arXiv subjects

Atsushi Kawamoto

Publications and source records attributed to Atsushi Kawamoto.

At least 19 recordsLinked to original sources

arXiv2026

Modular fabrication and design of thick rigid-foldable origami metamaterials

Origami metamaterials offer significant potential for stiff deployable structures However, fabricating load-bearing cellular structures from thick panels introduces geometric interference at non-manifold junctions. Conventional thick-panel fabrication often disrupt ideal kinematics, thereby compromising smooth motion and scalability. This study proposes a modular fabrication framework that preserves one-degree-of-freedom rigid-folding kinematics in thick and non-manifold origami metamaterials. By decomposing non-manifold junctions into a hierarchy of stacked, modular hinged panels, our approach successfully accommodates synchronized hinge motions using scissor-like linkages. Exploiting this representation, we implement a graph-based topology optimization framework that tailors macroscopic stiffness while preserving folding connectivity. We demonstrate this approach by fabricating optimized prototypes that deploy seamlessly with a one-degree-of-freedom motion. Furthermore, we demonstrate engineering scalability through the large-scale construction of extensive deployable systems assembled from modular panels, which exhibit high load-bearing capacity. These results pave the way for the practical fabrication of structural, large-scale deployable metamaterials.

cs.CE↗

arXiv2026

Automated design of soft-rigid hybrid robots for dynamic locomotion

Rigid-bodied robots often lack compliance needed to adapt to unstructured environments, while fully soft robots, though highly adaptable, struggle with scalability and load capacity. In nature, musculoskeletal systems balance strength and flexibility by integrating hard and soft tissues. Inspired by this principle, we present an automated design method for soft-rigid hybrids that optimizes a freeform soft-body shape, a stiff truss layout, and multi-channel actuation. Our differentiable simulator couples the material point method (MPM) for deformable bodies with extended position-based dynamics (XPBD) for truss elements, enabling gradient-based search. The optimization generates truss skeletons that transmit actuation forces to the soft body. We fabricate the optimized design and evaluate it on a walking task. Experiments reproduce the walking mode predicted by the optimization, which does not emerge without the skeleton. Modal analysis further suggests that the skeleton enables deformation modes near the actuation frequency that promote effective stride generation.

cs.CE↗

arXiv2026

Mott Intermittency at the Metal-Insulator Boundary

The resistivity maximum at a temperature $T=T_{\mathrm{max}}$ is a recurring feature of bandwidth-tuned Mott systems, yet its meaning remains controversial: is it a coherence-incoherence crossover of an electronically homogeneous metal, or does it mark the onset of transport through a mixed landscape of metallic and insulating regions? Even more debated is whether a true phase-coexistence regime survives in the relevant parameter range, or whether apparent inhomogeneity is merely extrinsic. Here we address these questions by moving beyond temperature sweeps and probe charge transport in the time domain. Near $T=T_{\mathrm{max}}$, we find that the resistance of a model system, a quasi-two-dimensional Mott spin liquid material, exhibits clear random-telegraph switching between discrete levels over long timescales. The statistics of the switching - sharp two-level behavior with thermally activated dwell times - point to a mesoscopic "current-controlling" region that dynamically toggles between metallic and insulating states, intermittently opening and closing the dominant conduction channel. This characteristic fluctuating dynamics provides direct evidence for intrinsic metal-insulator coexistence and establishes $T\sim T_{\mathrm{max}}$ as the regime of Mott intermittency, where transport is governed by stochastic domain switching rather than quasiparticle decoherence.

cond-mat.str-el↗

arXiv2025

Universal relation between residual resistivity and A coefficient in correlated metals

The effects of strong electronic correlations and disorder are crucial for emergent phenomena such as unconventional superconductivity, metal-insulator transitions, and quantum criticality. While both are omnipresent in real materials, their individual impacts on charge transport remain elusive. To disentangle their respective roles, we have independently varied the degree of randomness and the strength of electronic correlations -- by chemical substitution and physical pressure, respectively -- within the metallic phase nearby a Mott-insulating state. We find a distinct correlation dependence of the disorder-dependent residual resistivity $ρ_0$ in the Fermi-liquid regime $ρ(T)=ρ_0 + A T^2$, where $A\propto (m^{\star}/m)^2$ quantifies the electronic mass enhancement. Contrary to conventional expectations, we observe that at fixed disorder level $ρ_0$ grows linearly with $A$. This scaling can be understood in terms of chemical-potential fluctuations with variance $σ_μ^2$, yielding $ρ_0 \propto A\,σ_μ^2$. By comparing our findings to transport data on other organic Mott systems, oxides, heavy-fermion compounds, and moiré materials, we demonstrate that this new relation between residual resistivity and mass enhancement is a universal feature of correlated metals.

