arXiv subjects

Akira Onuki

Publications and source records attributed to Akira Onuki.

At least 19 recordsLinked to original sources

arXiv2026

Global space correlations of polarization, charge density, and electric field in electrolytes under the fixed-potential condition

We examine the thermal fluctuations of the polarization $p$, the charge density $ρ$, and the electric field $E$ in dilute electrolytes inserted between pararell metallic electrodes, where we fix the applied potential difference $Φ_a$ between the two electrodes. If the film thickness $H$ is shorter than the Debye screening length $κ^{-1}$, the space correlation of the polarization $p_z$ and the electric field $E_z$ along the surface normal (in the $z$ direction) acuire global components inversely proportional to the film volume $V$, which vary slowly along the $z$ axis and are homogeneous in the $xy$ plane. The areal charge density on each electrode surface also has a component homogeneous on the surface, which produces the global electric fluctuations. On the other hand, if $H$ much exceeds $κ^{-1}$, the global correlations of $p_z$ and $ρ$ become small in the bulk region outside the electric double layers, but that of $E_z$ remains almost unchanged by ions in the whole cell at fixed $Φ_a$. The dielectric constant $ε_{\rm eff}$ depends on $H$ and $κ$ and is expressed in terms of the fluctuation variances of $p_z$ and $ρ$ and that of the noblocal surface charge density at fixed $Φ_a$.

cond-mat.soft↗

arXiv2026

Theory of central peak and acoustic anomaly in cubic BaTiO3 close to ferroelectric transition

We present a Ginzburg-Landau theory on statics and dynamics of BaTiO$_3$-type ferroelectrics in the paraelectric phase with the cubic structure, where the order parameter is the polarization $\bi p$. Unique effects are caused by the electrostrictive (ES) coupling between ${\bi p}$ and the elastic displacement $\bi u$. We show that the ES coupling gives rise to a central peak in the Fourier-Laplace transform of the displacement time-correlation function at small wave numbers. It emerges and grows with a narrow width as the transition is approached. Such central peaks have long been observed in a number of scattering experiments in various ferroelectrics, but their origin has not been well understood. From the acoustic part of the displacement dynamic correlation we obtain the frequency-dependent elastic moduli $C_{11}^*(ω)$, $C_{12}^*(ω)$, and $C_{44}^*(ω)$, whose singular parts arise from the ES coupling, We then calculate the singular sound velocity and attenuation. In the central peak and the elastic moduli, the frequency $ω$ appears in the scaled form $ωτ_D$, where $τ_D$ is the Debye relaxation time in the frequency-dependent dielectric constant. {Keywords}: ferroelectric transition, central peak, acoustic anomaly, electrostrictive coupling

cond-mat.mtrl-sci↗

arXiv2025

Static and dynamic theory of polarization under internal and directing electric fields: Fixed-charge and fixed-potential conditions

We present a continuum theory on statics and dynamics of polar fluids, where the orientational polarization ${\bi p}_1$ and the induced polarization ${\bi p}_2$ are governed by the Onsager directing field ${\bi E}_d$ and the Lorentz internal field $\bi F$, respectively. We start with a dielectric free energy functional $\cal F$ with a cross term $\propto \int\hspace{-0.5mm} d{\bi r}~{\bi p}_1\cdot{\bi p}_2$, which was proposed by Felderhof $[$J. Phys. C: Solid State Phys. {\bf 12}, 2423 (1979)$]$. With this cross-coupling, our theory can yield the theoretical results by Onsager and Kirkwood. We also present dynamic equations using the functional derivatives $δ{\cal F}/δ{\bi p}_i$ to calculate the space-time correlations of ${\bi p}_i$. We then obtain analytic expressions for various frequency-dependent quantities including the Debye formula. We find that the fluctuations of the total polarization drastically depend on whether we fix the electrode charge or the applied potential difference between parallel metal electrodes. In the latter fixed-potential condition, we obtain a nonlocal (long-range) polarization correlation inversely proportional to the cell volume $V$, which is crucial to understand the dielectric response. It is produced by nonlocal charge fluctuations on the electrode surfaces and is sensitive to the potential drops in the Stern layers in small systems. These nonlocal correlations in the bulk and on the surfaces are closely related due to the global constraint of fixed potential difference. We also add some results in other boundary conditions including the periodic one, where nonlocal correlations also appear.

