arXiv subjects

Kazuhiko Seki

Publications and source records attributed to Kazuhiko Seki.

At least 37 records · Page 2Linked to original sources

arXiv2015

Coil--globule transition of a polymer involved in excluded-volume interactions with macromolecules

Polymers adopt extended coil and compact globule states according to the balance between entropy and interaction energies. The transition of a polymer between an extended coil state and compact globule state can be induced by changing thermodynamic force such as temperature to alter the energy/entropy balance. Previously, this transition was theoretically studied by taking into account the excluded-volume interaction between monomers of a polymer chain using the partition function. For binary mixtures of a long polymer and short polymers, the coil-globule transition can be induced by changing the concentration of the shorter polymers. Here we investigate the transition caused by short polymers by generalizing the partition function of the long polymer to include the excluded-volume effect of short polymers. The coil-globule transition is studied as a function of the concentration of mixed polymers by systematically varying Flory's chi-parameters. We show that the transition is caused by the interplay between the excluded-volume interaction and the dispersion state of short polymers in the solvent. We also reveal that the same results can be obtained by combining the mixing entropy and elastic energy if the volume of a long polymer is properly defined.

cond-mat.soft↗

arXiv2015

Relationship between Entropy and Diffusion: A statistical mechanical derivation of Rosenfeld expression for a rugged energy landscape

Diffusion-a measure of dynamics, and entropy-a measure of disorder in the system, are found to be intimately correlated in many systems, and the correlation is often strongly non-linear. We explore the origin of this complex dependence by studying diffusion of a point Brownian particle on a model potential energy surface characterized by ruggedness. If we assume that the ruggedness has a Gaussian distribution then for this model, one can obtain the excess entropy exactly for any dimension. By using the expression for the mean first passage time (MFPT), we present a statistical mechanical derivation of the well-known and well-tested scaling relation proposed by Rosenfeld between diffusion and excess entropy. In anticipation that Rosenfeld diffusion-entropy scaling (RDES) relation may continue to be valid in higher dimensions (where the mean first passage time approach is not available), we carry out an effective medium approximation (EMA) based analysis of the effective transition rate, and hence of the effective diffusion coefficient. We show that the EMA expression can be used to derive the RDES scaling relation for any dimension higher than unity. However, RDES is shown to break down in the presence of spatial correlation among the energy landscape values.

cond-mat.soft↗

arXiv2015

Theoretical limit of power conversion efficiency for organic and hybrid halide perovskite photovoltaics

We calculated the maximum power conversion efficiency as a function of the optical band gap for organic photovoltaic (PV) cells by assuming that charge separation is accompanied by the energy loss required to dissociate strongly bound charge pairs. The dissociation energy can be estimated from the relationship between the open circuit voltage (VOC) and the optical band gap (Eg). By analyzing the published data on VOC and Eg, the dissociation energy can be estimated. The result could be used as a guide for selecting donor and acceptor materials. We also studied the theoretical limit of power conversion efficiency of hybrid halide perovskite by taking into account the energy loss involved in the carrier transfer from the perovskite phase to the metal oxide charge transport layer.

cond-mat.mtrl-sci↗

arXiv2014

Transition from distributional to ergodic behavior in an inhomogeneous diffusion process: Method revealing an unknown surface diffusivity

Diffusion of molecules in cells plays an important role in providing a biological reaction on the surface by finding a target on the membrane surface. The water retardation (slow diffusion) near the target assists the searching molecules to recognize the target. Here, we consider effects of the surface on the diffusivity in three-dimensional diffusion processes, where diffusion on the surface is slower than that in bulk. We show that the ensemble-averaged mean square displacements increase linearly with time when the desorption rate from the surface is finite even when the diffusion on the surface is subdiffusion. Moreover, this slow diffusion on the surface affects the fluctuations of the time-averaged mean square displacements (TAMSDs). We find that fluctuations of the TAMSDs remain large when the measurement time is smaller than a characteristic relaxation time, and decays according to an increase of the measurement time for a relatively large measurement time. Therefore, we find a transition from non-ergodic (distributional) to ergodic diffusivity in a target search process. Moreover, this fluctuation analysis provides a method to estimate an unknown surface diffusivity.

cond-mat.stat-mech↗

arXiv2014

Diffusion in a rough potential revisited

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 $(\varepsilon)$ of randomness in a Gaussian random landscape by simulations and theoretical analysis. We first show that the elegant expression of Zwanzig [PNAS, 85, 2029 (1988)] for $D(\varepsilon)$ 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, that quantitatively reproduces the simulation results and which reduces to Zwanzig's form only in the limit of infinite spatial correlation. We construct a continuous Gaussian field with inherent correlation to establish the effect of spatial correlation on random walk. The presence of TSTs at large ruggedness $(\varepsilon \gg k_{B}T)$ give rise to an apparent breakdown of ergodicity of the type often encountered in glassy liquids.

