Showing papers by "Makoto Tsubota published in 2012"
TL;DR: Quantum hydrodynamics in superfluid helium and atomic Bose-Einstein condensates (BECs) has been recently one of the most important topics in low temperature physics.
Abstract: Quantum hydrodynamics in superfluid helium and atomic Bose-Einstein condensates (BECs) has been recently one of the most important topics in low temperature physics In these systems, a macroscopic wave function appears because of Bose-Einstein condensation, which creates quantized vortices Turbulence consisting of quantized vortices is called quantum turbulence (QT) The study of quantized vortices and QT has increased in intensity for two reasons The first is that recent studies of QT are considerably advanced over older studies, which were chiefly limited to thermal counterflow in 4He, which has no analogue with classical traditional turbulence, whereas new studies on QT are focused on a comparison between QT and classical turbulence The second reason is the realization of atomic BECs in 1995, for which modern optical techniques enable the direct control and visualization of the condensate and can even change the interaction; such direct control is impossible in other quantum condensates like superfluid helium and superconductors Our group has made many important theoretical and numerical contributions to the field of quantum hydrodynamics of both superfluid helium and atomic BECs In this article, we review some of the important topics in detail The topics of quantum hydrodynamics are diverse, so we have not attempted to cover all these topics in this article We also ensure that the scope of this article does not overlap with our recent review article (arXiv:10045458), "Quantized vortices in superfluid helium and atomic Bose--Einstein condensates", and other review articles
107 citations
TL;DR: In this paper, the authors studied a mechanism to create a vorton or three-dimensional skyrmion in phase-separated two-component BECs with the order parameters of the two condensates.
Abstract: We study a mechanism to create a vorton or three-dimensional skyrmion in phase-separated two-component BECs with the order parameters ${\ensuremath{\Psi}}_{1}$ and ${\ensuremath{\Psi}}_{2}$ of the two condensates. We consider a pair of a domain wall (brane) and an antidomain wall (antibrane) stretched by vortices (strings), where the ${\ensuremath{\Psi}}_{2}$ component with a vortex winding is sandwiched by two domains of the ${\ensuremath{\Psi}}_{1}$ component. The vortons appear when the domain wall pair annihilates. Experimentally, this can be realized by preparing the phase separation in the order ${\ensuremath{\Psi}}_{1}$, ${\ensuremath{\Psi}}_{2}$, and ${\ensuremath{\Psi}}_{1}$ components, where the nodal plane of a dark soliton in ${\ensuremath{\Psi}}_{1}$ component is filled with the ${\ensuremath{\Psi}}_{2}$ component with vorticity. By selectively removing the filling ${\ensuremath{\Psi}}_{2}$ component gradually with a resonant laser beam, the collision of the brane and antibrane can be made, creating vortons.
53 citations
TL;DR: In this paper, the authors theoretically study counterflow instability and turbulence in a spin-1 spinor Bose-Einstein condensate by the Gross-Pitaevskii equation and the Bogoliubov--de Gennes equation.
Abstract: We theoretically study counterflow instability and turbulence in a spin-1 spinor Bose--Einstein condensate by the Gross--Pitaevskii equation and the Bogoliubov--de Gennes equation. Our study considers (i) the dynamics induced by the counterflow of two components with different magnetic quantum numbers, which leads to turbulence with spin degrees of freedom, and (ii) the properties of the turbulence. For (i), the behavior of the condensate induced by the counterflow strongly depends on whether the spin-dependent interaction is ferromagnetic or antiferromagnetic, leading to different behaviors for the dispersion relation and the spin density vector, $etc$. For (ii), we numerically calculate the spectrum of the spin-dependent interaction energy, which also depends on the spin-dependent interaction. The spectrum of the spin-dependent interaction energy in the ferromagnetic case exhibits a -7/3 power law, whereas that in the antiferromagnetic case does not. The -7/3 power law can be explained by scaling analysis.
40 citations
TL;DR: It is shown theoretically that a domain-wall annihilation in two-component Bose-Einstein condensates causes tachyon condensation accompanied by spontaneous symmetry breaking in a two-dimensional subspace, which provides insights into how the extra dimensions influence subspatial phase transition in higher-dimensional space.
Abstract: We show theoretically that a domain-wall annihilation in two-component Bose-Einstein condensates causes tachyon condensation accompanied by spontaneous symmetry breaking in a two-dimensional subspace. Three-dimensional vortex formation from domain-wall annihilations is considered a kink formation in subspace. Numerical experiments reveal that the subspatial dynamics obey the dynamic scaling law of phase-ordering kinetics. This model is experimentally feasible and provides insights into how the extra dimensions influence subspatial phase transition in higher-dimensional space.
32 citations
TL;DR: In this paper, the authors performed a numerical study of spin turbulence in a two-dimensional trapped spin-1 spinor Bose-Einstein condensate, focusing on the energy spectrum.
