arXiv subjects

Makoto Tsubota

Publications and source records attributed to Makoto Tsubota.

At least 109 records · Page 6Linked to original sources

arXiv2009

Internal Josephson Effects in Spinor Dipolar Bose--Einstein Condensates

We theoretically study the internal Josephson effect, which is driven by spin exchange interactions and magnetic dipole-dipole interactions, in a three-level system for spin-1 Bose--Einstein condensates, obtaining novel spin dynamics. We introduce single spatial mode approximations into the Gross--Pitaevskii equations and derive the Josephson type equations, which are analogous to tunneling currents through three junctions between three superconductors. From an analogy with two interacting nonrigid pendulums, we identify unique varied oscillational modes, called the 0--$π$, 0--$running$, $running$--$running$, $2nπ& running$--$2π$, $single nonrigid pendulum$, and $two rigid pendulums$ phase modes. These Josephson modes in the three states are expected to be found in real atomic Bose gas systems.

cond-mat.quant-gas↗

arXiv2009

Dynamics of Charged Quantized Vortices

We study theoretically the dynamics of charged quantized vortices (CQVs). CQVs (ion-vortex complexes) have been used as an important probe in the field of quantum hydrodynamics. Recent experimental studies of quantum turbulence at very low temperatures utilize CQVs. In this work we propose the equation of motion of CQVs based on the vortex filament model. An analytical solution for a charged vortex ring shows how it expands under an electric field. Numerical simulation reveals the characteristic behavior of CQVs under an electric field.

cond-mat.other↗

arXiv2009

Numerical Studies of Counterflow Turbulence, Velocity Distribution of Vortices

We performed the numerical simulation of quantum turbulence produced by thermal counterflow in superfluid $^{4}${\rm He} by using the vortex filament model. The pioneering work was made by Schwarz, which has two defects. One is neglecting non-local terms of the Biot-Savart integral (localized induction approximation, LIA), and the other is the unphysical mixing procedure in order to sustain the statistically steady state of turbulence. We succeeded in making the statistically steady state without the LIA and the mixing. This state shows the characteristic relation $L=γ^2 v_{ns}^2$ between the line-length-density $L$ and the counterflow relative velocity $v_{ns}$ with the quantitative agreement of the coefficient $γ$ with some typical observations. We compare our numerical results to the observation of experiment by Paoletti {\it et al}, where thermal couterflow was visualized by solid hydrogen particles.

cond-mat.other↗

arXiv2009

Drag force on an oscillating object in quantum turbulence

This paper reports results of the computation of the drag force exerted on an oscillating object in quantum turbulence in superfluid $^4$He. The drag force is calculated on the basis of numerical simulations of quantum turbulent flow about the object. The drag force is proportional to the square of the magnitude of the oscillation velocity, which is similar to that in classical turbulence at high Reynolds number. The drag coefficient is also calculated, and its value is found to be of the same order as that observed in previous experiments. The correspondence between quantum and classical turbulences is further clarified by examining the turbulence created by oscillating objects.

cond-mat.other↗

arXiv2009

Spacetime analogue of Bose-Einstein condensates: Bogoliubov-de Gennes formulation

We construct quantum field theory in an analogue curved spacetime in Bose-Einstein condensates based on the Bogoliubov-de Gennes equations, by exactly relating quantum particles in curved spacetime with Bogoliubov quasiparticle excitations in Bose-Einstein condensates. Here, we derive a simple formula relating the two, which can be used to calculate the particle creation spectrum by solving the time-dependent Bogoliubov-de Gennes equations. Using our formulation, we numerically investigate particle creation in an analogue expanding Universe which can be expressed as Bogoliubov quasiparticles in an expanding Bose-Einstein condensate. We obtain its spectrum, which follows the thermal Maxwell-Boltzmann distribution, the temperature of which is experimentally attainable. Our derivation of the analogy is useful for general Bose-Einstein condensates and not limited to homogeneous ones, and our simulation is the first example of particle creations by solving the Bogoliubov-de Gennes equation in an inhomogeneous condensate.

cond-mat.other↗

arXiv2009

Quantum Monte Carlo simulations for the Bose-Hubbard model with random chemical potential; localized Bose-Einstein condensation without superfluidity

The hardcore-Bose-Hubbard model with random chemical potential is investigated using quantum Monte Carlo simulation. We consider two cases of random distribution of the chemical potential: a uniformly random distribution and a correlated distribution. The temperature dependences of the superfluid density, the specific heat, and the correlation functions are calculated. If the distribution of the randomness is correlated, there exists an intermediate state, which can be thought of as a localized condensate state of bosons, between the superfluid state and the normal state.