physics.chem-ph↗

arXiv2014

Overall current-voltage characteristics of space charge controlled currents for thin films by a single carrier species

The Mott-Gurney equation (Child's law) has been frequently applied to measure the mobility of carrier transport layers. One of the main assumption in the Mott-Gurney theory is ignoring the diffusive currents. It was not obvious, however, whether the diffusive currents can be ignored for thin carrier transport layers. We obtained the current-voltage relation using analytical solutions of drift-diffusion equation coupled with the Poisson's equation. The integration constants were numerically determined using nonlinear equations obtained from boundary conditions.A simple analytical relation between the voltage and current was also derived. The analytical equation improved over the Mott-Gurney equation when the voltage is between 0.1 and 2 [V] at room temperature. By using published data, we show that both the mobility and the layer thickness can be simultaneously obtained by applying the analytical expression. The effect of diffusion on the current-voltage relation is explained by the movement of the virtual electrode formed by space charge accumulation.

cond-mat.mtrl-sci↗

arXiv2014

Diffusion coefficients in leaflets of bilayer membranes

We study diffusion coefficients of liquid domains by explicitly taking into account the two-layered structure called leaflets of the bilayer membrane. In general, the velocity fields associated with each leaflet are different and the layers sliding past each other cause frictional coupling. We obtain analytical results of diffusion coefficients for a circular liquid domain in a leaflet, and quantitatively study their dependence on the inter-leaflet friction. We also show that the diffusion coefficients diverge in the absence of coupling between the bilayer and solvents, even when the inter-leaflet friction is taken into account. In order to corroborate our theory, the effect of the inter-leaflet friction on the correlated diffusion is examined.

cond-mat.soft↗

arXiv2013

Effect of Energetic Disorder on the Open-Circuit Voltage in Organic Bulk Heterojunction Composites

Under open-circuit condition, the current is not extracted and the photogenerated carriers in principle disappear only by recombination. We study the open-circuit voltage $V_{\rm OC}$ and transient photovoltage under the effect of bulk recombination in a medium with energetic disorder by using the multiple trapping (MT) model. The key parameter in the MT model is the dispersion parameter $α$ given by the ratio of thermal energy to the characteristic energy of trap states. We show that $V_{\rm OC}$ depends linearly on the logarithm of the light intensity and the slope depends on the $α$ of the MT model. Under the continuous irradiation of light, the photovoltage response to the weak perturbation by a pulsed light obeys pseudo-first-order decay. The rate as a function of $V_{\rm OC}$ is independent of the dispersion parameter. However, it obeys the power law as a function of light intensity, and the exponent is given by $1/(1+α)$, which reduces to 1/2 in the absence of energetic disorder.

cond-mat.mtrl-sci↗

arXiv2013

Growth kinetics of circular liquid domains on vesicles by diffusion-controlled coalescence

Motivated by recent experiments on multi-component membranes, the growth kinetics of domains on vesicles is theoretically studied. It is known that the steady-state rate of coalescence cannot be obtained by taking the long-time limit of the coalescence rate when the membrane is regarded as an infinite two-dimensional (2D) system. The steady-state rate of coalescence is obtained by explicitly taking into account the spherical vesicle shape. Using the expression of the 2D diffusion coefficient obtained in the limit of small domain size, an analytical expression for the domain growth kinetics is obtained when the circular shape is always maintained. For large domains, the growth kinetics is discussed by investigating the size dependence of the coalescence rate using the expression for the diffusion coefficient of arbitrary domain size.

cond-mat.soft↗

arXiv2013

Theory of Bulk Recombination in a Medium with Energetic Disorder under Continuous Irradiation with Excitation Light

We study bulk recombination in a medium with energetic disorder by using the multiple trapping (MT) model under continuous irradiation with excitation light. Charge densities are shown to obey power law as a function of the intensity of excitation light and the exponent is related to the dispersion parameter $α$ of the MT model. The theory is applied to analyze charge densities in bulk heterojunction photovoltaic cells measured by light-induced electron spin resonance (LESR). The values of the exponent are consistent with those obtained from the transient decay of charge densities after pulsed excitations.We show that the value of the exponent is robust against addition of Gaussian trapping energy distribution due to shallow trap states. We also show that open-circuit voltage ($V_{\rm OC}$) depends linearly on logarithm of the light intensity and the slope is equal to $k_B T/e$ when only shallow trap states are present. The same linear relation holds with a slope larger than $k_B T/e$ in the presence of an exponential trapping energy distribution due to deep trap states.

cond-mat.mtrl-sci↗

arXiv2012

Viscoelasticity of two-layer-vesicles in solution

The dynamic shape relaxation of the two-layer-vesicle is calculated. In additional to the undulation relaxation where the two bilayers move in the same direction, the squeezing mode appears when the gap between the two bilayers is small. At large gap, the inner vesicle relaxes much faster, whereas the slow mode is mainly due to the outer layer relaxation. We have calculated the viscoelasticity of the dilute two-layer-vesicle suspension. It is found that for small gap, the applied shear drives the undulation mode strongly while the slow squeezing mode is not much excited. In this limit the complex viscosity is dominated by the fast mode contribution. On the other hand, the slow mode is strongly driven by shear for larger gap. We have determined the crossover gap which depends on the interaction between the two bilayers. For a series of samples where the gap is changed systematically, it is possible to observe the two amplitude switchings.

