arXiv subjects

Hiromitsu Takeuchi

Publications and source records attributed to Hiromitsu Takeuchi.

At least 19 recordsLinked to original sources

arXiv2026

Anomalous minimization for critical velocity of superflow along a step potential

To reveal a microscopic mechanism for the anomalous minimization and dependence of the superfluid critical velocity on a moving obstacle potential in a atomic Bose-Einstein condensate [\href{https://link.aps.org/doi/10.1103/PhysRevA.91.053615}{Phys.~Rev.~A \textbf{91}, 053615 (2015)}], we introduce a considerably simplified model of superflow along a step potential. The energy spectrum and wave functions of the lowest-energy excitations in this system are well described by the semi-classical analysis based on the Bogoliubov theory. We found that the critical velocity is minimized and becomes zero when the potential height equals the hydrostatic chemical potential, which corresponds to the critical point of the local condensation phase transition inside the step potential. In a finite-size system, the critical velocity $v_\mathrm{c}$ obeys a power-law scaling with the system size $L_x$ as $v_\mathrm{c}\propto L_x^{-0.963}$. This criticality provides an explanation of the power-law scaling of the minimum critical velocity observed in the experiment.

cond-mat.quant-gas↗

arXiv2025

Effect of Population Imbalance on Vortex Mass in Superfluid Fermi Gases

One of the fundamental parameters associated with quantized vortices in superfluids is the vortex mass, which is the inertia of a vortex. As of yet, this mass has not been observed in a superfluid. However, ultracold Fermi gases provide a promising platform in which recently much experimental progress was made, offering tunability of the interaction as well as control on the single-vortex level. Not only can the scattering length be freely tuned, allowing exploration of the BEC-BCS crossover, but also an imbalance between different pseudospin states can be introduced. We study the effect of introducing this imbalance on the vortex mass, using a method based on an effective field theory for superfluid Fermi gases. We find that it is crucial to consider the imbalance in conjunction with nonzero temperatures; at some temperatures, the vortex mass is significantly enhanced while at others, the vortex mass is diminished. This pronounced temperature dependence highlights the need for careful tuning of experimental conditions and identifies favorable parameter regimes in which the vortex mass is likely to be observed.

cond-mat.quant-gas↗

arXiv2025

Vortex Mass in Superfluid Fermi Gases along the BEC-BCS Crossover

Vortex mass is a key concept in the study of superfluid dynamics, referring to the inertia of vortices in a superfluid, which affects their motion and behavior. Despite being an important quantity, the vortex mass has never been observed experimentally, and remains an unresolved issue in this field. As of now, a large body of research assumes that the vortex mass is a local parameter. In contrast, we present a calculation that suggests a logarithmic dependence on the system size, agreeing with some earlier predictions in the context of Bose gases. We analyze the problem using an effective field theory that describes ultracold atomic Fermi gases over the BEC-BCS crossover at both zero and nonzero temperatures. Our study reveals a strong dependence of the vortex mass on the scattering length; in particular, the vortex mass grows rapidly when moving towards the BCS side. Furthermore, we find that the system-size dependence of the vortex mass results in values an order of magnitude larger than those predicted by other models for realistic system sizes. This implies that the vortex mass could be observable in a wider parameter range than was previously expected. This is particularly relevant considering recent advances in experimental techniques that place the observation of vortex mass in superfluid Fermi gases within reach.

cond-mat.quant-gas↗

arXiv2025

Dynamic scaling of vorticity in phase-separating superfluid mixtures

Recently, it has been experimentally confirmed that non-equilibrium dynamics of phase separation in strongly ferromagnetic Bose-Einstein condensates of $^7$Li atoms obey the dynamic scaling law belonging to the binary-fluid universality class in the inertial hydrodynamic stage. The current work theoretically and numerically studies the dynamic scaling law of structure factor of vorticity in a phase-separating binary superfluid mixture, equivalent to the $^7$Li condensates in a strong limit of quadratic Zeeman shift. We found a dynamic scaling law for the structure factor based on our numerical observation that the peak of the energy spectrum from turbulence theory does not vary in time in the stage. Similarly to freely decaying turbulence, a power-law hierarchy exists in the energy spectrum in our system, and we proposed a decay law of the energy based on the dynamic scaling law by introducing the microscopic, high-wavenumber cutoff.