Abstract: We perform a numerical study of spin turbulence in a two-dimensional trapped spin-1 spinor Bose-Einstein condensate, focusing on the energy spectrum. The spin turbulence in the trapped system is generated by instability of the helical structure of the spin density vector in the initial state. Our numerical calculation finds that in the trapped system, the spectrum of the spin-dependent interaction energy in the ferromagnetic case exhibits a $\ensuremath{-}7/3$ power law, which was confirmed in a uniform system by our previous study. The relation between the $\ensuremath{-}7/3$ power law and the motion of the spin density vector is discussed by investigating the orbits of dynamical variables in the spin space.
23 citations
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TL;DR: In this paper, the authors theoretically studied the vortex formation from the collision of the domain walls in phase-separated two-component Bose-Einstein condensates, and revealed that the line density and the core size of the vortices depend on the initial distance between the walls.
Abstract: We theoretically study the vortex formation from the collision of the domain walls in phase-separated two-component Bose-Einstein condensates. The collision process mimics the tachyon condensation for the annihilation of D-brane and anti-D-brane in string theory. A pair annihilation leaves the quantized vortices with superflow along their core, namely `superflowing cosmic strings'. It is revealed that the line density and the core size of the vortices depend on the initial distance between the walls.
10 citations
TL;DR: In this article, the authors considered the brane-anti-brane annihilation in strongly segregated two-component condensates, where one component is sandwiched by two domains of the other component.
Abstract: In brane cosmology, the Big Bang is hypothesized to occur by the annihilation of the brane-anti-brane pair in a collision, where the branes are three-dimensional objects in a higher-dimensional Universe. Spontaneous symmetry breaking accompanied by the formation of lower-dimensional topological defects, e.g. cosmic strings, is triggered by the so-called `tachyon condensation', where the existence of tachyons is attributable to the instability of the brane-anti-brane system. Here, we discuss the closest analogue of the tachyon condensation in atomic Bose-Einstein condensates. We consider annihilation of domain walls, namely branes, in strongly segregated two-component condensates, where one component is sandwiched by two domains of the other component. In this system, the process of the brane annihilation can be projected effectively as ferromagnetic ordering dynamics onto a two-dimensional space. Based on this correspondence, three-dimensional formation of vortices from a domain-wall annihilation is considered to be a kink formation due to spontaneous symmetry breaking in the two-dimensional space. We also discuss a mechanism to create a `vorton' when the sandwiched component has a vortex string bridged between the branes. We hope that this study motivates experimental researches to realize this exotic phenomenon of spontaneous symmetry breaking in superfluid systems.
8 citations
17 Dec 2012
TL;DR: In this paper, the authors theoretically proposed a new method of making quantum turbulence from many dark solitons in atomic Bose-Einstein condensates by solving numerically the two-dimensional Gross-Pitaevskii equation.
Abstract: We theoretically propose a new method of making quantum turbulence from many dark solitons in atomic Bose-Einstein condensates. We solve numerically the two-dimensional Gross-Pitaevskii equation. We set initially many solitons so that they can form a square grid. A dark soliton is known to be stable in one-dimensional systems, but unstable in two- or three-dimensional systems and decay to vortices. Our simulation shows that these solitons decay to a lot of vortices which move around in the system and eventually lead to two-dimensional quantum turbulence. The probability distribution function of the superfluid velocity obeys a Gaussian distribution in the low-velocity region and a power-law distribution in the high-velocity region. The decay of the total number of vortices obeys a power-law for a relatively long period. This scenario may be experimentally realized through interference of Bose-Einstein condensates in a trap potential.
6 citations
TL;DR: In this paper, the decay of thermal counterflow quantum turbulence from a statistically steady state at T = 1.9[K] was simulated with the assumption that the normal fluid is at rest during the decay.
Abstract: We have simulated the decay of thermal counterflow quantum turbulence from a statistically steady state at T=1.9[K], with the assumption that the normal fluid is at rest during the decay. The results are consistent with the predictions of the Vinen equation (in essence the vortex line density (VLD) decays as t^{-1}). For the statistically steady state, we determine the parameter c_2, which connects the curvature of the vortex lines and the mean separation of vortices. A formula connecting the parameter \chi_2 of the Vinen equation with c_2 is shown to agree with the results of the simulations. Disagreement with experiment is discussed.
4 citations
TL;DR: In this article, the authors describe the recent developments as well as the motivation of studying quantum turbulence and quantized vortices in the field of physics and chemistry, and present a visualization technique to control and visualize directly the condensate and quantised vortice.
Abstract: Superfluid turbulence consisting of quantized vortices is called quantum turbulence (QT). Quantum turbulence and quantized vortices were discovered in superfluid $^4$He about 50 years ago, but innovation has occurred recently in this field. One is in the field of superfluid helium. Statistical quantities such as energy spectra and probability distribution function of the velocity field have been accessible both experimentally and numerically. Visualization technique has developed and succeeded in the direct visualization of quantized vortices. The other innovation is in the field of atomic Bose-Einstein condensation. The modern optical technique has enabled us to control and visualize directly the condensate and quantized vortices. Various kinds of hydrodynamic instability have been revealed. Even QT is realized experimentally. This article describes such recent developments as well as the motivation of studying QT.
4 citations
Posted Content•
TL;DR: In this paper, the authors numerically study spin turbulence in a two-dimensional trapped spin-1 spinor Bose-Einstein condensate, focusing on the energy spectrum.