cond-mat.soft↗

arXiv2011

Diffusion-mediated geminate reactions under excluded volume interactions

In this paper, influence of crowding by inert particles on the geminate reaction kinetics is theoretically investigated. Time evolution equations for the survival probability of a geminate pair are derived from the master equation taking into account the correlation among all diffusing particles. The results interpolate between low and high concentrations of the inert particles. Excluded volume interactions by the inert particles hinder the diffusive motion of reactants. When the initial distribution of the inert particles is uniform, the excluded volume interactions slow the decay of the survival probability of a geminate pair in contrast to the acceleration of the decay found for the target problem under the presence of the excluded volume interactions among reactants. We also obtain the escape probability for a non-uniform initial distribution of the inert particles and show that reaction yield is increased when the reaction proceeds in the presence of a positive density gradient of the inert particles which inhibits the escape of reactants. The effect can be interpreted as a cage effect.

cond-mat.stat-mech↗

arXiv2011

Diffusion coefficient of an inclusion in a liquid membrane supported by a solvent of arbitrary thickness

The diffusion coefficient of a circular shaped inclusion in a liquid membrane is investigated by taking into account the interaction between membranes and bulk solvents of arbitrary thickness. As illustrative examples, the diffusion coefficients of two types of inclusions - a circular domain composed of fluid with the same viscosity as the host membrane and that of a polymer chain embedded in the membrane are studied.The diffusion coefficients are expressed in terms of the hydrodynamic screening lengths which vary according to the solvent thickness. When the membrane fluid is dragged by the solvent of finite thickness, via stick boundary conditions, multiple hydrodynamic screening lengths together with the weight factors to the diffusion coefficients are obtained from the dispersion relation. The condition for which the diffusion coefficients can be approximated by the expression including only a single hydrodynamic screening length are also shown.

cond-mat.soft↗

arXiv2011

Dynamics of a polymer chain confined in a membrane

We present a Brownian dynamics theory with full hydrodynamics (Stokesian dynamics) for a Gaussian polymer chain embedded in a liquid membrane which is surrounded by bulk solvent and walls. The mobility tensors are derived in Fourier space for the two geometries, namely, a free membrane embedded in a bulk fluid, and a membrane sandwiched by the two walls. Within the preaveraging approximation, a new expression for the diffusion coefficient of the polymer is obtained for the free membrane geometry. We also carry out a Rouse normal mode analysis to obtain the relaxation time and the dynamical structure factor. For large polymer size, both quantities show Zimm-like behavior in the free membrane case, whereas they are Rouse-like for the sandwiched membrane geometry. We use the scaling argument to discuss the effect of excluded volume interactions on the polymer relaxation time.

cond-mat.soft↗

arXiv2010

Hydrodynamic effects on concentration fluctuations in multicomponent membranes

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.

cond-mat.soft↗

arXiv2010

Drag coefficient of a liquid domain in a two-dimensional membrane

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.

cond-mat.soft↗

arXiv2009

Reaction under vacancy-assisted diffusion at high quencher concentration

Theory of diffusion-mediated reactions is already established for the target problem in the dilute limit, where the immobile target is surrounded by many quenchers. For lattice random walks in the crowded situation, each quencher is surrounded by other quenchers differently. As a result, each quencher migrates differently in the presence of site blocking effects. However, in the conventional theory, such difference is ignored and quenchers are assumed to move independently of each other. In this paper, theory of diffusion-mediated reactions of target problem is developed by taking into account the site blocking effects for quencher migration and the difference in the configuration of quenchers around each quencher. Our result interpolates between those in high and low limits of quencher concentrations and is a lower bound of the survival probability. In the static limit, the exact result is reproduced for a localized sink. In the presence of diffusion, the approximation is better when intrinsic reaction rates are low.

cond-mat.stat-mech↗

arXiv2008

Non-equilibrium thermodynamic study of magnetization dynamics in the presence of spin-transfer torque

The dynamics of magnetization in the presence of spin-transfer torque was studied. We derived the equation for the motion of magnetization in the presence of a spin current by using the local equilibrium assumption in non-equilibrium thermodynamics. We show that, in the resultant equation, the ratio of the Gilbert damping constant, $α$, and the coefficient, $β$, of the current-induced torque, called non-adiabatic torque, depends on the relaxation time of the fluctuating field $τ_{c}$. The equality $α=β$ holds when $τ_c$ is very short compared to the time scale of magnetization dynamics. We apply our theory to current-induced magnetization reversal in magnetic multilayers and show that the switching time is a decreasing function of $τ_{c}$.

cond-mat.mes-hall↗