arXiv subjects

Hiroki Saito

Publications and source records attributed to Hiroki Saito.

At least 73 records · Page 4Linked to original sources

arXiv2014

Dynamics of a vortex dipole across a magnetic phase boundary in a spinor Bose-Einstein condensate

Dynamics of a vortex dipole in a spin-1 Bose-Einstein condensate in which magnetic phases are spatially distributed is investigated. When a vortex dipole travels from the ferromagnetic phase to the polar phase, or vice versa, it penetrates the phase boundary and transforms into one of the various spin vortex dipoles, such as a leapfrogging ferromagnetic-core vortex dipole and a half-quantum vortex dipole. Topological connections of spin wave functions across the phase boundary are discussed.

cond-mat.quant-gas↗

arXiv2014

Hysteresis in quantized vortex shedding

It is shown using numerical simulations that flow patterns around an obstacle potential moving in a superfluid exhibit hysteresis. In a certain velocity region, there is a bistability between stationary laminar flow and periodic vortex shedding. The bistability exists in two and three dimensional systems.

cond-mat.quant-gas↗

arXiv2014

Observation of Dipole-Induced Spin Texture in an $^{87}$Rb Bose-Einstein Condensate

We report the spin texture formation resulting from the magnetic dipole-dipole interaction in a spin-2 $^{87}$Rb Bose-Einstein condensate. The spinor condensate is prepared in the transversely polarized spin state and the time evolution is observed under a magnetic field of 90 mG with a gradient of 3 mG/cm using Stern-Gerlach imaging. The experimental results are compared with numerical simulations of the Gross-Pitaevskii equation, which reveals that the observed spatial modulation of the longitudinal magnetization is due to the spin precession in an effective magnetic field produced by the dipole-dipole interaction. These results show that the dipole-dipole interaction has considerable effects even on spinor condensates of alkali metal atoms.

cond-mat.quant-gas↗

arXiv2014

Control of spin current in a Bose gas by bang-bang pulses

We generate spin currents in an $^{87}$Rb spin-2 Bose-Einstein condensate by application of a magnetic field gradient. The spin current destroys the spin polarization, leading to a sudden onset of two-body collisions. In addition, the spin coherence, as measured by the fringe contrast using Ramsey interferometry, is reduced drastically but experiences a weak revival due to in-trap oscillations. The spin current can be controlled using periodic $π$ pulses (bang-bang control), producing longer spin coherence times. Our results show that spin coherence can be maintained even in the presence of spin currents, with applications to quantum sensing in noisy environments.

cond-mat.quant-gas↗

arXiv2014

The Kakeya maximal operator on the variable Lebesgue spaces

We shall verify the Kakeya (Nikodym) maximal operator $K_{N}$, $N\gg 1$, is bounded on the variable Lebesgue space $L^{p(\cdot)}(\mathbb{R}^2)$ when the exponent function $p(\cdot)$ is $N$-modified locally log-Hölder continuous and log-Hölder continuous at infinity.

math.CA↗

arXiv2013

Many-body dynamics of a Bose--Einstein condensate collapsing by quantum tunneling

The dynamics of a Bose-Einstein condensate of atoms having attractive interactions is studied using quantum many-body simulations. The collapse of the condensate by quantum tunneling is numerically demonstrated and the tunneling rate is calculated. The correlation properties of the quantum many-body state are investigated.

cond-mat.quant-gas↗

arXiv2013

Modulation of a quantized vortex street with a vibrating obstacle

Dynamics of a superfluid flow past an obstacle are investigated by solving the Gross-Pitaevskii equation numerically. For an appropriate velocity and size of the obstacle, quantized vortices are periodically generated in the wake, which form a Benard-von Karman vortex street. It is found that vibration of an obstacle modulates the vortex street breaking a symmetry.

cond-mat.quant-gas↗

arXiv2013

Dissipative structures of quantized vortices in a coherently pumped polariton superfluid

Steady states reached in a coherently pumped exciton-polariton superfluid are investigated. As the pump parameter is changed, the translational symmetry of the uniform system is spontaneously broken, and various steady patterns of quantized vortices are formed. This is peculiar to nonequilibrium dissipative systems with continuous pumping.

cond-mat.mes-hall↗

arXiv2013

Kibble-Zurek mechanism in a trapped ferromagnetic Bose-Einstein condensate

Spontaneous spin vortex formation in the magnetic phase transition of a trapped spin-1 Bose-Einstein condensate is investigated using mean-field theory. In a harmonic trapping potential, an inhomogeneous atomic density leads to spatial variations of the critical point, magnetization time scale, and spin correlation length. The Kibble-Zurek phenomena are shown to emerge even in such inhomogeneous systems, when the quench of the quadratic Zeeman energy is fast enough. For slow quench, the magnetized region gradually expands from the center of the trap pushing out spin vortices, which hinders the Kibble-Zurek mechanism from occurring. A harmonic trap with a plug potential is also taken into account.

