arXiv subjects

Takuya Saito

Publications and source records attributed to Takuya Saito.

At least 37 records · Page 2Linked to original sources

arXiv2015

Phase structure of two-color QCD at real and imaginary chemical potentials; lattice simulations and model analyses

We investigate the phase structure of two-color QCD at both real and imaginary chemical potentials mu, performing lattice simulations and analyzing the data with the Polyakov-loop extended Nambu--Jona-Lasinio (PNJL) model. Lattice QCD simulations are done on an 8^3 times 4 lattice with the clover-improved two-flavor Wilson fermion action and the renormalization-group improved Iwasaki gauge action. We test the analytic continuation of physical quantities from imaginary mu to real mu by comparing lattice QCD results calculated at real mu with the result of analytic function the coefficients of which are determined from lattice QCD results at imaginary mu. We also test the validity of the PNJL model by comparing model results with lattice QCD ones. The PNJL model is good in the deconfinement region, but less accurate in the transition and confinement regions. This problem is improved by introducing the baryon degree of freedom to the model. It is also found that the vector-type four-quark interaction is necessary to explain lattice data on the quark number density.

hep-lat↗

arXiv2014

Rate in Template-directed Polymer Synthesis

We discuss temporal efficiency of template-directed polymer synthesis, such as DNA replication and transcription, under a given template string. To weigh the synthesis speed and accuracy on the same scale, we propose a template-directed synthesis (TDS) rate, which contains an expression analogous to that for the Shannon entropy. Increasing the synthesis speed accelerates the TDS rate, but the TDS rate is lowered if the produced sequences are diversified. We apply the TDS rate to some production system models and investigate how the balance between the speed and the accuracy is affected by changes in the system conditions.

cond-mat.soft↗

arXiv2014

Heavy quark potential at finite imaginary chemical potential

We investigate chemical-potential ($μ$) dependence of the static-quark free energies in both the real and imaginary $μ$ regions, using the clover-improved two-flavor Wilson fermion action and the renormalization-group improved Iwasaki gauge action. Static-quark potentials are evaluated from Polyakov-loop correlators in the deconfinement phase and the imaginary $μ=iμ_{\rm I}$ region and extrapolated to the real $μ$ region with analytic continuation. As the analytic continuation, the potential calculated at imaginary $μ=iμ_{\rm I}$ is expanded into a Taylor-expansion series of $iμ_{\rm I}/T$ up to 4th order and the pure imaginary variable $iμ_{\rm I}/T$ is replaced by the real one $μ_{\rm R}/T$. At real $μ$, the 4th-order term weakens $μ$ dependence of the potential sizably. Also, the color-Debye screening mass is extracted from the color-singlet potential at imaginary $μ$, and the mass is extrapolated to real $μ$ by analytic continuation. The screening mass thus obtained has stronger $μ$ dependence than the prediction of the leading-order thermal perturbation theory at both real and imaginary $μ$.

hep-lat↗

arXiv2013

Color screening potential at finite density in two-flavor lattice QCD with Wilson fermions

We investigate chemical-potential (μ) dependence of static-quark free energies in both the real and imaginary μregions, performing lattice QCD simulations at imaginary μand extrapolating the results to the real μregion with analytic continuation. Lattice QCD calculations are done on a 16^{3}\times 4 lattice with the clover-improved two-flavor Wilson fermion action and the renormalization-group improved Iwasaki gauge action. Static-quark potential is evaluated from the Polyakov-loop correlation functions in the deconfinement phase. As the analytic continuation, the potential calculated at imaginary μ=iμ_{\rm I} is expanded into a Taylor-expansion series of iμ_{\rm I}/T up to 4th order and the pure imaginary variable iμ_{\rm I}/T is replaced by the real one μ_{\rm R}/T. At real μ, the 4th-order term weakens μdependence of the potential sizably. At long distance, all of the color singlet and non-singlet potentials tend to twice the single-quark free energy, indicating that the interactions between heavy quarks are fully color-screened for finite μ. For both real and imaginary μ, the color-singlet q{\bar q} and the color-antitriplet qq interaction are attractive, whereas the color-octet q{\bar q} and the color-sextet qq interaction are repulsive. The attractive interactions have stronger μ/T dependence than the repulsive interactions. The color-Debye screening mass is extracted from the color-singlet potential at imaginary μ, and the mass is extrapolated to real μby analytic continuation. The screening mass thus obtained has stronger μdependence than the prediction of the leading-order thermal perturbation theory at both real and imaginary μ.

