arXiv subjects

Masaya Kunimi

Publications and source records attributed to Masaya Kunimi.

32 records · Page 2Linked to original sources

arXiv2019

Nonequilibrium steady states of Bose-Einstein condensates with a local particle loss in double potential barriers

We investigate stability of non-equilibrium steady states of Bose-Einstein condensates with a local one-body loss in the presence of double potential barriers. We construct an exactly solvable mean-field model, in which the local loss and the potential barriers take the form of a delta function. Using the exact solutions of our model, we show that there are parameter regions in which two steady-state solutions are dynamically stable, i.e., the model exhibits bistability. We also find that unidirectional hysteresis phenomena appear when the local-loss rate is varied in some parameter region.

cond-mat.quant-gas↗

arXiv2019

Ultrafast creation of overlapping Rydberg electrons in an atomic BEC and Mott-insulator lattice

An array of ultracold atoms in an optical lattice (Mott insulator) excited to a state where single electron wave-functions spatially overlap would represent a new and ideal platform to simulate exotic electronic many-body phenomena in the condensed phase. However, this highly excited non-equilibrium system is expected to be so short-lived that it has eluded observation so far. Here, we demonstrate the first step toward its realization by exciting high-lying electronic (Rydberg) states of the atomic Mott insulator with a coherent ultrashort laser pulse. Beyond a threshold principal quantum number where Rydberg orbitals of neighboring lattice sites overlap with each other, the atoms efficiently undergo spontaneous Penning ionization resulting in a drastic change of ion-counting statistics, sharp increase of avalanche ionization and the formation of an ultracold plasma. These observations signal the actual creation of exotic electronic states with overlapping wave functions, which is further confirmed by a significant difference in ionization dynamics between a Bose-Einstein condensate and a Mott insulator.

physics.atom-ph↗

arXiv2019

Decay mechanisms of superflow of Bose-Einstein condensates in ring traps

We study the supercurrent decay of a Bose-Einstein condensate in a ring trap combined with a repulsive barrier potential. In recent experiments, Kumar {\it et al.} [Phys. Rev. A {\bf 95}, 021602(R) (2017)] have measured the dependence of the decay rate on the temperature and the barrier strength. However, the origin of the decay observed in the experiment remains unclear. We calculate the rate of supercurrent decay due to thermally activated phase slips (TAPS) by using the Kramers formula based on the Gross-Pitaevskii mean-field theory. The resulting decay rate is astronomically small compared to that measured in the experiment, thus excluding the possibility of TAPS as the decay mechanism. Alternatively, we argue that three-body losses can be relevant to the observed decay and predict that one can observe supercurrent decay via TAPS by decreasing the number of atoms.

cond-mat.quant-gas↗

arXiv2018

Semiclassical quench dynamics of Bose gases in optical lattices

We analyze the time evolution of the Bose-Hubbard model after a sudden quantum quench to a weakly interacting regime. Specifically, motivated by a recent experiment at Kyoto University, we numerically simulate redistribution of the kinetic and onsite-interaction energies at an early time, which was observed in non-equilibrium dynamics of ultracold Bose gases in a cubic optical lattice starting with a singly-occupied Mott-insulator state. In order to compute the short-time dynamics corresponding to the experimental situation, we apply the truncated-Wigner approximation (TWA) to the Bose-Hubbard model on a cubic lattice. We show that our semiclassical approach quantitatively reproduces the fast redistribution dynamics. We further analyze spatial spreading of density-density correlations at equal time in the Bose-Hubbard model on a square lattice with a large filling factor. When the system is initially prepared in a coherent state, we find that a propagation velocity of the correlation wave packet in the correlation function strongly depends on the final interaction strength, and it is bounded by twice the maximum group velocity of the elementary excitations. In contrast, when the system is initially in a Mott-insulator state, the propagation velocity of the wave packet is approximately independent of the final interaction strength.

