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Hiroshi Yokota

Publications and source records attributed to Hiroshi Yokota.

15 recordsLinked to original sources
arXiv2025

Fusion of two critical points and accelerated phase dynamics in orientational ternary mixtures

Motivated by intracellular phase separation, we theoretically investigate how molecular orientation and multi-component nature affect phase behavior. We construct a minimal model for a ternary mixture composed of isotropic (I), anisotropic (A), and solvent (s) components by combining the Flory-Huggins and Maier-Saupe theories. We obtain two main results from evaluating the phase behavior and the time evolution of the density fields. First, for certain interaction parameters, two distinct binodal lines appear in the plane of the volume fractions of the I- and A-components, and merge through their respective critical points. Second, rapid droplet formation emerges due to a weakly first-order phase transition, characterized by a discontinuity of the spinodal surface. The first result indicates the possibility of continuous transformation between the two phase-separated states. The second result suggests that anisotropic molecules can regulate phase separation kinetics. These findings might be physically general beyond biological systems.

cond-mat.soft↗
arXiv2022

Evaluation of loop formation dynamics in a chromatin fiber during chromosome condensation

Chromatin fibers composed of DNA and proteins fold into consecutive loops to form rod-shaped chromosome in mitosis. Although the loop growth dynamics is investigated in several studies, its detailed processes are unclear. Here, we describe the time evolution of the loop length for thermal-driven loop growth processes as an iterative map by calculating physical quantities involved in the processes. We quantify the energy during the chromatin loop formation by calculating the free energies of unlooped and looped chromatins using the Domb-Joyce model of a lattice polymer chain incorporating the bending elasticity for thermal-driven loop growth processes. The excluded volume interaction among loops is integrated by employing the mean-field theory. We compare the loop formation energy with the thermal energy and evaluate the growth of loop length via thermal fluctuation. By assuming the dependence of the excluded volume parameter on the loop length, we construct an iterative map for the loop growth dynamics. The map demonstrates that the growth length of the loop for a single reaction cycle decreases with time to reach the condensin size, where the loop growth dynamics can be less stochastic and be regarded as direct power stroke of condensin as a kind of motor proteins.

cond-mat.soft↗
arXiv2019

Nucleation theory of polymer crystallization with conformation entropy

Based on classical nucleation theory, we propose a couple of theoretical models for the nucleation of polymer crystallization, i.e. one for a single chain system (Model S) and the other for a multi-chain system (Model M). In these models, we assume that the nucleus is composed of tails, loops and a cylindrical ordered region, and we evaluate the conformation entropy explicitly by introducing a transfer matrix. Using these two models, we evaluate the occurrence probability of critical nucleus as a function of the polymer chain stiffness. We found that the critical nucleus in Model M is easier to occur than in Model S because, for semi-flexible chains, the nucleus in Model M can grow by adding a new polymer chain into the nucleus rather than to diminish the loop and tail parts as in the case of Model S.

cond-mat.soft↗
arXiv2017

Modeling induction period of polymer crystallization

We study the possibility of the spinodal decomposition in the induction period of the polymer crystallization. This phenomenon was first reported in an X-ray scattering experiment, and has still been controversial due to various experiments and theories that support or deny the phenomenon. In this article, we explain the condition for the spinodal decomposition to occur in polymer melts by deriving a Ginzburg-Landau model of the free energy as a functional of the density and the orientation of the segments, where we introduce the excluded volume and the nematic interactions through a combination of the random phase approximation and the transfer matrix for the polymer conformation. We show that, upon elimination of the degrees of freedom of the orientation, the nematic interaction reduces to an effective attraction whose strength increases with the stiffness of the polymer chain. Such an attraction induces spinodal decomposion especially for stiff polymer chain case.

cond-mat.soft↗
arXiv2012

Structure of a thermal quasifermion in the QCD/QED Medium

In this paper we carried out a nonperturbative analysis of a thermal quasifermion in the chiral symmetric thermal QCD/QED medium by studying its self-energy function through the Dyson-Schwinger equation with the hard-thermal-loop resummed improved ladder kernel. Our analysis reveals several interesting results, two in some of which may force us to change the image of thermal quasifermions: (1) The thermal mass of a quasifermion begins to decrease as the strength of the coupling gets stronger and finally disappears in the strong coupling region, thus showing a property of a massless particle, and (2) its imaginary part (i.e., the decay width) persists to have $O(g^2T \log(1/g))$ behavior. These results suggest that in the recently produced strongly coupled quark-gluon plasma, the thermal mass of a quasiquark should vanish. Taking into account the largeness of the imaginary part, it seems very hard for a quark to exist as a qausiparticle in the strongly coupled quark-gluon plasma phase. Other important findings are as follows: (3) The collective plasmino mode disappears also in the strongly coupled system, and (4) there exists an ultrasoft third peak in the quasifermion spectral density at least in the weakly coupled QED/QCD plasma, indicating the existence of the ultrasoft fermionic mode.

