arXiv subjects

Hiroshi Koibuchi

Publications and source records attributed to Hiroshi Koibuchi.

At least 37 records · Page 2Linked to original sources

arXiv2014

Branched-polymer to inflated transition of self-avoiding fluid surfaces

We study phase transition of self-avoiding fluid surface model on dynamically triangulated lattices using the Monte Carlo simulation technique. We report the continuous transition between the branched polymer and inflated phases at ${\it Δ}p \!=\!0$, where ${\it Δ}p (=\!p_{\rm in}\!-\!p_{\rm out})$ is the pressure difference between the inner and outer sides of the surface. This transition is characterized by almost discontinuous change of the enclosed volume versus the variations of the bending rigidity $κ$ and the pressure difference ${\it Δ}p$. No surface fluctuation transition accompanies this transition up to the surface with the number of vertices $N\!=\!2562$.

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arXiv2014

Monte Carlo simulations of Landau-Ginzburg model for membranes

The Landau-Ginzburg (LG) model for membranes is numerically studied on triangulated spheres in ${\bf R}^3$. The LG model is in sharp contrast to the model of Helfrich-Polyakov (HP). The reason for this difference is that the curvature energy of the LG (HP) Hamiltonian is defined by means of the tangential (normal) vector of the surface. For this reason the curvature energy of the LG model includes the in-plane bending or shear energy component, which is not included in the curvature energy of the HP model. From the simulation data, we find that the LG model undergoes a first-order collapse transition. The results of the LG model in the higher dimensional spaces ${\bf R}^d (d>3)$ and on the self-avoiding surfaces in ${\bf R}^3$ are presented and discussed. We also study the David-Guitter (DG) model, which is a variant of the LG model, and find that the DG model undergoes a first-order transition. It is also found that the transition can be observed only on the homogeneous surfaces, which are composed of almost uniform triangles according to the condition that the induced metric ${\partial_a {\bf r}}\cdot{\partial_b {\bf r}}$ is close to $δ_{ab}$.

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arXiv2013

Monte Carlo studies of a Finsler geometric surface model

This paper presents a new type of surface models constructed on the basis of Finsler geometry. A Finsler metric is defined on the surface by using an underlying vector field, which is an in-plane tilt order. According to the orientation of the vector field, the Finsler length becomes dependent on both position and direction on the surface, and for this reason the parameters such as the surface tension and bending rigidity become anisotropic. To confirm that the model is well-defined, we perform Monte Carlo simulations under several isotropic conditions such as those given by random vector fields. The results are comparable to those of previous simulations of the conventional model. It is also found that a tubular phase appears when the vector field is constant. Moreover, we find that the tilts form the Kosterlitz-Thouless and low temperature configurations, which correspond to two different anisotropic phases such as disk and tubular, in the model in which the tilt variable is assumed to be a dynamical variable. This confirms that the model in this paper may be used as an anisotropic model for membranes.

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arXiv2013

Crumpled-to-tubule transition and shape transformations of a model of self-avoiding spherical meshwork

This paper analyzes a new self-avoiding (SA) meshwork model using the canonical Monte Carlo simulation technique on lattices that consist of connection-fixed triangles. The Hamiltonian of this model includes a self-avoiding potential and a pressure term. The model identifies a crumpled-to-tubule (CT) transition between the crumpled and tubular phases. This is a second-order transition, which occurs when the pressure difference between the inner and outer sides of the surface is close to zero. We obtain the Flory swelling exponents $ν_{{\rm R}^2}(=\!D_f/2)$ and $\barν_{\rm v}$ corresponding to the mean square radius of gyration $R_g^2$ and enclosed volume $V$, where $D_f$ is the fractal dimension. The analysis shows that $\barν_{\rm v}$ at the transition is almost identical to the one of the smooth phase of previously reported SA model which has no crumpled phase.