cond-mat.quant-gas↗

arXiv2012

Order-disorder oscillations in exciton-polariton superfluids

The dynamics of an exciton-polariton superfluid resonantly pumped in a semiconductor microcavity are investigated by mean-field theory. Modulational instability develops into crystalline order and then ordered and disordered states alternately form. A supersolid-like state is also found, in which superflow coexists with crystalline order at rest.

cond-mat.mes-hall↗

arXiv2012

Bénard-von Kármán vortex street in an exciton-polariton superfluid

The dynamics of an exciton--polariton superfluid resonantly injected into a semiconductor microcavity are investigated numerically. The results reveal that a Bénard--von Kármán vortex street is generated in the wake behind an obstacle potential, in addition to the generation of quantized vortex dipoles and dark solitons. The vortex street is shown to be robust against a disorder potential in a sample and it can be observed even in time-integrated measurements.

cond-mat.mes-hall↗

arXiv2011

Penetration of a vortex dipole across an interface of Bose-Einstein condensates

The dynamics of a vortex dipole in a quasi-two dimensional two-component Bose-Einstein condensate are investigated. A vortex dipole is shown to penetrate the interface between the two components when the incident velocity is sufficiently large. A vortex dipole can also disappear or disintegrate at the interface depending on its velocity and the interaction parameters.

cond-mat.quant-gas↗

arXiv2011

Symmetry breaking Rayleigh-Taylor instability in a two-component Bose-Einstein condensate

The interfacial instability and subsequent dynamics in a phase-separated two-component Bose-Einstein condensate with rotation symmetry are studied. When the interatomic interaction or the trap frequency is changed, the Rayleigh-Taylor instability breaks the rotation symmetry of the interface, which is subsequently deformed into nonlinear patterns including mushroom shapes.

cond-mat.quant-gas↗

arXiv2011

Controlled generation and manipulation of vortex dipoles in a Bose-Einstein condensate

We propose methods to generate and manipulate vortex dipoles in a Bose-Einstein condensate using Gaussian beams of red or blue-detuned laser. Velocity-controlled vortex dipoles are shown to be created and launched by a red-detuned beam and by two blue-detuned beams. Critical velocities for the vortex nucleation are investigated. The launched vortex dipoles can be trapped, curved, accelerated, and decelerated using Gaussian beams. Collisions between vortex dipoles are demonstrated.

cond-mat.quant-gas↗

arXiv2011

Capillary instability in a two-component Bose-Einstein condensate

Capillary instability and the resulting dynamics in an immiscible two-component Bose-Einstein condensate are investigated using the mean-field and Bogoliubov analyses. A long, cylindrical condensate surrounded by the other component is dynamically unstable against breakup into droplets due to the interfacial tension arising from the quantum pressure and interactions. A heteronuclear system confined in a cigar-shaped trap is proposed for realizing this phenomenon experimentally.

cond-mat.quant-gas↗

arXiv2011

Correlation between flavour violating decay of long-lived slepton and tau in the coannihilation scenario with Seesaw mechanism

We investigate flavour violating decays of the long-lived lightest slepton and the tau lepton in the coannihilation region of the Minimal Supersymmetric Standard Model with a Seesaw mechanism to generate neutrino masses. We consider a situation where the mass difference between the lightest neutralino, as the Lightest Supersymmetric particle (LSP), and the lightest slepton, as the Next-to-LSP, is smaller than the mass of tau lepton. In this situation, the lifetime of the lightest slepton is very long and it is determined by lepton flavour violating (LFV) couplings because the slepton mainly consists of the lighter stau and the flavour conserving 2-body decay is kinematically forbidden. We show that the lifetime can change many orders of magnitude by varying the Yukawa couplings entering the Seesaw mechanism. We also show that branching ratio of LFV tau decays are strongly correlated with the lightest slepton lifetime. Therefore the branching ratios of LFV tau decays can be determined or constrained by measuring the slepton lifetime at the LHC experiment.

hep-ph↗

arXiv2010

Dynamics of bubbles in a two-component Bose-Einstein condensate

The dynamics of a phase-separated two-component Bose-Einstein condensate are investigated, in which a bubble of one component moves through the other component. Numerical simulations of the Gross--Pitaevskii equation reveal a variety of dynamics associated with the creation of quantized vortices. In two dimensions, a circular bubble deforms into an ellipse and splits into fragments with vortices, which undergo the Magnus effect. The Bénard--von Kármán vortex street is also generated. In three dimensions, a spherical bubble deforms into toruses with vortex rings. When two rings are formed, they exhibit leapfrogging dynamics.

cond-mat.quant-gas↗

arXiv2010

Crossover between Kelvin-Helmholtz and counter-superflow instabilities in two-component Bose-Einstein condensates

Dynamical instabilities at the interface between two Bose--Einstein condensates that are moving relative to each other are investigated using mean-field and Bogoliubov analyses. Kelvin--Helmholtz instability is dominant when the interface thickness is much smaller than the wavelength of the unstable interface mode, whereas the counter-superflow instability becomes dominant in the opposite case. These instabilities emerge not only in an immiscible system but also in a miscible system where an interface is produced by external potential. Dynamics caused by these instabilities are numerically demonstrated in rotating trapped condensates.

cond-mat.quant-gas↗