cond-mat.soft↗

arXiv2024

Extension of Kirkwood-Buff theory: Partial enthalpies, fluctuations of energy density, temperature, and pressure, and solute-induced effects in a mixture solvent

We present a statistical mechanical theory of multi-component fluids, where we consider the correlation functions of the number densities and the energy density in the grand canonical ensemble. In terms of their space integrals we express the partial volumes ${\bar v}_i$, the partial enthalpies ${\bar H}_i$, and other thermodynamic derivatives. These ${\bar v}_i$ and ${\bar H}_i$ assume simple forms for binary mixtures and for ternary mixtures with a dilute solute. They are then related to the space-dependent thermal fluctuations of the temperature and the pressure. The space averages of these fluctuations are those introduced by Landau and Lifshits in the isothermal-isobaric ($T$-$p$) ensemble. We also give expressions for the long-range (nonlocal) correlations in the canonical and $T$-$p$ ensembles, which are inversely proportional to the system volume. For a mixture solvent, we examine the solvent-induced solute-solute attraction and the osmotic enthalpy changes due to the solute doping using the correlation function integrals.

cond-mat.stat-mech↗

arXiv2024

Ions and dipoles in electric field: Nonlinear polarization and field-dependent chemical reaction

We investigate electric-field effects in dilute electrolytes with nonlinear polarization. As a first example of such systems, we add a dipolar component with a relatively large dipole moment $μ_0$ to an aqueous electrolyte. As a second example, the solvent itself exhibits nonlinear polarization near charged objects. For such systems, we present a Ginzburg-Landau free energy and introduce field-dependent chemical potentials, entropy density, and stress tensor, which satisfy general thermodynamic relations. In the first example, the dipoles accumulate in high-field regions, as predicted by Abrashikin {\it et al}.$[$Phys.Rev.Lett. {\bf 99}, 077801 (2007)$]$. Finally, we consider the case, where Bjerrum ion pairs form a dipolar component with nonlinear polarization. The Bjerrum dipoles accumulate in high-field regions, while field-induced dissociation was predicted by Onsager $[$J. Chem. Phys.{\bf 2}, 599 (1934)$]$. We present an expression for the field-dependent association constant $K(E)$, which depends on the field strength nonmonotonically.

cond-mat.soft↗

arXiv2020

Theory of electrolytes including steric, attractive, and hydration interactions

We present a continuum theory of electrolytes composed of a waterlike solvent and univalent ions. First, we start with a density functional $\cal F$ for the coarse-grained solvent, cation, and anion densities, including the Debye-Hückel free energy, the Coulombic interaction, and the direct interactions among these three components. These densities fluctuate obeying the distribution $\propto \exp(- {\cal F}/k_BT)$. Eliminating the solvent density deviation in $\cal F$, we obtain the effective non-Coulombic interactions among the ions, which consist of the direct ones and the solvent-mediated ones. %where the latter are written in terms of the ion volumes %and are inversely proportional to the solvent compressibility. We then derive general expressions for the ion correlation, the apparent partial volume, and the activity and osmotic coefficients up to linear order in the average salt density $n_{\rm s}$. Secondly, we perform numerical analysis using the Mansoori-Carnahan-Starling-Leland model $[$J. Chem. Phys. {\bf 54}, 1523 (1971)$]$ for three-component hardspheres. The effective interactions sensitively depend on the cation and anion sizes due to competition between the steric and hydration effects, which are repulsive between small-large ion pairs and attractive between symmetric pairs. These agree with previous experiments and Collins' rule $[$Biophys. J. {\bf 72}, 65 (1997)$]$. We also give simple approximate expressions for the ionic interaction coefficients valid for any ion sizes.

cond-mat.soft↗

arXiv2019

Theory of Applying Heat Flow from Thermostatted Boundary Walls: Dissipative and Local-Equilibrium Responses and Fluctuation Theorems

We construct a microscopic theory of applying a heat flow from thermostatted boundary walls in the film geometry. We treat a classical one-component fluid, but our method is applicable to any fluids and solids. We express linear response of any variable ${\cal B}$ in terms of the time-correlation functions between $\cal B$ and the heat flows ${\cal J}_K$ from the thermostats to the particles. Furthermore, the surface variables ${\cal J}_K$ can be written in the form of space integrals of bulk quantities from the equations of motion. Owing to this surface-to-bulk relation, the steady-state response functions consist of dissipative and local-equilibrium parts, where the former gives rise to Fourier's law with Green's expression for the thermal conductivity. In the nonlinear regime, we derive the steady-state distribution in the phase space in the McLennan-Zubarev form from the first principles. Some fluctuation theorems are also presented.