hep-ph↗
arXiv2012

Vanishing Thermal Mass in the Strongly Coupled QCD/QED medium

In this paper we perform a nonperturbative analysis of a thermal quasifermion in thermal QCD/QED by studying its self-energy function through the Dyson-Schwinger equation with the hard-thermal-loop resummed improved ladder kernel. Our analysis reveals several interesting results, some of which may force us to change the image of the thermal quasifermion: (1) The thermal mass of a quasifermion begins to decrease as the coupling gets stronger and finally disappears in the strong coupling region,(2) the imaginary part of the chiral invariant mass function (i.e., the decay width of the quasifermion) persists to have $O(g^2 T \log (1/g))$ behavior. Present results suggest that in the recently produced strongly coupled quark-gluon-plasma, the thermal mass of a quasifermion should vanish. We also briefly comment on evidence of the existence of a massless, or an ultrasoft mode.

hep-ph↗
arXiv2007

Analysis of the Phase Structure of Thermal QED/QCD through the HTL Improved Ladder Dyson-Schwinger Equation --On the Gauge Dependence of the Solution--

We solved with a numerical procedure the HTL improved ladder DS equation for the retarded fermion self-energy function $Σ_R$ to study the spontaneous generation of fermion mass in thermal QCD/QED, and studied the gauge-dependence of the solution within a general covariant gauge where the gauge parameter $ξ$ is any constant number. With the numerical solutions thus obtained, we found the followings; i) The fermion wave function renormalization function $A(P)$ always deviates largely from unity even at the momentum where the mass is defined, thus the corresponding solutions explicitly contradict with the Ward-Takahashi identity. ii) As a result, the obtained solutions strongly depend on the choice of gauge parameters: the critical temperatures and the critical coupling constants significantly change gauge by gauge. In all gauges we studied in the present analysis, we could not find any solution, having a possibility to be consistent with the Ward-Takahashi identity. Thus we are forced to investigate the procedure to find a gauge which enables us to get a solution being consistent with the Ward-Takahashi identity, otherwise we can not obtain any physically sensible conclusions through the analysis of the point-vertex ladder DS equation no matter how the gauge propagator gets improved.

hep-ph↗
arXiv2002

Chiral Phase Transitions in QED at Finite Temperature: Dyson-Schwinger Equation Analysis in the Real Time Hard-Thermal-Loop Approximation

In order for clarifying what are the essential thermal effects that govern the chiral phase transition at finite temperature, we investigate, in the real-time thermal QED, the consequences of the Hard-Thermal-Loop (HTL) resummed Dyson-Schwinger equation for the physical fermion mass function $Σ_R$. Since $Σ_R$ is the mass function of an ``unstable'' quasi-particle in thermal field theories, it necessarily has non-trivial imaginary parts together with non-trivial wave function renormalization constants. In the present analysis we correctly respect this fact, and study, in the ladder approximation, the effect of HTL resummed gauge boson propagator. Our results with the use of numerical analysis, show the two facts; i) The chiral phase transition is of second order, since the fermion mass is dynamically generated at a critical value of the temperature $T_c$, or at the critical coupling constant $α_c$, without any discontinuity, and ii) the critical temperature $T_c$ at fixed value of $α$ is significantly lower than the previous results, namely the restoration of chiral symmetry occurs at lower temperature than previously expected. The second fact shows the importance of correctly taking the essential thermal effect into the analysis of chiral phase transition, which are, in the previous analyses, neglected due to the inappropriate approximations. The procedure how to maximally respect the gauge invariance in the present approximation, is also discussed.

hep-ph↗
arXiv2001

Chiral Phase Transitions in QCD at Finite Temperature: Hard-Thermal-Loop Resummed Dyson-Schwinger Equation in the Real Time Formalism

