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Hiroki Saito

Publications and source records attributed to Hiroki Saito.

At least 55 records · Page 3Linked to original sources
arXiv2017

Moving obstacle potential in a spin-orbit-coupled Bose-Einstein condensate

We investigate the dynamics around an obstacle potential moving in the plane-wave state of a pseudospin-$1/2$ Bose-Einstein condensate with Rashba spin-orbit coupling. We numerically investigate the dynamics of the system and find that it depends not only on the velocity of the obstacle but also significantly on the direction of obstacle motion, which are verified by a Bogoliubov analysis. The excitation diagram with respect to the velocity and direction is obtained. The dependence of the critical velocity on the strength of the spin-orbit coupling and the size of the obstacle is also investigated.

cond-mat.quant-gas↗
arXiv2017

Spinor dynamics in a mixture of spin-1 and spin-2 Bose-Einstein condensates

The spinor dynamics of Bose-Einstein condensates of 87Rb atoms with hyperfine spins 1 and 2 were investigated. A technique of simultaneous Ramsey interferometry was developed for individual control of the vectors of two spins with almost the same Zeeman splittings. The mixture of spinor condensates is generated in the transversely polarized spin-1 and the longitudinally polarized spin-2 states. The time evolution of the spin-1 condensate exhibits dephasing and rephasing of the Larmor precession due to the interaction with the spin-2 condensate. The scattering lengths between spin-1 and -2 atoms were estimated by comparison with the numerical simulation using the Gross-Pitaevskii equation. The proposed technique is expected to facilitate the further study of multiple spinor condensates.

cond-mat.quant-gas↗
arXiv2017

The Fefferman-Stein type inequalities for the multilinear strong maximal functions

Let $\vecω=( ω_{1},...,ω_{m})$ be a multiple weight and $\{Ψ_{j}\}^{m}_{j=1}$ be a sequence of Young functions. Let $\mathcal{M}_{\mathcal{R}}^{\vecΨ}$ be the multilinear strong maximal function with Orlicz norms which is defined by $$\mathcal{M}_{\mathcal{R}}^{\vecΨ}(\vec{f})(x)=\sup_{R\in \mathcal{R},R\ni x}\prod^{m}_{j=1}\|f_{j}\|_{Ψ_{j},R}$$ where the supremum is taken over all rectangles with sides parallel to the coordinate axes. If $Ψ_j(t)=t$, then $\mathcal{M}_{\mathcal{R}}^{\vec{t}}$ coincides with the multilinear strong mximal function $\mathcal{M}_{\mathcal{R}}$ defined and studied by Grafakos et al. In this paper, we first investigated the Fefferman-Stein type inequality for $\mathcal{M}_{\mathcal{R}}^{\vecΨ}$ when $\vecω$ satisfies the $A_{\infty,\mathcal{R}}$ condition. Then, for arbitrary $\vecω\geq 0$( each $ ω_{j}\ge 0$), the Fefferman-Stein type inequality for the multilinear strong maximal function $\mathcal{M}_{\mathcal{R}} $ associated with rectangles will be given.

math.CA↗
arXiv2017

Vortex pairs in a spin-orbit coupled Bose-Einstein condensate

Static and dynamic properties of vortices in a two-component Bose-Einstein condensate with Rashba spin-orbit coupling are investigated. The mass current around a vortex core in the plane-wave phase is found to be deformed by the spin-orbit coupling, and this makes the dynamics of the vortex pairs quite different from those in a scalar Bose-Einstein condensate. The velocity of a vortex-antivortex pair is much smaller than that without spin-orbit coupling, and there exist stationary states. Two vortices with the same circulation move away from each other or unite to form a stationary state.

