arXiv subjects

Shunsuke Yabunaka

Publications and source records attributed to Shunsuke Yabunaka.

At least 19 recordsLinked to original sources

arXiv2026

Global Fixed Point Potentials in the Abelian Higgs Model with $N$ flavors

Existence of charged fixed points in the Abelian Higgs model with $N$ flavors in $d$ dimensions is studied using the functional renormalization group. We numerically solve the coupled fixed point equations for the scale dependent charge and the non-perturbative effective potential for the scalar field. We show that the $ε=4-d$ expansion, famously successful in theories with $O(N)$ symmetry, fails to produce reliable results when taking the $ε\rightarrow 1$ limit. By determining global fixed point potentials, it is shown that the critical flavor number at which charged fixed points appear, modifies significantly compared to the perturbative treatment. In $d=3$, signs of a richer fixed point structure with presumably multicritical fixed points are also found. Discussions include subtleties of the gauge fixing and the corresponding modified Ward-Takahashi identities, including the possibility of a nonzero dimensionless photon mass at the infrared fixed point.

hep-ph↗

arXiv2026

Mass generation at a fixed point: A Functional Renormalization Group Study of the tricritical O($N$) model in $d=3$ and $N=\infty$

Renormalization group (RG) fixed points are commonly associated with scale invariance and a divergent correlation length. We show that this connection can fail in the tricritical $O(N)$ model in three dimensions in the limit $N\to\infty$. Revisiting the line of fixed points identified by Bardeen, Moshe, and Bander, we use the functional renormalization group to clarify the mechanism leading to mass generation at its singular endpoint (the BMB fixed point). We demonstrate that the generated mass is nonuniversal and originates from the nonanalytic structure of the effective potential. We show that the critical exponent $ν$ which takes the value $ν= 1/2$ along the regular part of the BMB line, that is, for $0 \leq λ< λ_{\rm BMB}$, jumps to $ν= 1/3$ on the singular part of this line with the BMB FP, corresponding to $λ= λ_{\rm BMB}$, being the pivotal point between these two regimes. We also show how its singular potential emerges dynamically along the renormalization flow.

cond-mat.stat-mech↗

arXiv2025

Drag Coefficient in Near-Critical Binary Mixtures: Solving Hydrodynamic Fields with Improved Numerics

We calculate the drag coefficient of a spherical particle suspended in a near-critical binary fluid mixture. To capture the scaling behavior associated with critical adsorption in the strong adsorption regime, we employ the framework of local renormalized functional theory. Previous theoretical studies encountered numerical difficulties when attempting to solve the coupled hydrodynamic and chemical potential equations, expressed as integral equations, for systems with large bulk correlation lengths. These difficulties limited direct comparison with experimental results. In this study, we overcome those limitations by reformulating the hydrodynamic equations as a set of ordinary differential equations using a compactified radial coordinate. This approach enables more stable numerical computation and facilitates the implementation of appropriate boundary conditions at large distances from the particle. As a result, we successfully compute the drag coefficient over a broader range of bulk correlation lengths than in previous works and compare our theoretical predictions with available experimental data.

cond-mat.soft↗

arXiv2025

Global fixed point potential approach to frustrated antiferromagnets

We revisit the critical behavior of classical frustrated systems using the nonperturbative renormalization group (NPRG) equation. Our study is performed within the local potential approximation of this equation to which is added the flow of the field renormalization. Our flow equations are functional to avoid possible artifacts coming from the field expansion of the fixed point potential which consists in keeping only a limited number of coupling constants. We explain in detail our numerical implementation, its advantages and the difficulties encountered in the vicinity of $d=2$. For $N$-component spins, the function $N_c(d)$ separating the regions of first and second order transitions in the $(d,N)$ plane is computed for $d$ between 4 and 2.3. Our results confirm what was previously found with cruder approximations of the NPRG equation and contradict both the fixed dimension perturbative approach and some of the results obtained within the conformal bootstrap approach.

cond-mat.stat-mech↗

arXiv2023

Universal direction in thermoosmosis of a near-critical binary fluid mixture

We consider thermoosmosis of a near-critical binary fluid mixture, lying in the one-phase region, through a capillary tube in the presence of preferential adsorption of one component. The critical composition is assumed in the two reservoirs linked by the tube. With coarse-grained approach, we evaluate the flow field induced by the thermal force density. We predict a universal property; if the mixture is near the upper (lower) consolute point, the flow direction is the same as (opposite to) the direction of the temperature gradient, irrespective of which component is adsorbed onto the wall.

cond-mat.soft↗

arXiv2023

Thermoosmosis of a near-critical binary fluid mixture: a general formulation and universal flow direction

