Kazuhiko Seki - Academia.edu (original) (raw)

Papers by Kazuhiko Seki

Journal of Chemical Physics, May 17, 2016

Diffusion in one dimensional rugged energy landscape (REL), is predicted to be pathologically dif... more Diffusion in one dimensional rugged energy landscape (REL), is predicted to be pathologically different (from any higher dimension) with much larger chance of encountering broken ergodicity (D. L. Stein and C. M. Newman, AIP Conf. Proc. 1479, 620 (2012)). However, no quantitative study of this difference has been reported, despite prevalence of multidimensional physical models in literature (like a high dimensional funnel guiding protein folding/unfolding). Paradoxically, some theoretical studies of these phenomena still employ a one dimensional diffusion description for analytical tractability. We explore the dimensionality dependent diffusion on REL by carrying out an effective medium approximation based analytical calculations and compare them with the available computer simulation results. We find that at intermediate level of ruggedness (assumed to have _____________________________ a)

arXiv (Cornell University), Nov 3, 2009

Using a hydrodynamic theory that incorporates a momentum decay mechanism, we calculate the drag c... more Using a hydrodynamic theory that incorporates a momentum decay mechanism, we calculate the drag coefficient of a circular liquid domain of finite viscosity moving in a two-dimensional membrane. We derive an analytical expression for the drag coefficient which covers the whole range of domain sizes. Several limiting expressions are discussed. The obtained drag coefficient decreases as the domain viscosity becomes smaller with respect to the outer membrane viscosity. This is because the flow induced in the domain acts to transport the fluid in the surrounding matrix more efficiently.

Meeting abstracts of the Physical Society of Japan, Mar 3, 2011

Applied Physics Letters, Feb 23, 2009

Recently power law dynamics was observed in the fluorescence decay from semiconductor nanocrystal... more Recently power law dynamics was observed in the fluorescence decay from semiconductor nanocrystals and a model was proposed and analyzed on the basis the of Monte Carlo simulation to clarify the relation between the power law fluorescence decay and the power law blinking. However, analysis of the model on the basis of the Monte Carlo simulation is not decisive. In this paper we present the analytical solution of the model. The analytical result is used to prove rigorously that the exponent of the power law blinking and that of the power law fluorescence decay are the same.

Journal of Chemical Physics, Oct 8, 2003

We study the kinetics of the recombination reaction in subdiffusive media, where the displacement... more We study the kinetics of the recombination reaction in subdiffusive media, where the displacement of reactants r(t) follows 〈r2(t)〉∝tα with 0<α<1. We derive a rigorous fractional reaction–diffusion equation from a continuous time random walk model and calculate the kinetics of recombination reaction on the basis of this equation. The survival probability of a particle starting at r0 has an asymptotic time dependence of t−α/2 for both the perfectly absorbing and the partially reflecting boundary conditions. The change in the boundary condition alters only the coefficient for the asymptotic time dependence. The asymptotic time dependence of the survival probability is confirmed by the numerical simulations and supported by the results of a lattice model.

EPL, Oct 15, 1997

An analytically solvable linear model for stochastic resonance driven by multiplicative noise of ... more An analytically solvable linear model for stochastic resonance driven by multiplicative noise of the Gaussian type is presented. A maximum of the signal-to-noise ratio is observed both as a function of the noise intensity and as a function of the autocorrelation time. The maximum appears for low frequencies of the applied external oscillating field immediately as soon as noise correlation is introduced, and disappears as the correlation or the frequency is increased. No resonance occurs for white noise.

arXiv (Cornell University), Nov 26, 2010

We investigate the hydrodynamic effects on the dynamics of critical concentration fluctuations in... more We investigate the hydrodynamic effects on the dynamics of critical concentration fluctuations in multicomponent fluid membranes. Two geometrical cases are considered; (i) confined membrane case and (ii) supported membrane case. We numerically calculate the wavenumber dependence of the effective diffusion coefficient by changing the temperature and/or the thickness of the bulk fluid. For some limiting cases, the result is compared with the previously obtained analytical expression. An analogy of the multicomponent membrane to 2D microemulsion is explored for the confined membrane geometry.