cond-mat.quant-gas↗

arXiv2025

Proper Orthogonal Decomposition of a Superfluid Turbulent Wake

Superfluid turbulent wakes behind a square prism are studied theoretically and numerically by proper orthogonal decomposition (POD). POD is a data science approach that can efficiently extract the principal vibration modes of a physical system, and is widely used in hydrodynamics, including applications in wake structure analysis. It is not straightforward to apply the conventional POD method to superfluid wake systems, as the superfluid velocity field diverges at the center of a vortex whose circulation is quantized. We successfully established a POD method by applying appropriate blurring to the vorticity distribution in a two-dimensional superfluid wake. It is shown that a coherent structure corresponding to two parallel arrays of alternating quantum vortex bundles, called the "quasi-classical" K\'arm\'an vortex street, is latent as a distinctive major mode in the superfluid turbulent wakes that were naively thought to be "irregular". Since our method is also effective for fluid density, it can be applied to the experimental data analysis for ultra-cold atomic gases.

cond-mat.quant-gas↗

arXiv2024

Critical velocity for wake vortex generation behind a plate in a superflow

We study theoretically the critical velocity $U_c$ for quantum vortex generation by a thin plate-shaped obstacle moving through a uniform Bose-Einstein condensate. Our results based on the Gross-Pitaevskii theory reveal that the critical velocity monotonically decreases with increasing plate size $L$. In the limit of large $L$, the critical velocity is asymptotic to $L^{-1/2}$ predicted by the potential flow theory for an incompressible ideal fluid with a phenomenological length correction. As $L$ decreases, however, the incompressible analysis breaks down quantitatively. By performing a perturbative analysis to incorporate compressibility into the potential flow theory, we have successfully reproduced the numerical results analytically over a wide parameter range. It is also shown that the critical velocity increases with finite plate thickness.

cond-mat.quant-gas↗

arXiv2024

Universal description of massive point vortices and verification methods of vortex inertia in superfluids

Vortex mass, which is the inertia of a quantum vortex, has never been observed in superfluids and is a long-standing problem in low temperature physics. The impact of the mass is considered negligible in typical experiments with superfluid $^4$He. Recent developments of experimental techniques for manipulating quantum vortices in superfluid atomic gases have enabled us to test this problem more accurately. By introducing the vortex mass time and length as universal scales to many-body problems of massive quantum vortices, the theoretical description is formulated in the simplest manner and is universally applicable to different quantum fluids, including fermionic and multicomponent superfluids. There are two branches, the cyclotron and massless branches, for the circular motion of a pair of like-sign vortices. Finding a stable cyclotron branch for the motion of vortices is a clear evidence of vortex mass and superfluid $^3$He-B is the specific example of a system where this phenomena could be observed. The impact of the mass on the massless branch is small but can be enhanced by taking the difference in the two-body dynamics of point vortices with different initial conditions. Our results imply that the vortex mass is a direct cause of the splitting instability of a doubly quantized vortex at absolute zero and that the vortex mass length characterizes the final state after the instability. It is also demonstrated that a pair of massive vortices with opposite circulations has a critical distance characterized by the vortex mass length, below which they are spontaneously annihilated without thermal fluctuations.

cond-mat.quant-gas↗

arXiv2024

Stable singular fractional skyrmion spin texture from the quantum Kelvin-Helmholtz instability

Topology profoundly influences diverse fields of science, providing a powerful framework for classifying phases of matter and predicting nontrivial excitations, such as solitons, vortices, and skyrmions. These topological defects are typically characterized by integer numbers, called topological charges, representing the winding number in their order parameter field. The classification and prediction of topological defects, however, become challenging when singularities are included within the integration domain for calculating the topological charge. While such exotic nonlinear excitations have been proposed in the superfluid $^3$He-A phase and spinor Bose-Einstein condensate of atomic gases, experimental observation of these structures and studies of their stability have long been elusive. Here we report the observation of a singular skyrmion that goes beyond the framework of topology in a ferromagnetic superfluid. The exotic skyrmions are sustained by undergoing anomalous symmetry breaking associated with the eccentric spin singularity and carry half of the elementary charge, distinctive from conventional skyrmions or merons. By successfully realizing the universal regime of the quantum Kelvin-Helmholtz instability, we identified the eccentric fractional skyrmions, produced by emission from a magnetic domain wall and a spontaneous splitting of an integer skyrmion with spin singularities. The singular skyrmions are stable and can be observed after 2~s of hold time. Our results confirm the universality between classical and quantum Kelvin-Helmholtz instabilities and broaden our understanding on complex nonlinear dynamics of nontrivial texture beyond skyrmion in topological quantum systems.