We consider a binary fluid mixture, which lies in the one-phase region near the demixing critical point, and study its transport through a capillary tube linking two large reservoirs. We assume that short-range interactions cause preferential adsorption of one component on the tube's wall. The adsorption layer can become much thicker than the molecular size, which enables us to apply hydrodynamics based on a coarse-grained free-energy functional. For linear transport phenomena induced by gradients of the pressure, composition, and temperature along a cylindrical tube, we obtain the formulas of the Onsager coefficients to extend our previous results on isothermal transport, assuming the critical composition in the middle of each reservoir in the reference equilibrium state. Among the linear transport phenomena, we focus on thermoosmosis -- mass flow due to a temperature gradient. We explicitly derive a formula for the thermal force density, which is nonvanishing in the adsorption layer and causes thermoosmosis. This formula for a near-critical binary fluid mixture is an extension of the conventional formula for a one-component fluid, expressed in terms of local excess enthalpy. We predict that the direction of thermoosmotic flow of a mixture near the upper (lower) consolute point is the same as (opposite to) that of the temperature gradient, irrespective of which component is adsorbed on the wall. Our procedure would also be applied to dynamics of a soft material, whose mesoscopic inhomogeneity can be described by a coarse-grained free-energy functional.

cond-mat.soft↗

arXiv2023

A fixed point can hide another one: the nonperturbative behavior of the tetracritical fixed point of the O($N$) models at large $N$

We show that at $N=\infty$ and below its upper critical dimension, $d<d_{\rm up}$, the critical and tetracritical behaviors of the O($N$) models are associated with the same renormalization group fixed point (FP) potential. Only their derivatives make them different with the subtleties that taking their $N\to\infty$ limit and deriving them do not commute and that two relevant eigenperturbations show singularities. This invalidates both the $ε-$ and the $1/N-$ expansions. We also show how the Bardeen-Moshe-Bander line of tetracritical FPs at $N=\infty$ and $d=d_{\rm up}$ can be understood from a finite-$N$ analysis.

cond-mat.stat-mech↗

arXiv2022

Incompleteness of the large-$N$ analysis of the $O(N)$ models: Nonperturbative cuspy fixed points and their nontrivial homotopy at finite $N$

We summarize the usual implementations of the large $N$ limit of $O(N)$ models and show in detail why and how they can miss some physically important fixed points when they become singular in the limit $N\to\infty$. Using Wilson's renormalization group in its functional nonperturbative versions, we show how the singularities build up as $N$ increases. In the Wilson-Polchinski version of the nonperturbative renormalization group, we show that the singularities are cusps, which become boundary layers for finite but large values of $N$. The corresponding fixed points being never close to the Gaussian, are out of reach of the usual perturbative approaches. We find four new fixed points and study them in all dimensions and for all $N>0$ and show that they play an important role for the tricritical physics of $O(N)$ models. Finally, we show that some of these fixed points are bi-valued when they are considered as functions of $d$ and $N$ thus revealing important and nontrivial homotopy structures. The Bardeen-Moshe-Bander phenomenon that occurs at $N=\infty$ and $d=3$ is shown to play a crucial role for the internal consistency of all our results.

hep-th↗

arXiv2022

Isothermal transport of a near-critical binary fluid mixture through a capillary tube with the preferential adsorption

We study isothermal transport of a nonelectrolyte binary fluid mixture, which lies in the homogeneous phase near the demixing critical point, through a capillary tube connecting two reservoirs. Usually, one component is preferentially adsorbed onto the tube wall, and the adsorption becomes significant owing to large osmotic susceptibility. The mixture flowing out of the tube is rich in the preferred component when flow is driven by the pressure difference between the reservoirs. When flow is driven by the mass-fraction difference, the total mass flow occurs in the presence of the preferential adsorption, which means that diffusioosmosis emerges. These phenomena can be regarded as cross effects linked by the reciprocal relation. We also study these phenomena numerically by using the hydrodynamics based on the coarse-grained free-energy functional, which was previously obtained in terms of the renormalized local functional theory. It is shown in particular that the conductance, or the total mass flow rate under a given mass-fraction difference, in diffusioosmosis can change non-monotonically with the temperature.

cond-mat.soft↗

arXiv2021

Real-time observation of charge-spin cooperative dynamics driven by a nonequilibrium phonon environment

Quantum dots are recognized as a suitable platform for studying thermodynamic phenomena involving single electronic charges and spins in nano-scale devices. However, such a thermodynamic system is usually driven by electron reservoirs at different temperatures, not by a lattice temperature gradient. We report on experimental observations of charge-spin cooperative dynamics in transitions of two-electron spin states in a GaAs double quantum dot located in a non-equilibrium phonon environment. Enhancements in the spin-flip processes are observed, originating from phonon excitation combined with the spin-orbit interaction. In addition, due to the spatial gradient of phonon density between the dots, the spin-flip rate during an inter-dot electron tunnel from a hot to a cold dot is more enhanced than in the other direction, resulting in accumulation of parallel spin states in the double dot.

