arXiv subjects

Yuya Tanaka

Publications and source records attributed to Yuya Tanaka.

14 recordsLinked to original sources

arXiv2026

Distal-Stable Beam for Continuum Robots

Continuum robots are well suited for constrained environments but suffer from low distal stiffness, resulting in large posture errors under external loads. In this paper, we propose a novel structural primitive, the Distal-Stable Beam, which achieves a strong stiffness gradient through purely geometric design, maintaining compliance in the intermediate section while ensuring high distal rigidity. The structure consists of two parallel rods and one convergent rod constrained by guide disks, introducing geometric coupling that suppresses deformation modes and preserves distal posture. Experiments show that the distal stiffness is 12 times higher than at the center, corresponding to an approximately 100-fold improvement over a conventional cantilever beam. The proposed mechanism enables simultaneous compliance and distal stability without active stiffness modulation, providing a new design approach for continuum robots requiring both safety and precision.

cs.RO↗

arXiv2025

Does Fermi Level Alignment Hold Across Organic Interfaces? -- An Investigation Using a Rotary Kelvin Probe

Understanding energy level alignment at organic interfaces is crucial for optimizing the performance of organic devices. Interface dipole and band bending significantly influence carrier recombination and generation mechanisms. A method of simulating energy level alignment at metal/organic and organic/organic interfaces by assuming a thermal equilibrium model has been proposed, but its validation against experimental methods is still limited. In this study, the work function change in the $\alpha$-NPD/HAT-CN/Au interface was measured as a typical donor/acceptor system using a rotary Kelvin probe (RKP). Our findings demonstrate good agreement with simulations only at metal/organic interfaces which have "active" charge transfer. It is suggested that thermal equilibrium is not achieved simply by depositing the film under dark condition, and some treatment to supply carriers, such as exposure to UV light, is necessary for accurate evaluation. At the organic/organic interface, the the experimental results did not agree with thermal equilibrium model, highlighting the need to consider substrate-driven carrier supply and polarization effects when evaluating energy level alignment.

cond-mat.mtrl-sci↗

arXiv2025

Determining the density of in-gap states in organic semiconductors: A pitfall of photoelectron yield spectroscopy

Accurate determination of low-density electronic states in the bandgap (in-gap states) is crucial for optimizing the performance of organic optoelectronic devices. Derivative photoelectron yield spectroscopy (PYS) is employed to estimate the density of states (DOS) of in-gap states. However, low-energy photons in PYS can generate excitons and anions in organic semiconductors, raising questions about whether derivative PYS spectra truly represent the DOS. We revealed that PYS signals originate from the single-quantum external photoelectron effect (SQEPE) of in-gap states, SQEPE of the singly occupied molecular orbital (SOMO) of anions, and the biphotonic electron emission (BEE) effect via exciton fusion. Because BEE signals mask the DOS contribution, derivative PYS misestimates the DOS of in-gap states. In contrast, constant final state yield spectroscopy (CFS-YS) reliably determines the DOS by separating these components. For a tris(8-hydroxyquinoline) aluminum (Alq3) film, CFS-YS revealed the DOS of in-gap and SOMO states over six orders of magnitude, clarifying why the Alq3 layer works effectively in organic light-emitting diodes. In the devices, BEE can act as carrier-generation and degradation processes, and CFS-YS can also probe it. We provide the practical guidelines of low-energy photon measurements for DOS determination, such as measurements of photon-flux dependency.

cond-mat.mtrl-sci↗

arXiv2025

Boundedness and asymptotic stability in a model for tuberculosis granuloma formation

This paper deals with a problem which describes tuberculosis granuloma formation \begin{align*} \begin{cases} u_t = \Delta u - \nabla \cdot (u \nabla v) - uv - u + \beta, &x \in \Omega,\ t>0, \\ v_t = \Delta v + v -uv + \mu w, &x \in \Omega,\ t>0, \\ w_t = \Delta w + uv - wz - w, &x \in \Omega,\ t>0, \\ z_t = \Delta z - \nabla \cdot (z \nabla w) + f(w)z -z, &x \in \Omega,\ t>0 \end{cases} \end{align*} under homogeneous Neumann boundary conditions and initial conditions, where $\Omega \subset \mathbb{R}^n$ ($n\ge 2$) is a smooth bounded domain, $\beta,\mu>0$ and $f$ is some function, and shows that if initial data are small in some sense then the solution $(u,v,w,z)$ of the problem exists globally and convergences to $(\beta,0,0,0)$ exponentially when $\beta>1$ and the reproduction number $R_0 := \frac{\mu \beta + 1}{\beta}$ satisfies $R_0<1$.

math.AP↗

arXiv2025

To what extent does the consideration of positive total flux influence the dynamics of Keller-Segel-type models?