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arXiv2012

In-plane deformation of a triangulated surface model with metric degrees of freedom

Using the canonical Monte Carlo simulation technique, we study a Regge calculus model on triangulated spherical surfaces. The discrete model is statistical mechanically defined with the variables $X$, $g$ and $ρ$, which denote the surface position in ${\bf R}^3$, the metric on a two-dimensional surface $M$ and the surface density of $M$, respectively. The metric $g$ is defined only by using the deficit angle of the triangles in {$M$}. This is in sharp contrast to the conventional Regge calculus model, where {$g$} depends only on the edge length of the triangles. We find that the discrete model in this paper undergoes a phase transition between the smooth spherical phase at $b to infty$ and the crumpled phase at $b to 0$, where $b$ is the bending rigidity. The transition is of first-order and identified with the one observed in the conventional model without the variables $g$ and $ρ$. This implies that the shape transformation transition is not influenced by the metric degrees of freedom. It is also found that the model undergoes a continuous transition of in-plane deformation. This continuous transition is reflected in almost discontinuous changes of the surface area of $M$ and that of $X(M)$, where the surface area of $M$ is conjugate to the density variable $ρ$.

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arXiv2011

First-order phase transition of triangulated surfaces on a spherical core

We study an intrinsic curvature model defined on fixed-connectivity triangulated lattices enclosing a spherical core by using the canonical Monte Carlo simulation technique. We find that the model undergoes a discontinuous transition of shape transformation between the smooth state and a collapsed state even when the core radius $R$ is sufficiently large; the transition depends on $R$. The origin of the multitude of transitions is considered to be a degeneracy of the collapsed states. We also find that the Gaussian bond potential $S_1/N$, which is the sum of bond length squares, discontinuously changes at the transition. The discontinuity in $S_1/N$ implies a possibility of large fluctuations of the distance between lipids, or the density of lipids, in biological membranes such as giant vesicles or liposomes enclosing some materials.

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arXiv2010

Shape transformations of a model of self-avoiding triangulated surfaces of sphere topology

We study a surface model with a self-avoiding (SA) interaction using the canonical Monte Carlo simulation technique on fixed-connectivity (FC) triangulated lattices of sphere topology. The model is defined by an area energy, a deficit angle energy, and the SA potential. A pressure term is also included in the Hamiltonian. The volume enclosed by the surface is well defined because of the self-avoidance. We focus on whether or not the interaction influences the phase structure of the FC model under two different conditions of pressure ${\it Δ} p$; zero and small negative. The results are compared with the previous results of the self-intersecting model, which has a rich variety of phases; the smooth spherical phase, the tubular phase, the linear phase, and the collapsed phase. We find that the influence of the SA interaction on the multitude of phases is almost negligible except for the evidence that no crumpled surface appears under ${\it Δ} p\=\0$ at least even in the limit of zero bending rigidity $α\to \0$. The Hausdorff dimension is obtained in the limit of $α\to \0$ and compared with previous results of SA models, which are different from the one in this paper.

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arXiv2010

Flat histogram Monte Carlo simulations of triangulated fixed-connectivity surface models

Using the Wang-Landau flat histogram Monte Carlo (FHMC) simulation technique, we were able to study two types of triangulated spherical surface models in which the two-dimensional extrinsic curvature energy is assumed in the Hamiltonian. The Gaussian bond potential is also included in the Hamiltonian of the first model, but it is replaced by a hard-wall potential in the second model. The results presented in this paper are in good agreement with the results previously reported by our group. The transition of surface fluctuations and collapsing transition were studied using the canonical Metropolis Monte Carlo simulation technique and were found to be of the first-order. The results obtained in this paper also show that the FHMC technique can be successfully applied to triangulated surface models. It is non-trivial whether the technique is applicable or not to surface models because the simulations are performed on relatively large surfaces.

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arXiv2010

Monte Carlo studies of triangulated spherical surfaces in the two-dimensional space

We numerically study a triangulated surface model in R^2 by taking into account a viewpoint of string model. The models are defined by a mapping X from a two-dimensional surface M to R^2, where the mapping X and the metric g of M are the dynamical variables. The sum over g in the partition function is simulated by the sum over bond lengths and deficit angles by using the Regge calculus technique, and the sum over g is defined to be performed independently of the sum over X. We find that the model undergoes a first-order transition of surface fluctuations, which accompanies a collapsing transition, and that the transitions are reflected in the internal geometry of surface. Fluid surface models are also studied on dynamically triangulated surfaces, and the transitions are found to be of second order. The order of the transition remains unchanged from that of the conventional model defined only by the variable X both in the fixed-connectivity and the fluid models.