Journal of Physical Chemistry C, Jul 28, 2020

This paper presents a theoretical and computational study of charge-carrier transport in organic ... more This paper presents a theoretical and computational study of charge-carrier transport in organic solids in the presence of Gaussian energy disorder. A simulation methodology is developed to calculate the equilibrium low-field charge-carrier mobility with high precision irrespective of the magnitude of disorder. Using the simulation results obtained for several lattice models, we give accurate expressions of the mobility in the considered systems, which also relate this quantity to its value in a corresponding system of no disorder. We discuss the dependence of mobility on the lattice coordination number and dimensionality. Our theory can be applied to interpret experimental data on charge-carrier transport and related processes, for example, photoconductivity. Our methodology can also provide the reference data for developing new analytical models.

Studies in Physical and Theoretical Chemistry, 1995

ABSTRACT

Journal of Physical Chemistry A, Mar 1, 2005

In this paper, we study the fluorescence fluctuation correlation function in structured fluids wh... more In this paper, we study the fluorescence fluctuation correlation function in structured fluids where the diffusion coefficients of probe molecules have different values depending on the distance from initial position, and we derive two simple expressions. Both of them reproduce the exact numerical results rather accurately. One of the expressions contains a time-dependent diffusion coefficient and has a clear physical meaning. We show a procedure to analyze experimental data using the time-dependent diffusion coefficient which results from crossover from free diffusion inside a mesh to hindered diffusion through mesh structures.

Physical review, 1993

The time evolution of the mean deviation of initially close trajectories in a stochastic dynamica... more The time evolution of the mean deviation of initially close trajectories in a stochastic dynamical system is investigated. It is shown both for additive and linearly coupled multiplicative noise that the mean deviation loses its dependence on initial conditions for long times. For shorter times a power law is found for certain types of additive noise processes, in sharp contrast to the exponential separation of initially nearby trajectories in deterministic chaotic systems. Exponential time evolution is obtained for linearly coupled multiplicative noise after an initial transient during which more complex regimes, including a superexponential stage, can take place.

The Journal of Physical Chemistry C, 2018

Sm2Ti2S2O5 (STSO) is a visible-light-responsive oxysulfide semiconductor photocatalyst with appli... more Sm2Ti2S2O5 (STSO) is a visible-light-responsive oxysulfide semiconductor photocatalyst with applications to water splitting. In this work, plate-like STSO particles were synthesized through a flux-assisted one-step method at various temperatures. The activities of these materials during photocatalytic and photoelectrochemical O2 evolution from aqueous solutions were investigated. Single-phase STSO with a single crystal habit was produced at 923 K, which is approximately 200 K lower than the temperatures required for previously reported methods, such as solid-state reactions and thermal sulfurization under a H2S flow. The STSO sample synthesized at the optimal temperature exhibited an AQE of 1.3±0.2% at 420 nm during photocatalytic sacrificial O2 evolution. This efficiency is twice the values reported for specimens prepared using conventional methods. An STSO/Ti/Sn electrode fabricated by the particle transfer method generated a photoanodic current and evolved O2 by water oxidation with a Faradaic efficiency of approximately 70±7%. The synthesis temperature yielding the highest activity was lower for photocatalytic O2 evolution than for photoelectrochemical O2 evolution. This work demonstrates the applicability of the flux method to the synthesis of well-crystallized oxysulfides having various particle sizes and intended for different uses.