cond-mat.str-el↗

arXiv2025

Failed superconductivity in a Mott spin liquid material

A central challenge for understanding unconventional superconductivity in most strongly correlated electronic materials is their complicated band structure and presence of competing orders. In contrast, quasi-two-dimensional organic spin liquids are single-band systems with superconductivity arising near the bandwidth-tuned Mott metal-insulator transition in the absence of other orders. Here, we study chemically substituted $κ$-organics in which superconducting fluctuations emerge in the phase coexistence region between the Mott insulator and the Fermi liquid. Using magnetotransport and ac susceptibility measurements, we find that global superconductivity fails to set in as temperature $T\rightarrow 0$. Our results indicate instead the presence of superconducting domains embedded in the metallic percolating cluster that undergo a magnetic field-tuned quantum superconductor-to-metal phase transition. Surprisingly, albeit consistent with the percolation picture, universal conductance fluctuations are seen at high fields in macroscopic samples. The observed interplay of the intrinsic inhomogeneity and quantum phase fluctuations provides a new insight into failed superconductivity, a phenomenon seen in various conventional and unconventional superconductors, including cuprates.

cond-mat.supr-con↗

arXiv2024

Gapped magnetic ground state in the spin-liquid candidate $κ$-(BEDT-TTF)$_2$Ag$_2$(CN)$_3$ suggested by magnetic spectroscopy

The nature of the magnetic ground state of highly frustrated systems remained puzzling to this day. Here, we have performed multifrequency electron spin resonance (ESR) measurements on a putative quantum spin liquid compound $κ$-(BEDT-TTF)$_2$Ag$_2$(CN)$_3$, which is a rare example of $S = 1/2$ spins on a triangular lattice. At high temperatures, the spin susceptibility exhibits a weak temperature dependence which can be described by the Heisenberg model with an antiferromagnetic exchange interaction of strength $J/k_B \approx 175$ K. At low temperatures, however, the rapid drop of the static spin susceptibility, together with monotonic decrease of the ESR linewidth indicates that strong singlet correlations develop below a pairing energy scale $T^*$ accompanied by a spin gap. On the other hand, a weak Curie-like spin susceptibility and the angular dependence of the linewidth suggest additional contribution from impurity spins. We propose the gradual formation of spin singlets with an inhomogeneous spin gap at low temperatures.

cond-mat.str-el↗

arXiv2024

Shell topology optimization based on level set method

This paper proposes a level set-based method for optimizing shell structures with large design changes in shape and topology. Conventional shell optimization methods, whether parametric or nonparametric, often only allow limited design changes in shape. In the proposed method, the shell structure is defined as the isosurface of a level set function. The level set function is iteratively updated based on the shape sensitivity on the surface mesh. Therefore, the proposed method can represent an arbitrary manifold surface while dealing with topological changes, for example, from a spherical surface to a toroidal surface. We applied the proposed method to the mean compliance minimization problems of 3D shell structural designs for dome, bending plate and cantilever beam examples to demonstrate its efficacy of the proposed method.

cs.CE↗

arXiv2024

Computational synthesis of locomotive soft robots by topology optimization

Locomotive soft robots (SoRos) have gained prominence due to their adaptability. Traditional locomotive SoRo design is based on limb structures inspired by biological organisms and requires human intervention. Evolutionary robotics, designed using evolutionary algorithms (EAs), have shown potential for automatic design. However, EA-based methods face the challenge of high computational cost when considering multiphysics in locomotion, including materials, actuations, and interactions with environments. Here, we present a design approach for pneumatic SoRos that integrates gradient-based topology optimization with multiphysics material point method (MPM) simulations. This approach starts with a simple initial shape (a cube with a central cavity). The topology optimization with MPM then automatically and iteratively designs the SoRo shape. We design two SoRos, one for walking and one for climbing. These SoRos are 3D printed and exhibit the same locomotion features as in the simulations. This study presents an efficient strategy for designing SoRos, demonstrating that a purely mathematical process can produce limb-like structures seen in biological organisms.

cs.CE↗

arXiv2024

Computational co-design of structure and feedback controller for locomoting soft robots

Soft robots have gained significant attention due to their flexibility and safety, particularly in human-centric applications. The co-design of structure and controller in soft robotics has presented a longstanding challenge owing to the complexity of the dynamics involved. Despite some pioneering work dealing with the co-design of soft robot structures and actuation, design freedom has been limited by stochastic design search approaches. This study proposes the simultaneous optimization of structure and controller for soft robots in locomotion tasks, integrating topology optimization-based structural design with neural network-based feedback controller design. Here, the feedback controller receives information about the surrounding terrain and outputs actuation signals that induce the expansion and contraction of the material. We formulate the simultaneous optimization problem under uncertainty in terrains and construct an optimization algorithm that utilizes automatic differentiation within topology optimization and neural networks. We present numerical experiments to demonstrate the validity and effectiveness of our proposed method.