cond-mat.stat-mech↗

arXiv2018

Theory of Shear Modulus in Glasses

We construct a linear response theory of applying shear deformations from boundary walls in the film geometry in Kubo's theoretical scheme. Our method is applicable to any solids and fluids. For glasses, we assume quasi-equilibrium around a fixed inherent state. Then, we obtain linear-response expressions for any variables including the stress and the particle displacements, even though the glass interior is elastically inhomogeneous. In particular, the shear modulus can be expressed in terms of the correlations between the interior stress and the forces from the walls. It can also be expressed in terms of the inter-particle correlations, as has been shown in the previous literature. Our stress relaxation function includes the effect of the boundary walls and can be used for inhomogeneous flow response. We show the presence of long-ranged, long-lived correlations among the fluctuations of the forces from the walls and the displacements of all the particles in the cell. We confirm these theoretical results numerically in a two-dimensional model glass. As an application, we describe propagation of transverse sounds after boundary wall motions using these time-correlation functions We also find resonant sound amplification when the frequency of an oscillatory shear approaches that of the first transverse sound mode.

cond-mat.soft↗

arXiv2018

Theory of nonionic hydrophobic solutes in mixture solvent: Solvent-mediated interaction and solute-induced phase separation

We present a theory of nonionic solutes in a mixture solvent composed of water-like and alcohol-like species. First, we show relationship among the solvation chemical potential, the partial volumes $v_i$, the Kirkwood-Buff integrals, the second osmotic virial coefficient, and the Gibbs transfer free energy. We examine how the solute density $n_3$ is coupled to the solvent densities $n_1$ and $n_2$ in thermodynamics. In the limit of small compressibility, we show that the space-filling condition $\sum_i v_i n_i=1$ nearly holds for inhomogeneous densities $n_i$, where the concentration fluctuations of the solvent can give rise to a large solute-solute attractive interaction. We also derive a solute spinodal density $n_3^{\rm spi}$ for solute-induced instability. Next, we examine gas-liquid and liquid-liquid phase transitions induced by a small amount of a solute using the Mansoori, Carnahan, Starling, and Leland model for hard-sphere mixtures $[${ J. Chem. Phys.} {\bf 54}, 1523 (1971)$]$. Here, we assume that the solvent is close to its gas-liquid coexistence and the solute interacts repulsively with the water-like species but attractively with the alcohol-like one. We calculate the binodal and spinodal curves in the phase diagrams and examine nucleation for these two phase transitions.

cond-mat.soft↗

arXiv2017

Critical adsorption profiles around a sphere and a cylinder in a fluid at criticality: Local functional theory

We study universal critical adsorption on a solid sphere and a solid cylinder in a fluid at bulk criticality, where preferential adsorption occurs. We use a local functional theory proposed by Fisher, de Gennes, and Au-Yang ($[$C. R. Acad. Sci. Paris Ser. B {\bf 287}, 207 (1978)$]$ and $[$Physica {\bf 101}A, 255 (1980)$]$). We calculate the mean order parameter profile $ψ(r)$, where $r$ is the distance from the sphere center and the cylinder axis, respectively. The resultant differential equation for $ψ(r)$ is solved exactly around a sphere and numerically around a cylinder. A strong adsorption regime is realized except for very small surface field $h_1$, where the surface order parameter $ψ(a)$ is determined by $h_1$ and is independent of the radius $a$. If $r$ considerably exceeds $a$, $ψ(r)$ decays as $r^{-(1+η)} $ for a sphere and $r^{-(1+η)/2} $ for a cylinder in three dimensions, where $η$ is the critical exponent in the order parameter correlation at bulk criticality.

cond-mat.soft↗

arXiv2017

Acoustic resonance in periodically sheared glass

Using molecular dynamics simulation, we study acoustic resonance in low-temperature glass by applying a small periodic shear at a boundary wall. Shear wave resonance occurs as the frequency $ω$ approaches $ω_\ell= πc_\perp\ell/L$ ($\ell=1, 2, 3,...)$. Here, $c_\perp$ is the transverse sound speed and $L$ is the cell length. At resonance, large-amplitude sound waves appear after many cycles even for very small applied strains. They then induce plastic events, which are heterogeneous in space and intermittent on time scales longer than the oscillation period $2π/ω$. From these irreversible particle motions, there arises strong dissipation suppressing the growth of sounds. After many resonant cycles, we observe a phenomenon of forced aging, where the shear modulus (measured after switching off the oscillation) is increased significantly.Sometimes, exceptionally large plastic events and system-size sliding motions induce a transition from resonant to off-resonant states. At resonance, translational diffusion becomes appreciable as well as aging due to enhanced configurational changes.