Since the introduction of the Keller-Segel model in 1970 to describe chemotaxis (the interactions between cell distributions u and chemical distributions v), there has been a significant proliferation of research articles exploring various extensions and modifications of this model within the scientific community. From a technical standpoint, the totality of results concerning these variants are characterized by the assumption that the total flux, involving both distributions, of the model under consideration is zero. This research aims to present a novel perspective by focusing on models with a positive total flux. Specifically, by employing Robin-type boundary conditions for u and v, we seek to gain insights into the interactions between cells and their environment, uncovering important dynamics such as how variations in boundary conditions influence chemotactic behavior. In particular, the choice of the boundary conditions is motivated by real-world phenomena and by the fact that the related analysis reveals some interesting properties of the system.

math.AP↗

arXiv2024

Understanding the anomalous thermoelectric behaviour of Fe-V-W-Al based thin films

We have investigated the thermoelectric and thermal behaviour of Fe-V-W-Al based thin films prepared using radio frequency magnetron sputtering technique at different base pressures (0.1 ~ 1.0 X 10-2 Pa) and on different substrates (n, p and undoped Si). Interestingly, at lower base pressure, formation of bcc type of Heusler structure was observed in deposited samples, whereas at higher base pressure, we have noted the development of non-Heusler amorphous structure in these samples. Our findings indicates that the moderately oxidized Fe-V-W-Al amorphous thin film deposited on n-Si substrate, possesses large magnitude of absoulte S ~ 1098 microvolt per kelvin near room temperature, which is almost the double the previously reported value for thin films. Additionally, the power factor indicated enormously large values ~ 33.9 milliwatt per meter per kelvin sqaure near 320 K. The thermal conductivity of the amorphous thin film is also found to be 2.75 watt per meter per kelvin, which is quite lower compared to bulk alloys. As a result, the maximum figure of merit is estimated to be extremely high i.e. ~ 3.9 near 320 K, which is among one of the highest reported values so far. The anomalously large value of Seebeck coefficient and power factor has been ascribed to formation of amorphous structure and composite effect of thin film and substrate.

cond-mat.mtrl-sci↗

arXiv2023

Switching particle systems for foraging ants showing phase transitions in path selections

Switching interacting particle systems studied in probability theory are the stochastic processes of hopping particles on a lattice made up of slow and fast particles, where the switching between these types of particles occurs randomly at a given transition rate. This paper explores how such stochastic processes involving multiple particles can model group behaviors of ants. Recent experimental research by the last author's group has investigated how ants switch between two types of primarily relied cues to select foraging paths based on the current situation. Here, we propose a discrete-time interacting random walk model on a square lattice, incorporating two types of hopping rules. Numerical simulation results demonstrate global changes in selected homing paths, transitioning from trailing paths of the `pheromone road' to nearly optimal paths depending on the switching parameters. By introducing two types of order parameters characterizing the dependence of homing duration distributions on switching parameters, we discuss these global changes as phase transitions in ant path selections. We also study critical phenomena associated with continuous phase transitions.

cond-mat.stat-mech↗

arXiv2023

Finite-time blow-up in a two species chemotaxis-competition model with degenerate diffusion

This paper is concerned with the two-species chemotaxis-competition model with degenerate diffusion, \[\begin{cases} u_t = Δu^{m_1} - χ_1 \nabla\cdot(u\nabla w) + μ_1 u (1-u-a_1v), &x\inΩ,\ t>0,\\% v_t = Δv^{m_2} - χ_2 \nabla\cdot(v\nabla w) + μ_2 v (1-a_2u-v), &x\inΩ,\ t>0,\\% 0 = Δw +u+v-\overline{M}(t), &x\inΩ,\ t>0, \end{cases}\] with $\int_Ωw(x,t)\,dx=0$, $t>0$, where $Ω:= B_R(0) \subset \mathbb{R}^n$ $(n\ge5)$ is a ball with some $R>0$; $m_1,m_2>1$, $χ_1,χ_2,μ_1,μ_2,a_1,a_2>0$; $\overline{M}(t)$ is the spatial average of $u+v$. The purpose of this paper is to show finite-time blow-up in the sense that there is $\widetilde{T}_{\rm max}\in(0,\infty)$ such that \[\limsup_{t \nearrow \widetilde{T}_{\rm max}} (\|u(t)\|_{L^\infty(Ω)} + \|v(t)\|_{L^\infty(Ω)})=\infty\] for the above model within a concept of weak solutions fulfilling a moment inequality which leads to blow-up. To this end, we also give a result on finite-time blow-up in the above model with the terms $Δu^{m_1}$, $Δv^{m_2}$ replaced with the nondegenerate diffusion terms $Δ(u+δ)^{m_1}$, $Δ(v+δ)^{m_2}$, where $δ\in(0,1]$.