cond-mat.stat-mech↗

arXiv2009

Vortex sheet in rotating two-component Bose-Einstein condensates

We investigate vortex states of immiscible two-component Bose-Einstein condensates under rotation through numerical simulations of the coupled Gross-Pitaevskii equations. For strong intercomponent repulsion, the two components undergo phase separation to form several density domains of the same component. In the presence of the rotation, the nucleated vortices are aligned between the domains to make up winding chains of singly quantized vortices, a vortex sheet, instead of periodic vortex lattices. The vortices of one component are located at the region of the density domains of the other component, which results in the serpentine domain structure. The sheet configuration is stable as long as the imbalance of the intracomponent parameter is small. We employ a planar sheet model to estimate the distance between neighboring sheets, determined by the competition between the surface tension of the domain wall and the kinetic energy of the superflow via quantized vortices. By comparing the several length scales in this system, the phase diagram of the vortex state is obtained.

cond-mat.other↗

arXiv2009

Quantum turbulence: From superfluid helium to atomic Bose-Einstein condensates

This article reviews recent developments in quantum fluid dynamics and quantum turbulence (QT) for superfluid helium and atomic Bose-Einstein condensates. Quantum turbulence was discovered in superfluid $^4$He in the 1950s, but the field moved in a new direction starting around the mid 1990s. Quantum turbulence is comprised of quantized vortices that are definite topological defects arising from the order parameter appearing in Bose-Einstein condensation. Hence QT is expected to yield a simpler model of turbulence than does conventional turbulence. A general introduction to this issue and a brief review of the basic concepts are followed by a description of vortex lattice formation in a rotating atomic Bose-Einstein condensate, typical of quantum fluid dynamics. Then we discuss recent developments in QT of superfluid helium such as the energy spectra and dissipative mechanisms at low temperatures, QT created by vibrating structures, and the visualization of QT. As an application of these ideas, we end with a discussion of QT in atomic Bose-Einstein condensates.

cond-mat.other↗

arXiv2008

Spin echo in spinor dipolar Bose-Einstein condensates

We theoretically propose and numerically realize spin echo in a spinor Bose--Einstein condensate (BEC). We investigate the influence on the spin echo of phase separation of the condensate. The equation of motion of the spin density exhibits two relaxation times. We use two methods to separate the relaxation times and hence demonstrate a technique to reveal magnetic dipole--dipole interactions in spinor BECs.

cond-mat.other↗

arXiv2008

Quantized vortices in atomic Bose-Einstein condensates

In this review, we give an overview of the experimental and theoretical advances in the physics of quantized vortices in dilute atomic-gas Bose--Einstein condensates in a trapping potential, especially focusing on experimental research activities and their theoretical interpretations. Making good use of the atom optical technique, the experiments have revealed many novel structural and dynamic properties of quantized vortices by directly visualizing vortex cores from an image of the density profiles. These results lead to a deep understanding of superfluid hydrodynamics of such systems. Typically, vortices are stabilized by a rotating potential created by a laser beam, magnetic field, and thermal gas. Finite size effects and inhomogeneity of the system, originating from the confinement by the trapping potential, yield unique vortex dynamics coupled with the collective excitations of the condensate. Measuring the frequencies of the collective modes is an accurate tool for clarifying the character of the vortex state. The topics included in this review are the mechanism of vortex formation, equilibrium properties, and dynamics of a single vortex and those of a vortex lattice in a rapidly rotating condensate.

cond-mat.other↗

arXiv2008

Quantum Turbulence

The present article reviews the recent developments in the physics of quantum turbulence. Quantum turbulence (QT) was discovered in superfluid $^4$He in the 1950s, and the research has tended toward a new direction since the mid 90s. The similarities and differences between quantum and classical turbulence have become an important area of research. QT is comprised of quantized vortices that are definite topological defects, being expected to yield a model of turbulence that is much simpler than the classical model. The general introduction of the issue and a brief review on classical turbulence are followed by a description of the dynamics of quantized vortices. Then, we discuss the energy spectrum of QT at very low temperatures. At low wavenumbers, the energy is transferred through the Richardson cascade of quantized vortices, and the spectrum obeys the Kolmogorov law, which is the most important statistical law in turbulence; this classical region shows the similarity to conventional turbulence. At higher wavenumbers, the energy is transferred by the Kelvin-wave cascade on each vortex. This quantum regime depends strongly on the nature of each quantized vortex. The possible dissipation mechanism is discussed. Finally, important new experimental studies, which include investigations into temperature-dependent transition to QT, dissipation at very low temperatures, QT created by vibrating structures, and visualization of QT, are reviewed. The present article concludes with a brief look at QT in atomic Bose-Einstein condensates.