cond-mat.soft↗

arXiv2017

Electric double layer composed of an antagonistic salt in an aqueous mixture: Local charge separation and surface phase transition

We examine an electric double layer containing an antagonistic salt in an aqueous mixture, where the cations are small and hydrophilic but the anions are large and hydrophobic. In this situation, a strong coupling arises between the charge density and the solvent composition. As a result, the anions are trapped in an oil-rich adsorption layer on a hydrophobic wall. % while the cations are expelled from it. We then vary the surface charge density $σ$ on the wall. For $σ>0$ the anions remain accumulated, but for $σ<0$ the cations are attracted to the wall with increasing $|σ|$. Furthermore, the electric potential drop $Ψ(σ)$ is nonmonotonic when the solvent interaction parameter $χ(T)$ exceeds a critical value $χ_c$ determined by the composition and the ion density in the bulk. This leads to a first order phase transition between two kinds of electric double layers with different $σ$ and common $Ψ$. In equilibrium such two layer regions can coexist. The steric effect due to finite ion sizes is crucial in these phenomena.

cond-mat.soft↗

arXiv2017

Ferroelectric glass of spheroidal dipoles with impurities: Polar nanoregions, response to applied electric field, and ergodicity breakdown

Using molecular dynamics simulation, we study dipolar glass in crystals composed of slightly spheroidal, polar particles and spherical, apolar impurities between metal walls. We present physical pictures of ferroelectric glass, which have been observed in relaxors, mixed crystals (such as KCN$_x$KBr$_{1-x}$), and polymers. Our systems undergo a diffuse transition in a wide temperature range, where we visualize polar nanoregions (PNRs) surrounded by impurities. In our simulation, the impurities form clusters and their space distribution is heterogeneous. The polarization fluctuations are enhanced at relatively high $T$ depending on the size of the dipole moment. They then form frozen PNRs as $T$ is further lowered into the nonergodic regime. As a result, the dielectric permittivity exhibits the characteristic features of relaxor ferroelectrics. We also examine nonlinear response to cyclic applied electric field and nonergodic response to cyclic temperature changes (ZFC$/$FC), where the polarization and the strain change collectively and heterogeneously. We also study antiferroelectric glass arising from molecular shape asymmetry. We use an Ewald scheme of calculating the dipolar interaction in applied electric field.

cond-mat.soft↗

arXiv2016

Ionization at a solid-water interface in an applied electric field: Charge regulation

We investigate ionization at a solid-water interface in applied electric field. We attach an electrode to a dielectric film bearing silanol or carboxyl groups with an areal density $Γ_0$, where the degree of dissociation $α$ is determined by the proton density in water close to the film. We show how $α$ depends on the density $n_0$ of NaOH in water and the surface charge density $σ_m$ on the electrode. For $σ_m>0$, the protons are expelled away from the film, leading to an increase in $α$. In particular, in the range $0<σ_m<eΓ_0$, self-regulation occurs to realize $α\cong σ_m/eΓ_0 $ for $n_0\ll n_c$, where $n_c$ is $0.01$ mol$/$L for silica surfaces and is $2\times 10^{-5}$ mol$/$L for carboxyl-bearing surfaces. We also examine the charge regulation with decreasing the cell thickness $H$ below the Debye length $κ^{-1}$, where a crossover occurs at the Gouy-Chapman length. In particular, when $σ_m \sim eΓ_0$ and $H\ll κ^{-1}$, the surface charges remain only partially screened by ions, leading to an electric field in the interior.

cond-mat.soft↗

arXiv2016

Structure Formation due to Antagonistic Salts

Antagonistic salts are composed of hydrophilic and hydrophobic ions. In a mixture solvent (water-oil) such ion pairs are preferentially attracted to water or oil, giving rise to a coupling between the charge density and the composition. First, they form a large electric double layer at a water-oil interface, reducing the surface tension and producing mesophases. Here, the cations and anions are loosely bound by the Coulomb attraction across the interface on the scale of the Debye screening length. Second, on solid surfaces, hydrophilic (hydrophobic) ions are trapped in a water-rich (oil-rich) adsorption layer, while those of the other species are expelled from the layer. This yields a solvation mechanism of local charge separation near a solid. In particular, near the solvent criticality, disturbances around solid surfaces can become oscillatory in space. In mesophases, we calculate periodic structures, which resemble those in experiments.