math.AP↗

arXiv2023

Critical mass phenomena in higher dimensional quasilinear Keller-Segel systems with indirect signal production

In this paper, we deal with quasilinear Keller--Segel systems with indirect signal production, $$\begin{cases} u_t = \nabla \cdot ((u+1)^{m-1}\nabla u) - \nabla \cdot (u \nabla v), &x \in Ω,\ t> 0,\\ 0 = Δv - μ(t) + w, &x \in Ω,\ t> 0,\\ w_t + w = u, &x \in Ω,\ t> 0, \end{cases}$$ complemented with homogeneous Neumann boundary conditions and suitable initial conditions, where $Ω\subset\mathbb R^n$ $(n\ge3)$ is a bounded smooth domain, $m\ge1$ and $$μ(t) := \frac{1}{|Ω|} w(\cdot, t) \qquad\mbox{for}\ t>0.$$ We show that in the case $m\ge2-\frac{2}{n}$, there exists $M_c>0$ such that if either $m>2-\frac{2}{n}$ or $\int_Ωu_0 2^\frac{n}{2}n^{n-1}ω_n$, then there exist radially symmetric initial data such that $\int_Ωu_0 = M$ and the solution blows up in finite or infinite time, where the blow-up time is infinite if $m=2-\frac2n$. In particular, if $m=2-\frac{2}{n}$ there is a critical mass phenomenon in the sense that $\inf\left\{M > 0 : \exists u_0 \text{ with } \int_Ωu_0 = M \text{ such that the corresponding solution blows up in infinite time}\right\}$ is a finite positive number.

math.AP↗

arXiv2022

Can chemotactic effects lead to blow-up or not in two-species chemotaxis-competition models?

This paper deals with the two-species chemotaxis-competition models \begin{align*} \begin{cases} u_t = d_1 Δu - χ_1 \nabla \cdot (u \nabla w) + μ_1 u (1- u^{κ_1-1} - a_1 v^{λ_1-1}), &\quad x \in Ω,\ t>0,\\ % v_t = d_2 Δv - χ_2 \nabla \cdot (v \nabla w) + μ_2 v (1- a_2 u^{λ_2-1} - v^{κ_2-1}), &\quad x \in Ω,\ t>0,\\ % 0 = d_3 Δw + αu + βv - h(u,v,w), &\quad x \in Ω,\ t>0, \end{cases} \end{align*} where $Ω\subset \mathbb{R}^n$ $(n\ge2)$ is a bounded domain with smooth boundary, and $h=γw$ or $h=\frac{1}{|Ω|}\int_Ω(αu+ βv)\,dx$. In the case that $κ_1=λ_1=κ_2=λ_2=2$ and $h=γw$, it is known that smallness conditions for the chemotacic effects lead to boundedness of solutions (Math.\ Methods Appl.\ Sci.; 2018; 41; 234--249). However, the case that the chemotactic effects are large seems not to have been studied yet; therefore it remains to consider the question whether the solution is bounded also in the case that the chemotactic effects are large. The purpose of this paper is to give a negative answer to this question.

math.AP↗

arXiv2021

Boundedness and finite-time blow-up in a quasilinear parabolic-elliptic chemotaxis system with logistic source and nonlinear production

This paper deals with the quasilinear parabolic-elliptic chemotaxis system with logistic source and nonlinear production, \begin{equation*} \begin{cases} u_t=\nabla \cdot (D(u) \nabla u) - \nabla \cdot (S(u)\nabla v) + λu - μu^κ, & x\inΩ,\ t>0, \\[1mm] 0=Δv - \overline{M_f}(t) + f(u), & x\inΩ,\ t>0, \end{cases} \end{equation*} where $λ>0$, $μ>0$, $κ>1$ and $\overline{M_f}(t):=\frac{1}{|Ω|}\int_Ω f(u(x,t))\,dx$, and $D$, $S$ and $f$ are functions generalizing the prototypes \begin{align*} D(u)=(u+1)^{m-1},\quad S(u)=u(u+1)^{α-1}\quad\mbox{and}\quad f(u)=u^\ell \end{align*} with $m\in\mathbb{R}$, $α>0$ and $\ell>0$. In the case $m=α=\ell=1$, Fuest (NoDEA Nonlinear Differential Equations Appl.; 2021; 28; 16) obtained conditions for $κ$ such that solutions blow up in finite time. However, in the above system boundedness and finite-time blow-up of solutions have been not yet established. This paper gives boundedness and finite-time blow-up under some conditions for $m$, $α$, $κ$ and $\ell$.