cond-mat.mes-hall↗

arXiv2020

The finite $N$ origin of the Bardeen-Moshe-Bander phenomenon and its extension at $N=\infty$ by singular fixed points

We study the $O(N)$ model in dimension three (3$d$) at large and infinite $N$ and show that the line of fixed points found at $N=\infty$ --the Bardeen-Moshe-Bander (BMB) line-- has an intriguing origin at finite $N$. The large $N$ limit that allows us to find the BMB line must be taken on particular trajectories in the $(d,N)$-plane: $d=3-α/N$ and not at fixed dimension $d=3$. Our study also reveals that the known BMB line is only half of the true line of fixed points, the second half being made of singular fixed points. The potentials of these singular fixed points show a cusp for a finite value of the field and their finite $N$ counterparts a boundary layer.

hep-th↗

arXiv2019

Drag Coefficient of a Rigid Spherical Particle in a Near-Critical Binary Fluid Mixture beyond the Regime of the Gaussian Model

The drag coefficient of a rigid spherical particle deviates from the Stokes law when it is put into a near-critical fluid mixture in the homogeneous phase with the critical composition. The deviation ($Δγ_{\rm d}$) is experimentally shown to depend approximately linearly on the correlation length far from the particle ($ξ_\infty$), and is suggested to be caused by the preferential attraction between one component and the particle surface. In contrast, the dependence was shown to be much steeper in the previous theoretical studies based on the Gaussian free-energy density. In the vicinity of the particle, especially when the adsorption of the preferred component makes the composition strongly off-critical, the correlation length becomes very small as compared with $ξ_\infty$. This spacial inhomogeneity, not considered in the previous theoretical studies, can influence the dependence of $Δγ_{\rm d}$ on $ξ_\infty$. To examine this possibility, we here apply the local renormalized functional theory, which was previously proposed to explain the interaction of walls immersed in a (near-)critical binary fluid mixture, describing the preferential attraction in terms of the surface field. The free-energy density in this theory, coarse-grained up to the local correlation length, has much complicated dependence on the order parameter, as compared with the Gaussian free-energy density. Still, a concise expression of the drag coefficient, which was derived in one of the previous theoretical studies, turns out to be available in the present formulation. We show that, as $ξ_{\infty}$ becomes larger, the dependence of $Δγ_{\rm d}$ on $ξ_\infty$ becomes distinctly gradual and close to the linear dependence.

cond-mat.soft↗

arXiv2019

Full Counting Statistics of Spin-Flip/Conserving Charge Transitions in Pauli-Spin Blockade

We investigate the full counting statistics (FCS) of spin-conserving and spin-flip charge transitions in Pauli-spin blockade regime of a GaAs double quantum dot. A theoretical model is proposed to evaluate all spin-conserving and spin-flip tunnel rates, and to demonstrate the fundamental relation between FCS and waiting time distribution. We observe the remarkable features of parity effect and a tail structure in the constructed FCS, which do not appear in the Poisson distribution, and are originated from spin degeneracy and coexistence of slow and fast transitions, respectively. This study is potentially useful for elucidating the spin-related and other complex transition dynamics in quantum systems.

cond-mat.mes-hall↗

arXiv2018

Why Might the Standard Large $N$ Analysis Fail in the O($N$) Model: The Role of Cusps in the Fixed Point Potentials

The large $N$ expansion plays a fundamental role in quantum and statistical field theory. We show on the example of the O$(N)$ model that at $N=\infty$, its traditional implementation misses in all dimensions below four some fixed points of the renormalization group. These new fixed points show singularities at $N=\infty$ in their effective potential that become a boundary layer at finite $N$. We show that they have a physical impact on the multicritcal physics of the $O(N$) model at finite $N$. We also show that the mechanism at play holds also for the O($N$)$\otimes$O(2) model and is thus probably generic.

cond-mat.stat-mech↗

arXiv2017

Surprises in the $O(N)$ models: nonperturbative fixed points, large $N$ limit and multi-criticality

We find that the multicritical fixed point structure of the O($N$) models is much more complicated than widely believed. In particular, we find new nonperturbative fixed points in three dimensions ($d=3$) as well as at $N=\infty$. These fixed points come together with an intricate double-valued structure when they are considered as functions of $d$ and $N$. Many features found for the O($N$) models are shared by the O($N)\otimes$O(2) models relevant to frustrated magnetic systems.

cond-mat.stat-mech↗

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

Emergence of epithelial cell density waves

Epithelial cell monolayers exhibit traveling mechanical waves. We rationalize this observation thanks to a hydrodynamic description of the monolayer as a compressible, active and polar material. We show that propagating waves of the cell density, polarity, velocity and stress fields may be due to a Hopf bifurcation occurring above threshold values of active coupling coefficients.

physics.bio-ph↗

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↗