Searcharxiv⌕ Search

arXiv subjects

H. Koibuchi

Publications and source records attributed to H. Koibuchi.

16 recordsLinked to original sources
arXiv2025

Turing patterns on polymerized membranes: a coarse-grained lattice modelling with internal degree of freedom for polymer direction

We numerically study Turing patterns (TPs) on two-dimensional surfaces with a square boundary in ${\bf R}^3$ using a surface model for polymerized membranes. The variables used to describe the membranes correspond to two distinct degrees of freedom: an internal degree of freedom for the polymer directions in addition to the positional degree of freedom. This generalised surface model enables us to identify a non-trivial interference between the TP system and the membranes. To this end, we employ a hybrid numerical technique, utilising Monte Carlo updates for membrane configurations and discrete time iterations for the FitzHugh-Nagumo type Turing equation. The simulation results clearly show that anisotropies in the mechanical deformation properties, particularly the easy axes associated with the stretching and bending of the membranes, determine the direction of the TPs to be perpendicular or parallel to the easy axes. Additionally, by calculating the dependence of the maximum entropy on the internal degree of freedom, we can obtain information on the relaxation with respect to the polymer structure. This crucial information serves to remind us that non-equilibrium configurations can be captured within the canonical Monte Carlo simulations.

cond-mat.soft↗
arXiv2024

Monte Carlo Studies on Geometrically Confined Skyrmions in Nanodots: Stability and Morphology under Radial Stresses

We numerically study the stability and morphology of geometrically confined skyrmions in nanodots using Finsler geometry (FG) modeling technique. The FG model dynamically implements anisotropies in ferromagnetic interaction, Dzyaloshinskii-Moriya interaction, and magneto-elastic coupling in response to mechanical stresses. Without the stresses, there exists a geometrically confined effect originating from the surface effect of small nanodots, in which skyrmions are stabilized under a low external magnetic field. This surface effect is enhanced by radial stresses, which significantly reduce the surface DMI compared to the bulk DMI. The radial stresses also alter the interactions to be anisotropic. Owing to these position- and direction-dependent interactions, incomplete skyrmions emerge at the center of the nanodots under the tensile stress. In addition to the incomplete skyrmions, target skyrmions are observed under the compressive stress. Our numerical results indicate that the strain-enhanced surface effect and the strain-induced interaction anisotropies suitably explain the skyrmion stability in nanodots with zero magnetic field.

cond-mat.str-el↗
arXiv2022

Origin of anisotropic diffusion in Turing Patterns

In this paper, we numerically study Turing patterns by the Finsler geometry (FG) modeling technique on thermally fluctuating triangular lattices, which are often used for modeling cell membranes or lipid membranes, focusing on the origin of diffusion anisotropy. The FG modeling prescription allows us to assume direction-dependent diffusion described by Laplacian. To implement such diffusion anisotropy in the FG modeling, we need an internal degree of freedom (IDF), which depends on direction and position and is controlled by some external forces or stimuli. For such a direction-dependent IDF, we use velocity directions corresponding to thermal fluctuations of the lattice vertices. We find that anisotropic Turing patterns emerge in the direction along which vertices fluctuate. In the simulations, direction-dependent diffusion coefficients are unnecessary for input, and instead, the IDF aligns the direction of vertex fluctuation along a direction implemented by external stimuli. Our results and techniques provide insight into the origin of diffusion anisotropy connected to Turing patterns.

nlin.PS↗
arXiv2022

Monte Carlo studies of skyrmion stabilization under geometric confinement and uniaxial strain

Geometric confinement (GC) of skyrmions in nanodomains plays a crucial role in skyrmion stabilization. This confinement effect decreases the magnetic field necessary for skyrmion formation and is closely related to the applied mechanical stresses. However, the mechanism of GC is unclear and remains controversial. Here, we numerically study the effect of GC on skyrmion stabilization and find that zero Dzyaloshinskii-Moriya interaction (DMI) coupling constants imposed on the boundary surfaces of small thin plates cause confinement effects, stabilizing skyrmions in the low-field region. Moreover, the confined skyrmions are further stabilized by tensile strains parallel to the plate, and the skyrmion phase extends to the low-temperature region. This stabilization occurs due to the bulk anisotropic DMI coupling constant caused by lattice deformations. Our simulation data are qualitatively consistent with reported experimental data on skyrmion stabilization induced by tensile strains applied to a thin plate of the chiral magnet ${\rm Cu_2OSeO_3}$.

cond-mat.str-el↗
arXiv2022

Effect of Geometric Confinement on the Stabilization of Skyrmions

In this paper, we study the stability of skyrmions (SKYs) caused by the geometric confinement (GC) effect observed in nano-domains in recent experiments, where SKYs appear only inside the boundary and is stable at the low magnetic field region. However, the mechanism of the GC effect is unclear for skyrmions. We numerically find that this effect is not observed in the standard model, in which the Dzyaloshinskii-Moriya interaction (DMI) energy is discretized on lattice edges, while the effect is observed in a discrete model where the DMI energy is defined on lattice volume elements. In the latter model, the DMI energy on the surfaces effectively becomes very small compared with that of the inside. From this observation, we study a model with zero DMI energy on the surfaces parallel to the external magnetic field and find that SKY is significantly stabilized in the sample in the low magnetic field region.

cond-mat.mtrl-sci↗
arXiv2020

Electromechanical properties of ferroelectric polymers: Finsler geometry modeling and a Monte Carlo study