cond-mat.quant-gas↗

arXiv2023

Quantum viscosity and the Reynolds similitude in quantum liquid He-II

Reynolds similitude, a key concept in hydrodynamics, states that two phenomena of different length scales with a similar geometry are physically identical. Flow properties are universally determined in a unified way in terms of the Reynolds number ${\cal R}$ (dimensionless, ratio of inertial to viscous forces in incompressible fluids). For example, the drag coefficient $c_D$ of objects with similar shapes moving in fluids is expressed by a universal function of ${\cal R}$. Certain studies introduced similar dimensionless numbers, that is, the superfluid Reynolds number ${\cal R}_s$, to characterize turbulent flows in superfluids. However, the applicablity of the similitude to inviscid quantum fluids is nontrivial as the original theory is applicable to viscous fluids. This study proposed a method to verify the similitude using current experimental techniques in quantum liquid He-II. A highly precise relation between $c_D$ and ${\cal R}_s$ was obtained in terms of the terminal speed of a macroscopic body falling in He-II at finite temperatures across the Knudsen (ballistic) and hydrodynamic regimes of thermal excitations. Reynolds similitude in superfluids can facilitate unified mutual development of classical and quantum hydrodynamics.

cond-mat.other↗

arXiv2022

Kelvin wave in miscible two-component Bose-Einstein condensates

We study the dispersion of Kelvin waves propagating along single- and half-quantum vortices in miscible two-component Bose-Einstein condensates based on the analysis of the Bogoloubov-de Genne equation. With the help of the interpolating formula connecting the dispersion relations at low- and high-wavenumber regime, we reveal the nontrivial dependence of the dispersion relation of the Kelvin waves on the intercomponent interaction through the change of the vortex core size of the vortical component. We also find the splitting of the Kelvin mode dispersion into gapless and gapfull branches when both components have overlapping single-quantized vortices.

cond-mat.quant-gas↗

arXiv2021

Spin-Current Instability at a Magnetic Domain Wall in a Ferromagnetic Superfluid: a Generation Mechanism of Eccentric Fractional Skyrmions

Spinful superfluids of ultracold atoms are ideal for investigating the intrinsic properties of spin current and texture because they are realized in an isolated, nondissipative system free from impurities, dislocations, and thermal fluctuations. This study theoretically reveals the impact of spin current on a magnetic domain wall in spinful superfluids. An exact wall solution is obtained in the ferromagnetic phase of a spin-1 Bose--Einstein condensate with easy-axis anisotropy at zero temperature. The bosonic-quasiparticle mechanics analytically show that the spin current along the wall becomes unstable if the velocity exceeds the critical spin-current velocities, leading to complicated situations because of the competition between transverse magnons and ripplons. Our direct numerical simulation reveals that this system has a mechanism to generate an eccentric fractional skyrmion, which has a fractional topological charge, but its texture is not similar to that of a meron. This mechanism is in contrast to the generation of conventional skyrmions in easy-axis magnets. The theoretical findings can be examined in the same situation as in a recent experiment on ultracold atoms. In terms of the universality of spontaneous symmetry breaking, unexplored similar phenomena are expected in different physical systems with the same broken symmetry.

cond-mat.quant-gas↗

arXiv2021

Pattern formation of quantum Kelvin-Helmholtz instability in binary superfluids

We study theoretically nonlinear dynamics induced by shear-flow instability in segregated two-component Bose-Einstein condensates in terms of the Weber number, defined by extending the past theory on the Kelvin-Helmholtz instability in classical fluids. Numerical simulations of the Gross-Pitaevskii equations demonstrate that dynamics of pattern formation is well characterized by the Weber number $We$, clarifying the microscopic aspects unique to the quantum fluid system. For $We \lesssim 1$, the Kelvin-Helmholtz instability induces flutter-finger patterns of the interface and quantized vortices are generated at the tip of the fingers. The associated nonlinear dynamics exhibits a universal behavior with respect to $We$. When $We \gtrsim 1$ in which the interface thickness is larger than the wavelength of the interface mode, the nonlinear dynamics is effectively initiated by the counter-superflow instability. In a strongly segregated regime and a large relative velocity, the instability causes transient zipper pattern formation instead of generating vortices due to the lack of enough circulation to form a quantized vortex per a finger. While, in a weakly segregating regime and a small relative velocity, the instability leads to sealskin pattern in the overlapping region, in which the frictional relaxation of the superflow cannot be explained only by the homogeneous counter-superflow instability. We discuss the details of the linear and nonlinear characteristics of this dynamical crossover from small to large Weber numbers, where microscopic properties of the interface become important for the large Weber number.