cond-mat.other↗

arXiv2007

Quantum Turbulence in a Trapped Bose-Einstein Condensate

We study quantum turbulence in trapped Bose-Einstein condensates by numerically solving the Gross-Pitaevskii equation. Combining rotations around two axes, we successfully induce quantum turbulent state in which quantized vortices are not crystallized but tangled. The obtained spectrum of the incompressible kinetic energy is consistent with the Kolmogorov law, the most important statistical law in turbulence.

cond-mat.dis-nn↗

arXiv2007

Zel'dovich-Starobinsky Effect in Atomic Bose-Einstein Condensates: Analogy to Kerr Black Hole

We consider circular motion of a heavy object in an atomic Bose-Einstein condensate (BEC) at $T=0{\rm K}$. Even if the linear velocity of the object is smaller than the Landau critical velocity, the object may radiate quasiparticles and thus experience the quantum friction. The radiation process is similar to Zel'dovich-Starobinskii (ZS) effect -- the radiation by a rotating black hole. This analogy emerges when one introduces the effective acoustic metric for quasiparticles. In the rotating frame this metric has an ergosurface, which is similar to the ergosurface in the metric of a rotating black hole. In a finite size BEC, the quasiparticle creation takes place when the ergosurface is within the condensate and occurs via quantum tunneling from the object into the ergoregion. The dependence of the radiation rate on the position of the ergosurface is investigated.

cond-mat.other↗

arXiv2006

Dynamical vortex phases in a Bose-Einstein condensate driven by a rotating optical lattice

We present simulation results of the vortex dynamics in a trapped Bose-Einstein condensate in the presence of a rotating optical lattice. Changing the potential amplitude and the relative rotation frequency between the condensate and the optical lattice, we find a rich variety of dynamical phases of vortices. The onset of these different phases is described by the force balance of a driving force, a pinning force and vortex-vortex interactions. In particular, when the optical lattice rotates faster than the condensate, an incommensurate effect leads to a vortex liquid phase supported by the competition between the driving force and the dissipation.

cond-mat.other↗

arXiv2006

Bosons in Disordered Optical Potentials

In this work we systematically investigate the condensate properties, superfluid properties and quantum phase transitions in interacting Bose gases trapped in disordered optical potentials. We numerically solve the Bose-Hubbard Hamiltonian exactly for different: (a) types of disorder, (b) disorder strengths, and (c) interatomic interactions. The three types of disorder studied are: quasiperiodic disorder, uniform random disorder and random speckle-type disorder. We find that the Bose glass, as identified by Fisher et al [Phys. Rev. B {\bf 40}, 546 (1989)], contains a normal condensate component and we show how the three different factors listed above affect it.

cond-mat.dis-nn↗

arXiv2006

Thermal dissipation in quantum turbulence

The microscopic mechanism of thermal dissipation in quantum turbulence has been numerically studied by solving the coupled system involving the Gross-Pitaevskii equation and the Bogoliubov-de Gennes equation. At low temperatures, the obtained dissipation does not work at scales greater than the vortex core size. However, as the temperature increases, dissipation works at large scales and it affects the vortex dynamics. We successfully obtained the mutual friction coefficients of the vortex dynamics as functions of temperature, which can be applied to the vortex dynamics in dilute Bose-Einstein condensates.

cond-mat.dis-nn↗

arXiv2006

Modulation instability and solitary wave formation in two-component Bose-Einstein condensates

We investigate nonlinear dynamics induced by the modulation instability of a two-component mixture in an atomic Bose-Einstein condensate. The nonlinear dynamics is examined using numerical simulations of the time-dependent coupled Gross-Pitaevskii equations. The unstable modulation grows from initially miscible condensates into various types of vector solitary waves, depending on the combinations of the sign of the coupling constants (intracomponent and intercomponent). We discuss the detailed features of the modulation instability, dynamics of solitary wave formation, and an analogy with the collapsing dynamics in a single-component condensate with attractive interactions.

cond-mat.other↗

arXiv2006

Boojums in Rotating Two-Component Bose-Einstein Condensates

A boojum is a topological defect that can form only on the surface of an ordered medium such as superfluid $^3$He and liquid crystals. We study theoretically boojums appearing between two phases with different vortex structures in two-component BECs where the intracomponent interaction is repulsive in one phase and attractive in the other. The detailed structure of the boojums is revealed by investigating its density distribution, effective superflow vorticity and pseudospin texture.

cond-mat.other↗