Polyvinylidene difluoride (PVDF) is a ferroelectric polymer characterized by negative strain along the direction of the applied electric field. However, the electromechanical response mechanism of PVDF remains unclear due to the complexity of the hierarchical structure across the length scales. As described in this letter, we employ the Finsler geometry model as a new solution to the aforementioned problem and demonstrate that the deformations observed through Monte Carlo simulations on 3D tetrahedral lattices are nearly identical to those of real PVDF. Specifically, the simulated mechanical deformation and polarization are similar to those observed experimentally.

cond-mat.soft↗
arXiv2006

Phase Transition of Extrinsic Curvature Surface Model on a Disk

An extrinsic curvature surface model is investigated by Monte Carlo simulations on a disk. We found that the model undergoes a first-order transition separating the smooth phase from the collapsed phase. The results in this paper together with the previous ones suggest that the order of the transition is independent of whether the surface is compact (closed) or non-compact (open).

cond-mat.stat-mech↗
arXiv2006

Phase transitions of an intrinsic curvature model on dynamically triangulated spherical surfaces with point boundaries

An intrinsic curvature model is investigated using the canonical Monte Carlo simulations on dynamically triangulated spherical surfaces of size upto N=4842 with two fixed-vertices separated by the distance 2L. We found a first-order transition at finite curvature coefficient α, and moreover that the order of the transition remains unchanged even when L is enlarged such that the surfaces become sufficiently oblong. This is in sharp contrast to the known results of the same model on tethered surfaces, where the transition weakens to a second-order one as L is increased. The phase transition of the model in this paper separates the smooth phase from the crumpled phase. The surfaces become string-like between two point-boundaries in the crumpled phase. On the contrary, we can see a spherical lump on the oblong surfaces in the smooth phase. The string tension was calculated and was found to have a jump at the transition point. The value of σis independent of L in the smooth phase, while it increases with increasing L in the crumpled phase. This behavior of σis consistent with the observed scaling relation σ\sim (2L/N)^ν, where ν\simeq 0 in the smooth phase, and ν=0.93\pm 0.14 in the crumpled phase. We should note that a possibility of a continuous transition is not completely eliminated.

cond-mat.stat-mech↗
arXiv2006

Phase transition of triangulated spherical surfaces supported by elastic chains with rigid junctions

A surface model with skeletons is investigated by using the canonical Monte Carlo simulations. The skeleton is composed of linear chains, which are joined to each other at the rigid junctions. A one-dimensional bending energy is defined on the linear chains, and no two-dimensional curvature energy is assumed on the surface. The model undergoes a first-order transition between the smooth phase and the crumpled phase. We conclude that the first-order transition of the surface model with skeletons is independent of whether the junctions are elastic or rigid.

cond-mat.stat-mech↗
arXiv2006

First-order transition of a compartmentalized surface model for fluid membranes

Dynamically triangulated surface model is found to undergo a first-order crumpling transition between the smooth phase and a crumpled phase. Triangulated spheres are divided into compartmentalized domains, whose boundary bonds remain unflipped in the Monte Carlo simulations. The vertices can freely diffuse only inside the compartments, and they are prohibited from the free diffusion over the surface due to the domain boundaries. The violation of reparametrization invariance of the Hamiltonian is a reason for the existence of the long-range order in the fluid surface model.

cond-mat.stat-mech↗
arXiv2006

Phase structure of intrinsic curvature models on dynamically triangulated disk with fixed boundary length

A first-order phase transition is found in two types of intrinsic curvature models defined on dynamically triangulated surfaces of disk topology. The intrinsic curvature energy is included in the Hamiltonian. The smooth phase is separated from a non-smooth phase by the transition. The crumpled phase, which is different from the non-smooth phase, also appears at sufficiently small curvature coefficient $α$. The phase structure of the model on the disk is identical to that of the spherical surface model, which was investigated by us and reported previously. Thus, we found that the phase structure of the fluid surface model with intrinsic curvature is independent of whether the surface is closed or open.

cond-mat.stat-mech↗
arXiv2006

Phase transitions of a tethered membrane model with intrinsic curvature on spherical surfaces with point boundaries

We found that the order for the crumpling transition of an intrinsic curvature model changes depending on the distance between two boundary vertices fixed on the surface of spherical topology. The model is a curvature one governed by an intrinsic curvature energy, which is defined on triangulated surfaces. It was already reported that the model undergoes a first-order crumpling transition without the boundary conditions on the surface. However, the dependence of the transition on such boundary condition is yet to be studied. We have studied in this paper this problem by using the Monte Carlo simulations on surfaces up to a size N=8412. The first-order transition changes to a second-order one if the distance increases.

cond-mat.stat-mech↗
arXiv2006

Phase transition of an extrinsic curvature model on tori

We show a numerical evidence that a tethered surface model with extrinsic curvature undergoes a first-order crumpling transition between the smooth phase and a non-smooth phase on triangulated tori. The results obtained in this Letter together with the previous ones on spherical surfaces lead us to conclude that the tethered surface model undergoes a first-order transition on compact surfaces.

cond-mat.stat-mech↗
arXiv2006

Phase Transition of a Skeleton Model for Surfaces

A spherical model of skeleton with junctions is investigated by Monte Carlo simulations. The model is governed by one-dimensional bending energy. The results indicate that the model undergoes a first-order transition separating the smooth phase from the crumpled phase. The existence of phase transition indicates that junctions play a non-trivial role in the transition.

cond-mat.stat-mech↗
arXiv2004

Phase transition of surface models with intrinsic curvature

It is reported that a surface model of Polyakov strings undergoes a first-order phase transition between smooth and crumpled (or branched polymer) phases. The Hamiltonian of the model contains the Gaussian term and a deficit angle term corresponding to the weight of the integration measure dX in the partition function.

cond-mat.stat-mech↗