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arXiv2009

Spherical surface models with directors

A triangulated spherical surface model is numerically studied, and it is shown that the model undergoes phase transitions between the smooth phase and the collapsed phase. The model is defined by using a director field, which is assumed to have an interaction with a normal of the surface. The interaction between the directors and the surface maintains the surface shape. The director field is not defined within the two-dimensional differential geometry, and this is in sharp contrast to the conventional surface models, where the surface shape is maintained only by the curvature energies. We also show that the interaction makes the Nambu-Goto model well-defined, where the bond potential is given by the area of triangles; the Nambu-Goto model is well-known as an ill-defined one even when the conventional two-dimensional bending energy is included in the Hamiltonian.

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arXiv2008

Non-trivial effect of the in-plane shear elasticity on the phase transitions of fixed-connectivity meshwork models

We numerically study the phase structure of two types of triangulated spherical surface models, which includes an in-plane shear energy in the Hamiltonian, and we found that the phase structure of the models is considerably influenced by the presence of the in-plane shear elasticity. The models undergo a first-order collapsing transition and a first-order (or second-order) transition of surface fluctuations; the latter transition was reported to be of second-order in the first model without the in-plane shear energy. This leads us to conclude that the in-plane elasticity strengthens the transition of surface fluctuations. We also found that the in-plane elasticity decreases the variety of phases in the second model without the in-plane energy. The Hamiltonian of the first model is given by a linear combination of the Gaussian bond potential, a one-dimensional bending energy, and the in-plane shear energy. The second model is obtained from the first model by replacing the Gaussian bond potential with the Nambu-Goto potential, which is defined by the summation over the area of triangles.

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arXiv2008

Shape transformation transitions in a model of fixed-connectivity surfaces supported by skeletons

A compartmentalized surface model of Nambu and Goto is studied on triangulated spherical surfaces by using the canonical Monte Carlo simulation technique. One-dimensional bending energy is defined on the skeletons and at the junctions, and the mechanical strength of the surface is supplied by the one-dimensional bending energy defined on the skeletons and junctions. The compartment size is characterized by the total number L^\prime of bonds between the two-neighboring junctions and is assumed to have values in the range from L^\prime=2 to L^\prime=8 in the simulations, while that of the previously reported model is characterized by L^\prime=1, where all vertices of the triangulated surface are the junctions. Therefore, the model in this paper is considered to be an extension of the previous model in the sense that the previous model is obtained from the model in this paper in the limit of L^\prime\to1 The model in this paper is identical to the Nambu-Goto surface model without curvature energies in the limit of L^\prime\to \infty and hence is expected to be ill-defined at sufficiently large L^\prime. One remarkable result obtained in this paper is that the model has a well-defined smooth phase even at relatively large L^\prime just as the previous model of L^\prime\to1. It is also remarkable that the fluctuations of surface in the smooth phase are crucially dependent on L^\prime; we can see no surface fluctuation when L^\prime\leq2, while relatively large fluctuations are seen when L^\prime\geq3.

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arXiv2008

Surface tension in an intrinsic curvature model with fixed one-dimensional boundaries

A triangulated fixed connectivity surface model is investigated by using the Monte Carlo simulation technique. In order to have the macroscopic surface tension τ, the vertices on the one-dimensional boundaries are fixed as the edges (=circles) of the tubular surface in the simulations. The size of the tubular surface is chosen such that the projected area becomes the regular square of area A. An intrinsic curvature energy with a microscopic bending rigidity b is included in the Hamiltonian. We found that the model undergoes a first-order transition of surface fluctuations at finite b, where the surface tension τdiscontinuously changes. The gap of τremains constant at the transition point in a certain range of values A/N^\prime at sufficiently large N^\prime, which is the total number of vertices excluding the fixed vertices on the boundaries. The value of τremains almost zero in the wrinkled phase at the transition point while τremains negative finite in the smooth phase in that range of A/N^\prime.