math.AP↗

arXiv2021

Blow-up in a quasilinear parabolic-elliptic Keller-Segel system with logistic source

This paper deals with the quasilinear parabolic-elliptic Keller-Segel system with logistic source, \begin{align*} u_t=Δ(u+1)^m - χ\nabla \cdot (u(u+1)^{α- 1} \nabla v) + λ(|x|) u - μ(|x|) u^κ, \quad 0=Δv - v + u, \quad x\inΩ,\ t>0, \end{align*} where $Ω:=B_{R}(0)\subset\mathbb{R}^n\ (n\ge3)$ is a ball with some $R>0$; $m>0$, $χ>0$, $α>0$ and $κ\ge1$; $λ$ and $μ$ are spatially radial nonnegative functions. About this problem, Winkler (Z. Angew. Math. Phys.; 2018; 69; Art. 69, 40) found the condition for $κ$ such that solutions blow up in finite time when $m=α=1$. In the case that $m=1$ and $α\in(0,1)$ as well as $λ$ and $μ$ are constant, some conditions for $α$ and $κ$ such that blow-up occurs were obtained in a previous paper (Math. Methods Appl. Sci.; 2020; 43; 7372-7396). Moreover, in the case that $m\ge1$ and $α=1$ Black, Fuest and Lankeit (arXiv:2005.12089[math.AP]) showed that there exists initial data such that solutions blow up in finite time under some conditions for $m$ and $κ$. The purpose of the present paper is to give conditions for $m\ge1$, $α>0$ and $κ\ge1$ such that solutions blow up in finite time.

math.AP↗

arXiv2021

Blow-up phenomena in a parabolic-elliptic-elliptic attraction-repulsion chemotaxis system with superlinear logistic degradation

This paper is concerned with the attraction-repulsion chemotaxis system with superlinear logistic degradation, \begin{align*} \begin{cases} u_t = Δu - χ\nabla\cdot(u \nabla v) + ξ\nabla\cdot (u \nabla w) + λu - μu^k, \quad &x \in Ω,\ t>0,\\[1.05mm] 0= Δv + αu - βv, \quad &x \in Ω,\ t>0,\\[1.05mm] 0= Δw + γu - δw, \quad &x \in Ω,\ t>0, \end{cases} \end{align*} under homogeneous Neumann boundary conditions, in a ball $Ω\subset \mathbb{R}^n$ ($n \ge 3$), with constant parameters $λ\in \mathbb{R}$, $k>1$, $μ, χ, ξ, α, β, γ, δ>0$. Blow-up phenomena in the system have been well investigated in the case $λ=μ=0$, whereas the attraction-repulsion chemotaxis system with logistic degradation has been not studied. Under the condition that $k>1$ is close to $1$, this paper ensures a solution which blows up in $L^\infty$-norm and $L^σ$-norm with some $σ>1$ for some nonnegative initial data. Moreover, a lower bound of blow-up time is derived.

math.AP↗

arXiv2021

Remarks on two connected papers about Keller-Segel systems with nonlinear production

These notes aim to provide a deeper insight on the specifics of two articles dealing with chemotaxis models with nonlinear production. More precisely, we are referring to the papers "Boundedness of solutions to a quasilinear parabolic-parabolic chemotaxis model with nonlinear signal production" by X. Tao, S. Zhou and M. Ding [J. Math. Anal. Appl. 474:1 (2019) 733-747] and "Boundedness for a fully parabolic Keller-Segel model with sublinear segregation and superlinear aggregation" by S. Frassu and G. Viglialoro [Acta Appl. Math. 171:1 (2021), 19]. These works, independently published in these last years, present results leaving open room for further improvement. Indeed, in the first a gap in the proof of the main claim appears, whereas the cornerstone assumption in the second is not sharp. In these pages we give a more complete picture to the relative underlying comprehension.

math.AP↗