Physical Review E, 1998

A periodically driven linear system subject to multiplicative correlated noise is considered. It ... more A periodically driven linear system subject to multiplicative correlated noise is considered. It has been argued recently by several authors that such a simple system exhibits stochastic resonance. By introducing a general type of composite stochastic process, bridging two previously considered limiting cases of dichotomous and Gaussian noise, it is proved that, indeed, the amplitude of the average of the driven linear process at long times shows a pronounced maximum both as a function of the noise strength and as a function of the autocorrelation time. However, this kind of stochastic resonant behavior can be experimentally observable only in a special case where the initial phase of the external forcing is somehow fixed. Additional averaging over the uniform distribution of the initial random phase, inherent in most physical systems, leads to that the periodic output vanishes identically at long times. Moreover, the system response is typically defined in terms of the power spectrum rather than the amplitude of the average. The output signal given by the spectral density corresponding to the frequency of the external forcing is calculated via the long-time phase-averaged correlation function. It appears that the output signal simply diverges upon approaching the second moment instability point with increasing noise strength. No stochastic resonance is observed for any parameter settings. Interestingly, the resonancelike behavior of the system response as a function of the autocorrelation time is retained.

Journal of the American Chemical Society, Feb 1, 2017

Development of sunlight-driven water splitting systems with high efficiency, scalability, and cos... more Development of sunlight-driven water splitting systems with high efficiency, scalability, and cost-competitiveness is a central issue for mass production of solar hydrogen as a renewable and storable energy carrier. Photocatalyst sheets comprising a particulate hydrogen evolution photocatalyst (HEP) and an oxygen evolution photocatalyst (OEP) embedded in a conductive thin film can realize efficient and scalable solar hydrogen production using Z-scheme water splitting. However, the use of expensive precious metal thin films that also promote reverse reactions is a major obstacle to developing a cost-effective process at ambient pressure. In this study, we present a standalone particulate photocatalyst sheet based on an earth-abundant, relatively inert, and conductive carbon film for efficient Z-scheme water splitting at ambient pressure. A SrTiO3:La,Rh/C/BiVO4:Mo sheet is shown to achieve unassisted pure-water (pH 6.8) splitting with a solar-to-hydrogen energy conversion efficiency (...

Physical Review E, 1993

The time evolution of the mean deviation of initially close trajectories in a stochastic dynamica... more The time evolution of the mean deviation of initially close trajectories in a stochastic dynamical system is investigated. It is shown both for additive and linearly coupled multiplicative noise that the mean deviation loses its dependence on initial conditions for long times. For shorter times a power law is found for certain types of additive noise processes, in sharp contrast to the exponential separation of initially nearby trajectories in deterministic chaotic systems. Exponential time evolution is obtained for linearly coupled multiplicative noise after an initial transient during which more complex regimes, including a superexponential stage, can take place.

The Journal of Chemical Physics, 2014

Rugged energy landscapes find wide applications in diverse fields ranging from astrophysics to pr... more Rugged energy landscapes find wide applications in diverse fields ranging from astrophysics to protein folding. We study the dependence of diffusion coefficient (D) of a Brownian particle on the distribution width (ɛ) of randomness in a Gaussian random landscape by simulations and theoretical analysis. We first show that the elegant expression of Zwanzig [Proc. Natl. Acad. Sci. U.S.A. 85, 2029 (1988)] for D(ɛ) can be reproduced exactly by using the Rosenfeld diffusion-entropy scaling relation. Our simulations show that Zwanzig's expression overestimates D in an uncorrelated Gaussian random lattice – differing by almost an order of magnitude at moderately high ruggedness. The disparity originates from the presence of “three-site traps” (TST) on the landscape – which are formed by the presence of deep minima flanked by high barriers on either side. Using mean first passage time formalism, we derive a general expression for the effective diffusion coefficient in the presence of TST...

The Journal of Physical Chemistry A, 2005

In this paper, we study the fluorescence fluctuation correlation function in structured fluids wh... more In this paper, we study the fluorescence fluctuation correlation function in structured fluids where the diffusion coefficients of probe molecules have different values depending on the distance from initial position, and we derive two simple expressions. Both of them reproduce the exact numerical results rather accurately. One of the expressions contains a time-dependent diffusion coefficient and has a clear physical meaning. We show a procedure to analyze experimental data using the time-dependent diffusion coefficient which results from crossover from free diffusion inside a mesh to hindered diffusion through mesh structures.