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arXiv2008

Phase structure of a surface model with many fine holes

We study the phase structure of a surface model by using the canonical Monte Carlo simulation technique on triangulated, fixed connectivity, and spherical surfaces with many fine holes. The size of a hole is assumed to be of the order of lattice spacing (or bond length) and hence can be negligible compared to the surface size in the thermodynamic limit. We observe in the numerical data that the model undergoes a first-order collapsing transition between the smooth phase and the collapsed phase. Moreover the Hasudorff dimension H remains in the physical bound, i.e., H<3 not only in the smooth phase but also in the collapsed phase at the transition point. The second observation is that the collapsing transition is accompanied by a continuous transition of surface fluctuations. This second result distinguishes the model in this paper and the previous one with many holes, whose size is of the order of the surface size, because the previous surface model with large-sized holes has only the collapsing transition and no transition of surface fluctuations.

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arXiv2008

Possible effects of tilt order on phase transitions of a fixed connectivity surface model

We study the phase structure of a phantom tethered surface model shedding light on the internal degrees of freedom (IDOF), which correspond to the three-dimensional rod like structure of the lipid molecules. The so-called tilt order is assumed as IDOF on the surface model. The model is defined by combining the conventional spherical surface model and the XY model, which describes not only the interaction between lipids but also the interaction between the lipids and the surface. The interaction strength between IDOF and the surface varies depending on the interaction strength between the variables of IDOF. We know that the model without IDOF undergoes a first-order transition of surface fluctuations and a first-order collapsing transition. We observe in this paper that the order of the surface fluctuation transition changes from first-order to second-order and to higher-order with increasing strength of the interaction between IDOF variables. On the contrary, the order of collapsing transition remains first-order and is not influenced by the presence of IDOF.

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arXiv2007

Shape transformations of a compartmentalized fluid surface

A surface model on compartmentalized spheres is studied by using the Monte Carlo simulation technique with dynamical triangulations. We found that the model exhibits a variety of phases: the spherical phase, the tubular phase, the planar phase, the wormlike planar phase, the wormlike long phase, the wormlike short phase, and the collapsed phase. It is also shown that almost all phases are separated from their neighboring phases by first-order transitions. Mechanical strength of the surface is given only by elastic skeletons, which are the compartment boundaries, and vertices diffuse freely inside the compartments. We confirm that the cytoskeletal structure and the lateral diffusion of vertices are an origin of such a variety of phases.

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arXiv2007

Shape transformation transitions of a tethered surface model

A surface model of Nambu and Goto is studied statistical mechanically by using the canonical Monte Carlo simulation technique on a spherical meshwork. The model is defined by the area energy term and a one-dimensional bending energy term in the Hamiltonian. We find that the model has a large variety of phases; the spherical phase, the planar phase, the long linear phase, the short linear phase, the wormlike phase, and the collapsed phase. Almost all two neighboring phases are separated by discontinuous transitions. It is also remarkable that no surface fluctuation can be seen in the surfaces both in the spherical phase and in the planar phase.

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arXiv2007

Phase transitions in a fluid surface model with a deficit angle term

Nambu-Goto model is investigated by using the canonical Monte Carlo simulation technique on dynamically triangulated surfaces of spherical topology. We find that the model has four distinct phases; crumpled, branched-polymer, linear, and tubular. The linear phase and the tubular phase appear to be separated by a first-order transition. It is also found that there is no long-range two-dimensional order in the model. In fact, no smooth surface can be seen in the whole region of the curvature modulus α, which is the coefficient of the deficit angle term in the Hamiltonian. The bending energy, which is not included in the Hamiltonian, remains large even at sufficiently large αin the tubular phase. On the other hand, the surface is spontaneously compactified into a one-dimensional smooth curve in the linear phase; one of the two degrees of freedom shrinks, and the other degree of freedom remains along the curve. Moreover, we find that the rotational symmetry of the model is spontaneously broken in the tubular phase just as in the same model on the fixed connectivity surfaces.

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