cond-mat.quant-gas↗

arXiv2017

Thermally activated phase slips of one-dimensional Bose gases in shallow optical lattices

We study the decay of superflow via thermally activated phase slips in one-dimensional Bose gases in a shallow optical lattice. By using the Kramers formula, we numerically calculate the nucleation rate of a thermally activated phase slip for various values of the filling factor and flow velocity in the absence of a harmonic trapping potential. Within the local density approximation, we derive a formula connecting the phase-slip nucleation rate with the damping rate of a dipole oscillation of the Bose gas in the presence of a harmonic trap. We use the derived formula to directly compare our theory with the recent experiment done by the LENS group [L. Tanzi, et al., Sci. Rep. {\bf 6}, 25965 (2016)]. From the comparison, the observed damping of dipole oscillations in a weakly correlated and small velocity regime is attributed dominantly to thermally activated phase slips rather than quantum phase slips.

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

Metastability, excitations, fluctuations, and multiple-swallowtail structures of a superfluid in a Bose-Einstein condensate in the presence of a uniformly moving defect

We solve the Gross-Pitaevskii (GP) and Bogoliubov equations to investigate the metastability of superfluidity in a Bose-Einstein condensate in the presence of a uniformly moving defect potential in a two-dimensional torus. We calculate the total energy and momentum as functions of the driving velocity of the moving defect and find metastable states with negative effective-mass near the critical velocity. We also find that the first excited energy (energy gap) in the finite-sized torus closes at the critical velocity, that it obeys one-fourth power-law scaling, and that the dynamical fluctuation of the density (amplitude of the order parameter) is strongly enhanced near the critical velocity. We confirm the validity of our results near the critical velocity by calculating the quantum depletion. We find an unconventional swallowtail structure (multiple-swallowtail structure) through calculations of the unstable stationary solutions of the GP equation.

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↗

arXiv2014

Metastable spin textures and Nambu-Goldstone modes of a ferromagnetic spin-1 Bose-Einstein condensate confined in a ring trap

We investigate the metastability of a ferromagnetic spin-1 Bose-Einstein condensate confined in a quasi-one-dimensional rotating ring trap by solving the spin-1 Gross-Pitaevskii equation. We find analytical solutions that exhibit spin textures. By performing linear stability analysis, it is shown that the solutions can become metastable states. We also find that the number of Nambu-Goldstone modes changes at a certain rotation velocity without changing the continuous symmetry of the order parameter.

cond-mat.quant-gas↗

arXiv2013

Precursor phenomena of nucleations of quantized vortices in the presence of a uniformly moving obstacle in Bose-Einstein condensates

We investigate excitations and fluctuations of Bose-Einstein condensates in a two-dimensional torus with a uniformly moving Gaussian potential by solving the Gross-Pitaevskii equation and the Bogoliubov equation. The energy gap $Δ$ between the current-flowing metastable state (that reduces to the ground state for sufficiently slowly-moving potential) and the first excited state vanishes when the moving velocity $v$ of the potential approaches a critical velocity v_c(>0). We find a scaling law $Δ\propto (1-|v|/v_c)^{1/4}$, which implies that a characteristic time scale diverges toward the critical velocity. Near the critical velocity, we show that low-energy local density fluctuations are enhanced. These behaviors can be regarded as precursor phenomena of the vortex nucleation.

cond-mat.quant-gas↗

arXiv2012

Mean-field and stability analysis of two-dimensional flowing soft-core bosons modeling a supersolid

The soft-core boson system is one of the simplest models of supersolids, which have both off-diagonal long-range order (Bose-Einstein condensation) and diagonal long-range order (crystalline order). Although this model has been studied from various points of view, studies of the stability of current-flowing states are lacking. Solving the Gross-Pitaevskii and Bogoliubov equations, we obtain excitation spectra in superfluid, supersolid, and stripe phases. On the basis of the results of the excitation spectra, we present a stability phase diagram that shows the region of the metastable superflow states for each phase.

cond-mat.quant-gas↗