Abstract: We numerically study spin turbulence in a two-dimensional trapped spin-1 spinor Bose--Einstein condensate, focusing on the energy spectrum. The spin turbulence in the trapped system is generated by instability of the helical structure of the spin density vector in the initial state. Our numerical calculation finds that in the trapped system the spectrum of the spin-dependent interaction energy in the ferromagnetic case exhibits a -7/3 power law, which was confirmed in a uniform system by our previous study. The relation between the -7/3 power law and the motion of the spin density vector is discussed by investigating the orbits of dynamical variables in the spin space.
01 Jan 2012
TL;DR: In this paper, the authors considered the brane annihilation in strongly segregated two-component condensates, where one compo- nent is sandwiched by two domains of the other component.
Abstract: Keywords Bose-Einstein condensates, tachyon condensates, brane annihilation Abstract In brane cosmology, the Big Bang is hypothesized to occur by the anni- hilation of the brane-anti-brane pair in a collision, where the branes are three- dimensional objects in a higher-dimensional Universe. Spontaneous symmetry breaking accompanied by the formation of lower-dimensional topological defects, e.g. cosmic strings, is triggered by the so-called 'tachyon con densation', where the existence of tachyons is attributable to the instability of the brane-anti-brane sys- tem. Here, we discuss the closest analogue of the tachyon condensation in atomic Bose-Einstein condensates. We consider annihilation of domain walls, namely branes, in strongly segregated two-component condensates, where one compo- nent is sandwiched by two domains of the other component. In this system, the process of the brane annihilation can be projected effectively as ferromagnetic ordering dynamics onto a two-dimensional space. Based on this correspondence, three-dimensional formation of vortices from a domain-wall annihilation is con- sidered to be a kink formation due to spontaneous symmetry breaking in the two-dimensional space. We also discuss a mechanism to create a 'vorton' when the sandwiched component has a vortex string bridged between the branes. We hope that this study motivates experimental researches to realize this exotic phe- nomenon of spontaneous symmetry breaking in superfluid syst ems.
17 Dec 2012
TL;DR: In this article, the authors studied numerically the time development of the energy spectrum of vortex rings in 3He-B and showed that for separated rings, the spectrum contains very little energy at small wave numbers.
Abstract: Bradley et al. studied experimentally the emission of vortex rings by a vibrating grid in superfluid 3He-B.1. They observed a sharp transition from ballistic propagation of vortex rings at low grid velocities to a cloud of quantum turbulence at higher velocities, the turbulence being generated by coalescence of the rings. This behaviour is consistent with the results of a full Biot-Savart numerical simulation with the vortex filament model.2 Bradley et al suggested that in the quantum turbulent regime a Kolmogorov energy spectrum develops at small wave numbers (presumably less than 2π/l, where l is the vortex line spacing) and they suggested that the observed rate of free decay of the turbulence is consistent with this idea. In this work we have studied numerically the time-development of the energy spectrum. For the separated rings the spectrum contains very little energy at small wave numbers. After the transition to turbulence the energy at small wavenumbers increases, but it remains much less than would be the case for a Kolmogorov spectrum. We consider why the assumptions underlying the numerical simulations do not lead to the generation of a Kolmogorov spectrum.
17 Dec 2012
TL;DR: In this article, the authors theoretically study the instability of countersuperflow, i.e., two counter-propagating miscible superflows, in uniform two-component Bose-Einstein condensates.
Abstract: We theoretically study the instability of countersuperflow, i.e., two counter-propagating miscible superflows, in uniform two-component Bose-Einstein condensates. When the relative velocity of the counterflow exceeds a critical value, the instability causes the nucleation and expansion of vortex rings. A lot of vortex reconnections are caused and lead to binary quantum turbulence, where both components become turbulent. Then we introduce the unique velocity in two-component Bose-Einstein condensates and investigate the probability distribution of the velocity in the binary quantum turbulence to obtain the probability distribution whose tail in the high-velocity region is considerably suppressed compared to single-component quantum turbulence.
17 Dec 2012
TL;DR: In this article, the authors reported the motions of vortex rings emitted from a quantum turbulence in superfluid 4He, by detecting vortex rings using a vortex-free vibrating wire as a detector.
Abstract: Motions of vortices are fundamental characteristics of quantum turbulence. These motions are expected to be governed only by quantized circulations in superfluids at the zero temperature limit. In the present paper, we report the motions of vortex rings emitted from a quantum turbulence in superfluid 4He, by detecting vortex rings using a vortex-free vibrating wire as a detector. The time of flights of vortex rings are distributed, because vortex rings are emitted in any direction from a turbulent region and the detector can respond only to a reachable vortex ring. By measuring time-of-flights many times, we find an exponential distribution of time-of-flights with a non-detection period, which corresponds to the fastest time of flights of vortex rings. For a larger generation power of vortex rings, a distribution of time-of-flights still shows a single exponential distribution, but a non-detection period becomes shorter. This result implies that sizes of emitted vortex rings are distributed dependently on the generation power of turbulence. The observed exponential distributions are confirmed by numerical simulations of the dynamics of vortex rings.