hep-lat↗

arXiv2013

Cis-Trans Dynamical Asymmetry in Driven Polymer Translocation

During polymer translocation driven by e.g. voltage drop across a nanopore, the segments in the cis-side is incessantly pulled into the pore, which are then pushed out of it into the trans-side. This pulling and pushing polymer segments are described in the continuum level by nonlinear transport processes known, respectively, as fast and slow diffusions. By matching solutions of both sides through the mass conservation across the pore, we provide a physical basis for the cis and trans dynamical asymmetry, a feature repeatedly reported in recent numerical simulations. We then predict how the total driving force is dynamically allocated between cis (pulling) and trans (pushing) sides, demonstrating that the trans-side event adds a finite-chain length effect to the dynamical scaling, which may become substantial for weak force and/or high pore friction cases.

cond-mat.soft↗

arXiv2012

Two phase picture in driven polymer translocation

Two phase picture is a simple and effective methodology to capture the nonequilibrium dynamics of polymer associated with tension propagation. When applying it to the driven translocation process, there is a point to be noted, as briefly discussed in our recent article [Phys. Rev. E 85, 061803 (2012)]. In this article, we address this issue in detail and modify our previous prediction [Euro. Phys. J. E 34, 135 (2011)] by adopting an alternative steady-state ansatz. The modified scaling prediction turns out to be the same as that of the iso-flux model recently proposed by Rowghanian and Grosberg [J. Phys. Chem. B 115, 14127-14135 (2011)].

cond-mat.soft↗

arXiv2012

Towards extremely dense matter on the lattice

QCD is expected to have a rich phase structure. It is empirically known to be difficult to access low temperature and nonzero chemical potential $μ$ regions in lattice QCD simulations. We address this issue in a lattice QCD with the use of a dimensional reduction formula of the fermion determinant. We investigate spectral properties of a reduced matrix of the reduction formula. Lattice simulations with different lattice sizes show that the eigenvalues of the reduced matrix follow a scaling law for the temporal size $N_t$. The properties of the fermion determinant are examined using the reduction formula. We find that as a consequence of the $N_t$ scaling law, the fermion determinant becomes insensitive to $μ$ as $T$ decreases, and $μ$-independent at T=0 for $μ<m_π/2$. The $N_t$ scaling law provides two types of the low temperature limit of the fermion determinant: (i) for low density and (ii) for high-density. The fermion determinant becomes real and the theory is free from the sign problem in both cases. In case of (ii), QCD approaches to a theory, where quarks interact only in spatial directions, and gluons interact via the ordinary Yang-Mills action. The partition function becomes exactly $Z_3$ invariant even in the presence of dynamical quarks because of the absence of the temporal interaction of quarks. The reduction formula is also applied to the canonical formalism and Lee-Yang zero theorem. We find characteristic temperature dependences of the canonical distribution and of Lee-Yang zero trajectory. Using an assumption on the canonical partition function, we discuss physical meaning of those temperature dependences and show that the change of the canonical distribution and Lee-Yang zero trajectory are related to the existence/absence of $μ$-induced phase transitions.

hep-lat↗

arXiv2012

Process Time Distribution of Driven Polymer Transport

We discuss the temporal distribution of dynamic processes in driven polymer transport inherent to flexible chains due to stochastic tension propagation. The stochasticity originates from the disordered initial configuration of an equilibrium polymer coil, which results in random paths for tension propagation. We consider the process time for when translocation occurs across a fixed pore and when stretching occurs by pulling the chain end. A scaling argument for the mean and standard deviation of the process time is provided using the two-phase picture for stochastic propagation. The two cases are found to differ remarkably. The process time distribution of the translocation exhibits substantial spreading even in the long-chain limit, unlike that found for the dynamics of polymer stretching. In addition, the process time distribution in the driven translocation is shown to have a characteristic asymmetric shape.

cond-mat.soft↗

arXiv2012

Detecting the superfluid critical momentum of Bose gases in optical lattices through dipole oscillations

We study stability of superflow of Bose gases in optical lattices by analyzing the Bose-Hubbard model within the Gutzwiller mean-field approximation. We calculate the excitation spectra of the homogeneous Bose-Hubbard model at unit filling to determine the critical momenta for the Landau and dynamical instabilities. These two critical momenta are shown to approach each other when the on-site interaction increases towards the Mott transition point. In order to make a direct connection with realistic experiments, we next take into account a parabolic trapping potential and compute the real-time dynamics of dipole oscillations induced by suddenly displacing the trap center. We consider the following two cases: standard softcore bosons, whose interparticle interactions include the on-site one only, and hardcore bosons with long-range dipole-dipole interactions. For both cases, we show that the dipole oscillation is significantly damped when the maximum local momentum exceeds a certain threshold, which quantitatively agrees with the critical momentum for the dynamical instability in the homogeneous system. In the case of dipolar hardcore bosons, the dynamical instability of dipole oscillations leads to the formation of checkerboard density waves in the superfluid phase near the boundary to the supersolid phase.