Physica A: Statistical Mechanics and its Applications, 1994

ABSTRACT

Journal of the Physical Society of Japan, 1990

Journal of Chemical Physics, May 17, 2016

Diffusion in one dimensional rugged energy landscape (REL), is predicted to be pathologically dif... more Diffusion in one dimensional rugged energy landscape (REL), is predicted to be pathologically different (from any higher dimension) with much larger chance of encountering broken ergodicity (D. L. Stein and C. M. Newman, AIP Conf. Proc. 1479, 620 (2012)). However, no quantitative study of this difference has been reported, despite prevalence of multidimensional physical models in literature (like a high dimensional funnel guiding protein folding/unfolding). Paradoxically, some theoretical studies of these phenomena still employ a one dimensional diffusion description for analytical tractability. We explore the dimensionality dependent diffusion on REL by carrying out an effective medium approximation based analytical calculations and compare them with the available computer simulation results. We find that at intermediate level of ruggedness (assumed to have _____________________________ a)

arXiv (Cornell University), Nov 3, 2009

Using a hydrodynamic theory that incorporates a momentum decay mechanism, we calculate the drag c... more Using a hydrodynamic theory that incorporates a momentum decay mechanism, we calculate the drag coefficient of a circular liquid domain of finite viscosity moving in a two-dimensional membrane. We derive an analytical expression for the drag coefficient which covers the whole range of domain sizes. Several limiting expressions are discussed. The obtained drag coefficient decreases as the domain viscosity becomes smaller with respect to the outer membrane viscosity. This is because the flow induced in the domain acts to transport the fluid in the surrounding matrix more efficiently.

Meeting abstracts of the Physical Society of Japan, Mar 3, 2011

Applied Physics Letters, Feb 23, 2009

Recently power law dynamics was observed in the fluorescence decay from semiconductor nanocrystal... more Recently power law dynamics was observed in the fluorescence decay from semiconductor nanocrystals and a model was proposed and analyzed on the basis the of Monte Carlo simulation to clarify the relation between the power law fluorescence decay and the power law blinking. However, analysis of the model on the basis of the Monte Carlo simulation is not decisive. In this paper we present the analytical solution of the model. The analytical result is used to prove rigorously that the exponent of the power law blinking and that of the power law fluorescence decay are the same.

Journal of Chemical Physics, Oct 8, 2003

We study the kinetics of the recombination reaction in subdiffusive media, where the displacement... more We study the kinetics of the recombination reaction in subdiffusive media, where the displacement of reactants r(t) follows 〈r2(t)〉∝tα with 0<α<1. We derive a rigorous fractional reaction–diffusion equation from a continuous time random walk model and calculate the kinetics of recombination reaction on the basis of this equation. The survival probability of a particle starting at r0 has an asymptotic time dependence of t−α/2 for both the perfectly absorbing and the partially reflecting boundary conditions. The change in the boundary condition alters only the coefficient for the asymptotic time dependence. The asymptotic time dependence of the survival probability is confirmed by the numerical simulations and supported by the results of a lattice model.

EPL, Oct 15, 1997

An analytically solvable linear model for stochastic resonance driven by multiplicative noise of ... more An analytically solvable linear model for stochastic resonance driven by multiplicative noise of the Gaussian type is presented. A maximum of the signal-to-noise ratio is observed both as a function of the noise intensity and as a function of the autocorrelation time. The maximum appears for low frequencies of the applied external oscillating field immediately as soon as noise correlation is introduced, and disappears as the correlation or the frequency is increased. No resonance occurs for white noise.

arXiv (Cornell University), Nov 26, 2010

We investigate the hydrodynamic effects on the dynamics of critical concentration fluctuations in... more We investigate the hydrodynamic effects on the dynamics of critical concentration fluctuations in multicomponent fluid membranes. Two geometrical cases are considered; (i) confined membrane case and (ii) supported membrane case. We numerically calculate the wavenumber dependence of the effective diffusion coefficient by changing the temperature and/or the thickness of the bulk fluid. For some limiting cases, the result is compared with the previously obtained analytical expression. An analogy of the multicomponent membrane to 2D microemulsion is explored for the confined membrane geometry.