cond-mat.quant-gas↗

arXiv2020

Phase diagram of vortices in the polar phase of spin-1 Bose-Einstein condensates

The phase diagram of lowest-energy vortices in the polar phase of spin-1 Bose--Einstein condensates is investigated theoretically. Singly quantized vortices are categorized by the local ordered state in the vortex core and three types of vortices are found as lowest-energy vortices, which are elliptic AF-core vortices, axisymmetric F-core vortices, and N-core vortices. These vortices are named after the local ordered state, ferromagnetic (F), antiferromagnetic (AF), broken-axisymmetry (BA), and normal (N) states apart from the bulk polar (P) state. The N-core vortex is a conventional vortex, in the core of which the superfluid order parameter vanishes. The other two types of vortices are stabilized when the quadratic Zeeman energy is smaller than a critical value. The axisymmetric F-core vortex is the lowest-energy vortex for ferromagnetic interaction, and it has an F core surrounded by a BA skin that forms a ferromagnetic-spin texture, as exemplified by the localized Mermin--Ho texture. The elliptic AF-core vortex is stabilized for antiferromagnetic interaction; the vortex core has both nematic-spin and ferromagnetic orders locally and is composed of the AF-core soliton spanned between two BA edges. The phase transition from the N-core vortex to the other two vortices is continuous, whereas that between the AF-core and F-core vortices is discontinuous. The critical point of the continuous vortex-core transition is computed by the perturbation analysis of the Bogoliubov theory and the Ginzburg--Landau formalism describes the critical behavior. The influence of trapping potential on the core structure is also investigated.

cond-mat.quant-gas↗

arXiv2020

Phase Diagram of Solitons in the Polar Phase of a Spin-1 Bose-Einstein Condensate

We theoretically study the structure of a stationary soliton in the polar phase of spin-1 Bose--Einstein condensate in the presence of quadratic Zeeman effect at zero temperature. The phase diagram of such solitons is mapped out by finding the states of minimal soliton energy in the defining range of polar phase. The states are assorted into normal, anti-ferromagnetic, broken-axisymmetry, and ferromagnetic phases according to the number and spin densities in the core. The order of phase transitions between different solitons and the critical behaviour of relevant continuous transitions are proved within the mean-field theory.

cond-mat.quant-gas↗

arXiv2020

Quantum Elliptic Vortex in a Nematic-Spin Bose-Einstein Condensate

We find a novel topological defect in a spin-nematic superfluid theoretically. A quantized vortex spontaneously breaks its axisymmetry, leading to an elliptic vortex in nematic-spin Bose-Einstein condensates with small positive quadratic Zeeman effect. The new vortex is considered the Joukowski transform of a conventional vortex. Its oblateness grows when the Zeeman length exceeds the spin healing length. This structure is sustained by balancing the hydrodynamic potential and the elasticity of a soliton connecting two spin spots, which are observable by in situ magnetization imaging. The theoretical analysis clearly defines the difference between half quantum vortices of the polar and antiferromagnetic phases in spin-1 condensates.

cond-mat.quant-gas↗

arXiv2020

Splitting instability of a doubly quantized vortex in superfluid Fermi gases

The splitting instability of a doubly-quantized vortex in the BEC-BCS crossover of a superfluid Fermi gas is investigated by means of a low-energy effective field theory. Our linear stability analysis and non-equilibrium numerical simulations reveal that the character of the instability drastically changes across the crossover. In the BEC-limit, the splitting of the vortex into two singly-quantized vortices occurs through the emission of phonons, while such an emission is completely absent in the BCS-limit. In the crossover-regime, the instability and phonon emission are enhanced, and the lifetime of a doubly-quantized vortex becomes minimal. The emitted phonon can be observed as a spiraling pattern amplified due to the rotational superradiance, known as a mechanism to carry away energy and angular momentum from a spinning black hole. We also investigate the influence of temperature, population imbalance, and three-dimensional effects.

cond-mat.quant-gas↗

arXiv2019

On internal structure of smaller domains in domain coarsening dynamics of spontaneous Z_2-symmetry breaking in two dimensions

The internal structure of domains smaller than the characteristic size in domain coarsening dynamics of $Z_2$ symmetry breaking is evaluated theoretically for different phase ordering systems in two dimensions. In the previous works on (non-) conserved Ising systems and binary superfluids, the statistical properties of smaller domains are analyzed by assuming that the contribution from its internal structure is negligible. It is shown that this assumption is justified analytically with respect to the statistical quantities, such as domain area, domain-wall length and superfluid circulation, according to the empirical dynamic scaling law for the smaller domains.

cond-mat.stat-mech↗

arXiv2019

Observation of wall-vortex composite defects in a spinor Bose-Einstein condensate

We report the observation of spin domain walls bounded by half-quantum vortices (HQVs) in a spin-1 Bose-Einstein condensate with antiferromagnetic interactions. A spinor condensate is initially prepared in the easy-plane polar phase, and then, suddenly quenched into the easy-axis polar phase. Domain walls are created via the spontaneous $\mathbb{Z}_2$ symmetry breaking in the phase transition and the walls dynamically split into composite defects due to snake instability. The end points of the defects are identified as HQVs for the polar order parameter and the mass supercurrent in their proximity is demonstrated using Bragg scattering. In a strong quench regime, we observe that singly charged quantum vortices are formed with the relaxation of free wall-vortex composite defects. Our results demonstrate a nucleation mechanism for composite defects via phase transition dynamics.

cond-mat.quant-gas↗