cond-mat.soft↗

arXiv2015

Density functional theory of gas-liquid phase separation in dilute binary mixtures

We examine statics and dynamics of phase-separated states of dilute binary mixtures using density functional theory. In our systems, the difference in the salvation chemical potential $Δμ_s$ between liquid and gas is considerably larger than the thermal energy $k_BT$ for each solute particle and the attractive interaction among the solute particles is weaker than that among the solvent particles. In these conditions, the saturated vapor pressure increases by an amount equal to the solute density in liquid multiplied by the large factor $k_BT \exp(Δμ_s/k_BT)$. As a result, phase separation is induced at low solute densities in liquid and the new phase remains in gaseous states, while the liquid pressure is outside the coexistence curve of the solvent. This explains the widely observed formation of stable nanobubbles in ambient water with a dissolved gas. We calculate the density and stress profiles across planar and spherical interfaces, where the surface tension decreases with increasing the interfacial solute adsorption. We realize stable solute-rich bubbles with radius about 30 nm, which minimize the free energy functional. We then study dynamics around such a bubble after a decompression of the surrounding liquid, where the bubble undergoes a damped oscillation. In addition, we present some exact and approximate expressions for the surface tension and the interfacial stress tensor.

cond-mat.soft↗

arXiv2015

Fluctuations of local electric field and dipole moments in water between metal walls

We examine the thermal fluctuations of the local electric field $E_k^{\rm loc}$ and the dipole moment $μ_k$ in liquid water at $T=298$ K between metal walls in electric field applied in the perpendicular direction. We use analytic theory and molecular dynamics simulation. In this situation, there is a global electrostatic coupling between the surface charges on the walls and the polarization in the bulk. Then, the correlation function of the polarization density $p_z(r)$ along the applied field contains a homogeneous part inversely proportional to the cell volume $V$. Accounting for the long-range dipolar interaction, we derive the Kirkwood-Fr$\ddot{\rm{o}}$hlich formula for the polarization fluctuations when the specimen volume $v$ is much smaller than $V$. However, for not small $v/V$, the homogeneous part comes into play in dielectric relations. We also calculate the distribution of $E_k^{\rm loc}$ in applied field. As a unique feature of water, its magnitude $|E_k^{\rm loc}|$ obeys a Gaussian distribution with a large mean value $E_0 \cong 17~$V$/$nm, which arises mainly from the surrounding hydrogen-bonded molecules. Since $|μ_k|E_0\sim 30 k_{\rm B}T$, $μ_k$ becomes mostly parallel to $E_k^{\rm loc}$. As a result, the orientation distributions of these two vectors nearly coincide, assuming the classical exponential form. In dynamics, the component of $μ_k(t)$ parallel to $E_k^{\rm loc}(t)$ changes on the timescale of the hydrogen bonds $\sim 5$ ps, while its smaller perpendicular component undergoes librational motions on timescales of 0.01 ps.

cond-mat.soft↗

arXiv2015

Bubble formation in water with addition of a hydrophobic solute

We show that phase separation can occur in a one-component liquid outside its coexistence curve (CX) with addition of a small amount of a solute. The solute concentration at the transition decreases with increasing the difference of the solvation chemical potential between liquid and gas. As a typical bubble-forming solute, we consider ${\rm O}_2$ in ambient liquid water, which exhibits mild hydrophobicity and its critical temperature is lower than that of water. Such a solute can be expelled from the liquid to form gaseous domains while the surrounding liquid pressure is higher than the saturated vapor pressure $p_{cx}$. This solute-induced bubble formation is a first-order transition in bulk and on a partially dried wall, while a gas film grows continuously on a completely dried wall. We set up a bubble free energy $ΔG$ for bulk and surface bubbles with a small volume fraction $ϕ$. It becomes a function of the bubble radius $R$ under the Laplace pressure balance. Then, for sufficiently large solute densities above a threshold, $ΔG$ exhibits a local maximum at a critical radius and a minimum at an equilibrium radius. We also examine solute-induced nucleation taking place outside CX, where bubbles larger than the critical radius grow until attainment of equilibrium.

cond-mat.soft↗