cond-mat.quant-gas↗
arXiv2016

Spin-orbit-coupled Bose-Einstein condensates held under toroidal trap

We study a quasispin-$1/2$ Bose-Einstein condensate with synthetically generated spin-orbit coupling in a toroidal trap, and show that the system has a rich variety of ground and metastable states. As the central hole region increases, i.e., the potential changes from harmonic-like to ring-like, the condensate exhibits a variety of structures, such as triangular stripes, flower-petal patterns, and counter-circling states. We also show that the rotating systems have exotic vortex configurations. In the limit of a quasi-one dimensional ring, the quantum many-body ground state is obtained, which is found to be the fragmented condensate.

cond-mat.quant-gas↗
arXiv2016

Double-Quantum Spin Vortices in SU(3) Spin-Orbit Coupled Bose Gases

We show that double-quantum spin vortices, which are characterized by doubly quantized circulating spin currents and unmagnetized filled cores, can exist in the ground states of SU(3) spin-orbit coupled Bose gases. It is found that the SU(3) spin-orbit coupling and spin-exchange interaction play important roles in determining the ground-state phase diagram. In the case of effective ferromagnetic spin interaction, the SU(3) spin-orbit coupling induces a three-fold degeneracy to the magnetized ground state, while in the antiferromagnetic spin interaction case, the SU(3) spin-orbit coupling breaks the ordinary phase rule of spinor Bose gases, and allows the spontaneous emergence of double-quantum spin vortices. This exotic topological defect is in stark contrast to the singly quantized spin vortices observed in existing experiments, and can be readily observed by the current magnetization-sensitive phase-contrast imaging technique.

cond-mat.quant-gas↗
arXiv2016

Twisted spin vortices in a spinor-dipolar Bose-Einstein condensate with Rashba spin-orbit coupling

We consider a spin-1 Bose-Einstein condensate with Rashba spin-orbit coupling and dipole-dipole interaction confined in a cigar-shaped trap. Due to the combined effects of spin-orbit coupling, dipole-dipole interaction, and trap geometry, the system exhibits a rich variety of ground-state spin structures, including twisted spin vortices. The ground-state phase diagram is determined with respect to the strengths of the spin-orbit coupling and dipole-dipole interaction.

cond-mat.quant-gas↗
arXiv2016

Self-rotation and synchronization in exciton-polariton condensates

Self-rotation occurs in an exciton-polariton condensate in a two-dimensional semiconductor microcavity pumped by a nonresonant Gaussian laser beam. A wave packet of the condensate spontaneously rotates around the center of the pumped region at a constant frequency breaking the rotation symmetry of the system. When two self-rotating condensates are created with an appropriate distance, synchronization occurs between the dynamics of the self-rotating condensates.

cond-mat.mes-hall↗
arXiv2016

Path-integral Monte Carlo study on a droplet of a dipolar Bose-Einstein condensate stabilized by quantum fluctuation

Motivated by the recent experiments [H. Kadau et al., Nature (London) 530, 194 (2016); I. Ferrier-Barbut et al., arXiv:1601.03318] and theoretical prediction (F. Wächtler and L. Santos, arXiv:1601.04501), the ground state of a dysprosium Bose-Einstein condensate with strong dipole-dipole interaction is studied using the path-integral Monte Carlo method. It is shown that quantum fluctuation can stabilize the condensate against dipolar collapse.

cond-mat.quant-gas↗
arXiv2016

Collision dynamics of Skyrmions in a two-component Bose-Einstein condensate

The dynamics of Skyrmions in a two-component Bose-Einstein condensate are numerically investigated in the mean-field theory. When two Skyrmions collide with each other, they are first united and then scattered into various states. For head-on collisions, Skyrmions with unit winding number are scattered. The collision dynamics with an impact parameter are shown to depend on the relative phase. These dynamic processes are characterized by integer winding numbers.

cond-mat.quant-gas↗
arXiv2015

Droplet formation in a Bose-Einstein condensate with strong dipole-dipole interaction