Journal of Physical Chemistry C, Jul 28, 2020

This paper presents a theoretical and computational study of charge-carrier transport in organic ... more This paper presents a theoretical and computational study of charge-carrier transport in organic solids in the presence of Gaussian energy disorder. A simulation methodology is developed to calculate the equilibrium low-field charge-carrier mobility with high precision irrespective of the magnitude of disorder. Using the simulation results obtained for several lattice models, we give accurate expressions of the mobility in the considered systems, which also relate this quantity to its value in a corresponding system of no disorder. We discuss the dependence of mobility on the lattice coordination number and dimensionality. Our theory can be applied to interpret experimental data on charge-carrier transport and related processes, for example, photoconductivity. Our methodology can also provide the reference data for developing new analytical models.

Studies in Physical and Theoretical Chemistry, 1995

ABSTRACT

Journal of Physical Chemistry A, Mar 1, 2005

In this paper, we study the fluorescence fluctuation correlation function in structured fluids wh... more In this paper, we study the fluorescence fluctuation correlation function in structured fluids where the diffusion coefficients of probe molecules have different values depending on the distance from initial position, and we derive two simple expressions. Both of them reproduce the exact numerical results rather accurately. One of the expressions contains a time-dependent diffusion coefficient and has a clear physical meaning. We show a procedure to analyze experimental data using the time-dependent diffusion coefficient which results from crossover from free diffusion inside a mesh to hindered diffusion through mesh structures.

Physical review, 1993

The time evolution of the mean deviation of initially close trajectories in a stochastic dynamica... more The time evolution of the mean deviation of initially close trajectories in a stochastic dynamical system is investigated. It is shown both for additive and linearly coupled multiplicative noise that the mean deviation loses its dependence on initial conditions for long times. For shorter times a power law is found for certain types of additive noise processes, in sharp contrast to the exponential separation of initially nearby trajectories in deterministic chaotic systems. Exponential time evolution is obtained for linearly coupled multiplicative noise after an initial transient during which more complex regimes, including a superexponential stage, can take place.

The Journal of Physical Chemistry C, 2018

Sm2Ti2S2O5 (STSO) is a visible-light-responsive oxysulfide semiconductor photocatalyst with appli... more Sm2Ti2S2O5 (STSO) is a visible-light-responsive oxysulfide semiconductor photocatalyst with applications to water splitting. In this work, plate-like STSO particles were synthesized through a flux-assisted one-step method at various temperatures. The activities of these materials during photocatalytic and photoelectrochemical O2 evolution from aqueous solutions were investigated. Single-phase STSO with a single crystal habit was produced at 923 K, which is approximately 200 K lower than the temperatures required for previously reported methods, such as solid-state reactions and thermal sulfurization under a H2S flow. The STSO sample synthesized at the optimal temperature exhibited an AQE of 1.3±0.2% at 420 nm during photocatalytic sacrificial O2 evolution. This efficiency is twice the values reported for specimens prepared using conventional methods. An STSO/Ti/Sn electrode fabricated by the particle transfer method generated a photoanodic current and evolved O2 by water oxidation with a Faradaic efficiency of approximately 70±7%. The synthesis temperature yielding the highest activity was lower for photocatalytic O2 evolution than for photoelectrochemical O2 evolution. This work demonstrates the applicability of the flux method to the synthesis of well-crystallized oxysulfides having various particle sizes and intended for different uses.