Chiral phase transition in thermal QCD is studied by using the Dyson-Schwinger (DS) equation in the real time hard thermal loop approximation. Our results on the critical temperature and the critical coupling are significantly different from those in the preceding analyses in the ladder DS equation, showing the importance of properly taking into account the essential thermal effects, namely the Landau damping and the unstable nature of thermal quasiparticles.

hep-ph↗
arXiv2001

N-Point Vertex Functions, Ward-Takahashi Identities and Dyson-Schwinger Equations in Thermal QCD/QED in the Real Time Hard-Thermal-Loop Approximation

In this paper we calculated the n-point hard-thermal-loop (HTL) vertex functions in QCD/QED for n= 2, 3 and 4 in the physical representation in the real time formalism (RTF). The result showed that the n-point HTL vertex functions can be classified into two groups, a) those with odd numbers of external retarded indices, and b) the others with even numbers of external retarded indices. The n-point HTL vertex functions with one retarded index, which obviously belong to the first group a), are nothing but the HTL vertex functions that appear in the imaginary time formalism (ITF), and vise versa. All the HTL vertex functions belonging to the first group a) are of $O(g^2T^2)$ , and satisfy among them the simple QED-type Ward-Takahashi identities, as in the ITF. Those vertex functions belonging to the second group b) never appear in the ITF, namely their existence is characteristic of the RTF, and their HTL's have the high temperature behavior of $O(g^2T^3)$, one-power of T higher than usual. Despite this difference we could verify that those HTL vertex functions belonging to the second group b) also satisfy among themselves the QED-type Ward-Takahashi identities, thus guaranteeing the gauge invariance of the HTL's in the real time thermal QCD/QED.

hep-ph↗
arXiv1999

Massive $ϕ^4$ Model at Finite Temperature -- Resummayion Procedure la RG Improvement --

In this paper the phase structure of the massive $λϕ^4$ model at finite temperature ($T \neq 0$) is investigated by applying a resummation method inspired by the renormalization-group (RG) improvement to the one-loop effective potential. The resummation method a la RG-improvement is shown to work quite succesfully by resumming up systematically large correction-terms of $O(λT/μ)$ and of $O(λ(T/μ)^2)$. The temperature-dependent phase transition of the model is shown to proceed through the second order transition. The critical exponents are determined analytically and are compared with those in other analyses.

hep-ph↗
arXiv1998

Phase Structure of the Massive Scalar phi^4 Model at Finite Temperature -- Resummation Procedure a la RG improvement --

In this paper a resummation method inspired by the renormalization-group improvement is applied to the one-loop effective potential (EP) in massive scalar $ϕ^4$ model at $T\neq0$. By investigating the phase structure of the model at $T \neq 0$ we get the following observations; i) Starting from the perturbative calculations with the theory renormalized at an arbitrary mass-scale $μ$ and at an arbitrary temperature $T_0$, we can in principle fully resum terms of $O(λT/μ)$ together with terms of $O(λ(T/μ)^2)$. The key idea is to fix the arbitrary RS-parameters so as to make both of one-loop radiative corrections to the mass as well as to the coupling vanish, ensuring the function form of the EP to be determined up to the next-to-leading order of large correction terms, thus absorbing completely those terms of $O(λ(T/μ)^2)$ and of $O(λT/μ)$. ii) If we start from the theory renormalized at the temperature of the environment $T$, the $O(λ(T/μ)^2)$-term resummation can be simply completed through the $T$-renormalization itself. With the lack of freedom we can set only one RS-fixing condition to absorb the large terms of $O(λT/μ)$, thus only the partial resummation of these terms can be carried out. iii) In the above two analyses the temperature-dependent phase transition of the model is shown with the analytic evaluation to proceed through the second order transition. The critical exponents are estimated analytically and are compared with those in other analyses. iv) Our resummation method does not work if we start from the theory renormalized at T=0.

hep-ph↗
arXiv1997

Effective Potential at Finite Temperature: RG improvement vs. High temperature expansion

We have applied the recently proposed renormalization group improvement procedure of the finite temperature effective potential, and have investigated extensively the phase structure of the massive scalar $ϕ^4$ model, showing that the $ϕ^4$ model has a rich 3-phase structure at $T \neq 0$, two of them are not seen in the ordinary perturbative analysis. Temperature dependent phase transition in this model is shown to be strongly the first order.

hep-ph↗