Motivated by the recent experiment [H. Kadau \textit{et al.}, arXiv:1508.05007 (2015)], we study roton instability and droplet formation in a Bose-Einstein condensate of $^{164}$Dy atoms with strong magnetic dipole-dipole interaction. We numerically solve the cubic-quintic Gross-Pitaevskii equation with dipole-dipole interaction, and show that the three-body interaction plays a significant role in the formation of droplet patterns. We numerically demonstrate the formation of droplet patterns and crystalline structures, decay of droplets, and hysteresis behavior, which are in good agreement with the experiment. Our numerical simulations provide the first prediction on the values of the three-body interaction in a $^{164}$Dy Bose-Einstein condensate. We also predict that the droplets remain stable during the time-of-flight expansion. From our results, further experiments investigating the three-body interaction in dipolar quantum gases are required.

cond-mat.quant-gas↗
arXiv2015

Non-equilibrium dynamics induced by miscible-immiscible transition in binary Bose-Einstein condensates

The non-equilibrium spatial dynamics in a two-component Bose-Einstein condensate were excited by controlled miscible-immiscible transition, in which immiscible condensates with domain structures are transferred to the miscible condensates by changing the internal state of 87Rb atoms. The subsequent evolution exhibits the oscillation of spatial structures involving component mixing and separation. We show that the larger total energy of the miscible system results in a higher oscillation frequency. This investigation introduces a new technique to control the miscibility and the spatial degrees of freedom in atomic Bose-Einstein condensates.

cond-mat.quant-gas↗
arXiv2015

Bouncing motion and penetration dynamics in multicomponent Bose-Einstein condensates

We investigate dynamic properties of bouncing and penetration in colliding binary and ternary Bose-Einstein condensates comprised of different Zeeman or hyperfine states of 87Rb. Through the application of magnetic field gradient pulses, two- or three-component condensates in an optical trap are spatially separated and then made to collide. The subsequent evolutions are classified into two categories: repeated bouncing motion and mutual penetration after damped bounces. We experimentally observed mutual penetration for immiscible condensates, bouncing between miscible condensates, and domain formation for miscible condensates. From numerical simulations of the Gross-Pitaevskii equation, we find that the penetration time can be tuned by slightly changing the atomic interaction strengths.

cond-mat.quant-gas↗
arXiv2015

Suppression of relative flow by multiple domains in two-component Bose-Einstein condensates

We investigate flow properties of immiscible Bose-Einstein condensates composed of two different Zeeman spin states of 87Rb. Spatially overlapping two condensates in the optical trap are prepared by application of a resonant radio frequency pulse, and then the magnetic field gradient is applied in order to produce the atomic flow. We find that the spontaneous multiple domain formation arising from the immiscible nature drastically changes the fluidity. The homogeneously overlapping condensates readily separate under the magnetic field gradient, and they form stable configuration composed of the two layers. In contrast, the relative flow between two condensates are largely suppressed in the case where the magnetic field gradient is applied after spontaneous domain formation.

cond-mat.quant-gas↗
arXiv2015

Upper bound of one-magnon excitation and lower bound of effective mass for ferromagnetic spinor Bose and Fermi gases

Using a variational method, we derive an exact upper bound for one-magnon excitation energy in ferromagnetic spinor gases, which limits the quantum corrections to the effective mass of a magnon to be positive. We also derive an upper bound for one-magnon excitation energy in lattice systems. The results hold for both Bose and Fermi systems in $d$ dimensions as long as the interaction is local and invariant under spin rotation.

cond-mat.quant-gas↗
arXiv2015

Energy shift of magnons in a ferromagnetic spinor-dipolar Bose-Einstein condensate

Motivated by the recent experiment performed by the Berkeley group [G. E. Marti {\it et al.}, Phys. Rev. Lett. {\bf 113}, 155302 (2014)], we consider the dynamics of magnons in a spin-1 spinor-dipolar Bose-Einstein condensate, using mean-field theory. We show that the effective mass of a magnon is increased by the magnetic dipole-dipole interaction, as observed in the experiment. The magnon mass is also decreased by changing the direction of the magnetic field. The increase and decrease in the magnon mass manifest themselves in the acceleration of the magnons.

cond-mat.quant-gas↗