Physical Review E, 1998

A periodically driven linear system subject to multiplicative correlated noise is considered. It ... more A periodically driven linear system subject to multiplicative correlated noise is considered. It has been argued recently by several authors that such a simple system exhibits stochastic resonance. By introducing a general type of composite stochastic process, bridging two previously considered limiting cases of dichotomous and Gaussian noise, it is proved that, indeed, the amplitude of the average of the driven linear process at long times shows a pronounced maximum both as a function of the noise strength and as a function of the autocorrelation time. However, this kind of stochastic resonant behavior can be experimentally observable only in a special case where the initial phase of the external forcing is somehow fixed. Additional averaging over the uniform distribution of the initial random phase, inherent in most physical systems, leads to that the periodic output vanishes identically at long times. Moreover, the system response is typically defined in terms of the power spectrum rather than the amplitude of the average. The output signal given by the spectral density corresponding to the frequency of the external forcing is calculated via the long-time phase-averaged correlation function. It appears that the output signal simply diverges upon approaching the second moment instability point with increasing noise strength. No stochastic resonance is observed for any parameter settings. Interestingly, the resonancelike behavior of the system response as a function of the autocorrelation time is retained.

Journal of the American Chemical Society, Feb 1, 2017

Development of sunlight-driven water splitting systems with high efficiency, scalability, and cos... more Development of sunlight-driven water splitting systems with high efficiency, scalability, and cost-competitiveness is a central issue for mass production of solar hydrogen as a renewable and storable energy carrier. Photocatalyst sheets comprising a particulate hydrogen evolution photocatalyst (HEP) and an oxygen evolution photocatalyst (OEP) embedded in a conductive thin film can realize efficient and scalable solar hydrogen production using Z-scheme water splitting. However, the use of expensive precious metal thin films that also promote reverse reactions is a major obstacle to developing a cost-effective process at ambient pressure. In this study, we present a standalone particulate photocatalyst sheet based on an earth-abundant, relatively inert, and conductive carbon film for efficient Z-scheme water splitting at ambient pressure. A SrTiO3:La,Rh/C/BiVO4:Mo sheet is shown to achieve unassisted pure-water (pH 6.8) splitting with a solar-to-hydrogen energy conversion efficiency (...

Physical Review E, 1993

The time evolution of the mean deviation of initially close trajectories in a stochastic dynamica... more The time evolution of the mean deviation of initially close trajectories in a stochastic dynamical system is investigated. It is shown both for additive and linearly coupled multiplicative noise that the mean deviation loses its dependence on initial conditions for long times. For shorter times a power law is found for certain types of additive noise processes, in sharp contrast to the exponential separation of initially nearby trajectories in deterministic chaotic systems. Exponential time evolution is obtained for linearly coupled multiplicative noise after an initial transient during which more complex regimes, including a superexponential stage, can take place.

The Journal of Chemical Physics, 2014

Rugged energy landscapes find wide applications in diverse fields ranging from astrophysics to pr... more Rugged energy landscapes find wide applications in diverse fields ranging from astrophysics to protein folding. We study the dependence of diffusion coefficient (D) of a Brownian particle on the distribution width (ɛ) of randomness in a Gaussian random landscape by simulations and theoretical analysis. We first show that the elegant expression of Zwanzig [Proc. Natl. Acad. Sci. U.S.A. 85, 2029 (1988)] for D(ɛ) can be reproduced exactly by using the Rosenfeld diffusion-entropy scaling relation. Our simulations show that Zwanzig's expression overestimates D in an uncorrelated Gaussian random lattice – differing by almost an order of magnitude at moderately high ruggedness. The disparity originates from the presence of “three-site traps” (TST) on the landscape – which are formed by the presence of deep minima flanked by high barriers on either side. Using mean first passage time formalism, we derive a general expression for the effective diffusion coefficient in the presence of TST...

The Journal of Physical Chemistry A, 2005

In this paper, we study the fluorescence fluctuation correlation function in structured fluids wh... more In this paper, we study the fluorescence fluctuation correlation function in structured fluids where the diffusion coefficients of probe molecules have different values depending on the distance from initial position, and we derive two simple expressions. Both of them reproduce the exact numerical results rather accurately. One of the expressions contains a time-dependent diffusion coefficient and has a clear physical meaning. We show a procedure to analyze experimental data using the time-dependent diffusion coefficient which results from crossover from free diffusion inside a mesh to hindered diffusion through mesh structures.

Physica A: Statistical Mechanics and its Applications, 1994

ABSTRACT

Journal of the Physical Society of Japan, 1990