conservation, various spin structures such as staggered magnetic domains, and helical and concentric ring structures are formed, depending on the geometry of the trapping potential.

cond-mat.other↗
arXiv2005

Symmetry Breaking and Enhanced Condensate Fraction in a Matter-Wave Bright Soliton

An exact diagonalization study reveals that a matter-wave bright soliton and the Goldstone mode are simultaneously created in a quasi-one-dimensional attractive Bose-Einstein condensate by superpositions of quasi-degenerate low-lying many-body states. Upon formation of the soliton the maximum eigenvalue of the single-particle density matrix increases dramatically, indicating that a fragmented condensate converts into a single condensate as a consequence of the breaking of translation symmetry.

cond-mat.other↗
arXiv2004

Bose-Einstein droplet in free space

We show that a droplet of a Bose-Einstein condensate can be dynamically stabilized in free space by rapid oscillations of interatomic interactions between attractive and repulsive through, e.g., the Feshbach resonance. Energy dissipation, which is present in realistic situations, is found to play a crucial role to suppress dynamical instabilities inherent in nonlinear nonequilibrium systems.

cond-mat.other↗
arXiv2004

Bloch Structures in a Rotating Bose-Einstein Condensate

A rotating Bose-Einstein condensate is shown to exhibit a Bloch band structure without the need of periodic potential. Vortices enter the condensate by a mechanism similar to the Bragg reflection, if the frequency of a rotating drive or the strength of interaction is adiabatically changed. A localized state analogous to a gap soliton in a periodic system is predicted near the edge of the Brillouin zone.

cond-mat.soft↗
arXiv2003

Split-merge cycle, fragmented collapse, and vortex disintegration in rotating Bose-Einstein condensates with attractive interactions

The dynamical instabilities and ensuing dynamics of singly- and doubly-quantized vortex states of Bose-Einstein condensates with attractive interactions are investigated using full 3D numerical simulations of the Gross-Pitaevskii equation. With increasing the strength of attractive interactions, a series of dynamical instabilities such as quadrupole, dipole, octupole, and monopole instabilities emerge. The most prominent instability depends on the strength of interactions, the geometry of the trapping potential, and deviations from the axisymmetry due to external perturbations. Singly-quantized vortices split into two clusters and subsequently undergo split-merge cycles in a pancake-shaped trap, whereas the split fragments immediately collapse in a spherical trap. Doubly-quantized vortices are always unstable to disintegration of the vortex core. If we suddenly change the strength of interaction to within a certain range, the vortex splits into three clusters, and one of the clusters collapses after a few split-merge cycles. The vortex split can be observed using a current experimental setup of the MIT group.

cond-mat↗
arXiv2003

Stability of the quantized circulation of an attractive Bose-Einstein condensate in a rotating torus

We investigate rotational properties of a system of bosons with attractive interactions confined in a one-dimensional torus. Two kinds of ground states, uniform-density and bright-soliton states, are obtained analytically as functions of the strength of interaction and of the rotational frequency of the torus. The quantization of circulation appears in the uniform-density state, but disappears upon formation of the soliton. By comparison with the results of exact diagonalization of the many-body Hamiltonian, we show that the Bogoliubov theory is valid at absolute zero over a wide range of parameters. At finite temperature we employ the exact diagonalization method to examine how thermal fluctuations smear the plateaus of the quantized circulation. Finally, by rotating the system with an axisymmetry-breaking potential, we clarify the process in which the quantized circulation becomes thermodynamically stabilized.

cond-mat↗
arXiv2003

A Consistent Picture of a Collapsing Bose-Einstein Condensate

We present a consistent picture of the recent JILA experiments on the collapse of a Bose-Einstein condensate (BEC) [E. A. Donley, et al., Nature 412, 295 (2001)] based on a generalized Gross-Pitaevskii equation that incorporates the effect of a three-body loss. The questions we address in this paper are: (1) Why does a BEC survive, albeit partially, after the collapse? (2) Why does the number of the remnant BEC atoms observed at JILA exceed the critical value? (3) What is the origin of the atom burst? (4) What is the origin of the atom jet and is it coherent? We review our predictions on pattern formation in the course of collapse and on the split instability of vortices that can test our collapsing theory.

cond-mat↗
arXiv2003

Measurement-induced spin squeezing in a cavity

Spin squeezing of an ensemble of N atoms in a high-Q cavity is shown to be enhanced by continuous measurement of photons in the cavity. A degree of spin squeezing proportional to N^{-1/3} is attained as a quasi-steady state over a broad set of initial states of the atoms and photons.

cond-mat↗
arXiv2002

Quantum phase transition in one-dimensional Bose-Einstein condensates with attractive interactions

Motivated by the recent development of the Feshbach technique, we studied the ground and low-lying excited states of attractive Bose-Einstein condensates on a one-dimensional ring as a function of the strength of interactions. The Gross-Pitaevskii mean-field theory predicts a quantum phase transition between a uniform condensate and a bright soliton, and a gapless singular cusp in the Bogoliubov excitation spectrum at the critical point. However, the exact diagonalization reveals the presence of an energy gap at the critical point, where the singularity is smeared by quantum fluctuations.

cond-mat↗
arXiv2002

Split Instability of a Vortex in an Attractive Bose-Einstein Condensate

An attractive Bose-Einstein condensate with a vortex splits into two pieces via the quadrupole dynamical instability, which arises at a weaker strength of interaction than the monopole and the dipole instabilities. The split pieces subsequently unite to restore the original vortex or collapse.

cond-mat↗