cond-mat.quant-gas↗

arXiv2012

Dragging a polymer in a viscous fluid: steady-state and transient

We study the conformation and dynamics of a single polymer chain that is pulled by a constant force applied at its one end with the other end free. Such a situation is relevant to the growing technology of manipulating individual macromolecules, which offers a paradigm research for probing far-from-equilibrium responses of long flexible biological polymers. We first analyze the Rouse model for the Gaussian chains for which the exact analytical results can be obtained. More realistic features such as the finite extensibility, the excluded volume and the hydrodynamic interactions are taken into account with the help of the scaling argument, which leads to various nontrivial predictions such as the stretching-force-dependent friction constants. We elucidate (i) generalized dynamical equations of state describing extension/friction laws in steady-state and (ii) the tension propagation laws in the transient process. We point out that the time evolutions of the dynamic friction in the transient process crucially depend on the experimental protocol, i.e., either constant force or velocity ensemble, which might be detectable in experiments using giant DNAs and chromosomes.

cond-mat.soft↗

arXiv2011

Dynamical Diagram and Scaling in Polymer Driven Translocation

By analyzing the real space nonequilibrium dynamics of polymers, we elucidate the physics of driven translocation and propose its dynamical scaling scenario analogous to that in the surface growth phenomena. We provide a detailed account of the previously proposed tension-propagation formulation and extend it to cover the broader parameter space relevant to real experiments; In addition to a near-equilibrium regime, we identify three distinct nonequilibrium regimes reflecting the steady-state property of a dragged polymer with the finite extensibility. Finite size effects are also pointed out. These elements are shown to be crucial for the appropriate comparison with experiments and simulations.

cond-mat.soft↗

arXiv2011

Lattice QCD and High Baryon Density State

We report our recent studies on the finite density QCD obtained from lattice QCD simulation with clover-improved Wilson fermions of two flavor and RG-improved gauge action. We approach the subject from two paths, i.e., the imaginary and real chemical potentials.

hep-lat↗

arXiv2010

Finite-Width Bundle is Most Stable in a Solution with Salt

We applied the mean-field approach to a columnar bundle assembled by the parallel arrangement of stiff polyelectrolyte rods in a salt bath. The electrostatic potential can be divided into two regions: inside the bundle for condensed counter-ions, and outside the bundle for free small ions. To determine the distribution of condensed counter-ions inside the bundle, we use a local self-consistent condition that depends on the charge density, the electrostatic potential, and the net polarization. The results showed that, upon bundle formation, the electric charge of polyelectrolytes, even those inside the bundle, tend to survive in an inhomogeneous manner, and thus their width remains finite under thermal equilibrium because of the long-range effect of charge instability.

cond-mat.soft↗

arXiv2009

Wilson fermions with imaginary chemical potential

We study the phase structure of imaginary chemical potential. We calculate the Polyakov loop using clover-improved Wilson action and renormalization improved gauge action. We obtain a two-state signals indicating the first order phase transition for $β= 1.9, μ_I = 0.2618, κ=0.1388$ on $8^3\times 4$ lattice volume We also present a result of the matrix reduction formula for the Wilson fermion.

hep-lat↗

arXiv2009

Coulomb gauge gluon propagator on anisotropic lattices

We calculate the transverse and the time-time components of the Coulomb-gauge gluon propagator in SU(3) lattice Yang-Mills theory both on isotropic and anisotropic lattices. The problem of scaling violation observed on the isotropic lattice is drastically reduced as the anisotropy increases; namely, the system approaches the Hamiltonian limit. In the infrared region, the transverse gluon propagator exhibits a turnover and the temporal gluon propagator shows divergent behavior.

hep-lat↗

arXiv2006

Quark propagators at finite temperature with the clover action

We study properties of the finite temperature quark propagator by using the SU(3) quenched lattice simulation in the Landau gauge and report numerical results of the standard Wilson quark case as well as the improved clover one. The mass function in the deconfinement phase is different from that of the confinement phase, especially at low momentum regions.

hep-lat↗

arXiv2004

Why is the condensed phase of DNA preferred at higher temperature? DNA compaction in the presence of a multivalent cation

Upon the addition of multivalent cations, a giant DNA chain exhibits a large discrete transition from an elongated coil into a folded compact state. We performed single-chain observation of long DNAs in the presence of a tetravalent cation (spermine), at various temperatures and monovalent salt concentrations. We confirmed that the compact state is preferred at higher temperatures and at lower monovalent salt concentrations. This result is interpreted in terms of an increase in the net translational entropy of small ions due to ionic exchange between higher and lower valence ions.

cond-mat.soft↗

arXiv2004

Heavy $qq$ interaction at finite temperature

The first lattice QCD numerical study of heavy quark-quark potentials at finite temperature is reported. Using the quenched approximation, we evaluate the color anti-symmetric and symmetric potentials.

hep-lat↗