cs.CE↗

arXiv2024

Pressure-induced nearly perfect rectangular lattice and superconductivity in an organic molecular crystal (DMET-TTF)$_2$AuBr$_2$

External pressure and associated changes in lattice structures are key to realizing exotic quantum phases such as high-$T_{\rm c}$ superconductivity. While applying external pressure is a standard method to induce novel lattice structures, its impact on organic molecular crystals has been less explored. Here we report a unique structural phase transition in (DMET-TTF)$_2$AuBr$_2$ under pressure. By combining advanced high-pressure techniques and $ab$ $initio$ calculations, we elucidate that (DMET-TTF)$_2$AuBr$_2$ undergoes a transition from a quasi-one-dimensional lattice to a nearly perfect rectangular lattice at 0.9 GPa. This transition leads to the realization of an antiferromagnetic Mott insulator with $T_{\rm N}=66$ K, the highest $T_{\rm N}$ in low-dimensional molecular crystal solids to date. Upon increasing the pressure, the antiferromagnetic ordering is suppressed, and a superconducting phase with $T_{\rm c}=4.8$ K emerges around 6 GPa. Our study reveals the significant impact of external pressure on lattice structures of organic molecular crystals and highlights the intricate relationship between geometrical frustration and superconductivity. Our findings also pave the way for realizing functional organic molecular crystals through changes in lattice structures by pressure.

cond-mat.str-el↗

arXiv2023

Stiff deployable structures via coupling of thick Miura-ori tubes along creases

Origami-based structures play an important role in the realization of deployable mechanisms and unique mechanical properties via programmable deformation by folding. Among origami-based structures, tessellation by the coupling of origami tubes enriches the variations in geometry and mechanical properties. However, thickness accommodation is a critical problem in engineering applications involving the coupling of thick origami tubes. To solve this problem, this study proposes the coupling of thick Miura-ori tubes along the creases for facile fabrication, which sustains the one-degree-of-freedom (DOF) motion of thick Miura-ori tubes owing to the local mirror symmetry around the coupling interfaces. Furthermore, the coupling method contributes to the high stiffness of the coupled Miura-ori tubes, as evidenced by the wide gap in the eigenvalues between the one-DOF mode and the elastic modes obtained by the bar-and-hinge models. Finally, meter-scale coupled Miura-ori tubes were fabricated to demonstrate one-DOF motion and high stiffness. The findings of this study enable the rapid construction of structures by one-DOF motion and enhancement of transportability via flat-foldability.

physics.app-ph↗

arXiv2023

4D topology optimization: Integrated optimization of the structure and self-actuation of soft bodies for dynamic motions

Topology optimization is a powerful tool utilized in various fields for structural design. However, its application has primarily been restricted to static or passively moving objects, mainly focusing on hard materials with limited deformations and contact capabilities. Designing soft and actively moving objects, such as soft robots equipped with actuators, poses challenges due to simulating dynamics problems involving large deformations and intricate contact interactions. Moreover, the optimal structure depends on the object's motion, necessitating a simultaneous design approach. To address these challenges, we propose "4D topology optimization," an extension of density-based topology optimization that incorporates the time dimension. This enables the simultaneous optimization of both the structure and self-actuation of soft bodies for specific dynamic tasks. Our method utilizes multi-indexed and hierarchized density variables distributed over the spatiotemporal design domain, representing the material layout, actuator layout, and time-varying actuation. These variables are efficiently optimized using gradient-based methods. Forward and backward simulations of soft bodies are done using the material point method, a Lagrangian-Eulerian hybrid approach, implemented on a recent automatic differentiation framework. We present several numerical examples of self-actuating soft body designs aimed at achieving locomotion, posture control, and rotation tasks. The results demonstrate the effectiveness of our method in successfully designing soft bodies with complex structures and biomimetic movements, benefiting from its high degree of design freedom.

cs.CE↗

arXiv2023

Homogenization and Inverse Problems for Fractional Diffusion Equations

We consider the homogenization for time-fractional diffusion equations in a periodic structure and we derive the homogenized time-fractional diffusion equation. Then we discuss the determination of the constant diffusion coefficient by minimum data. Combining the results obtained above, we investigate the inverse problems of determining the diffusion coefficient for the homogenized equations from the data in the periodic structure and vice versa, that is, we investigate the inverse problem of determining the diffusion coefficient for the periodic equations from the data in the homogenized structure.

math.AP↗

arXiv2023

Topology optimization of locomoting soft bodies using material point method

Topology optimization methods have widely been used in various industries, owing to their potential for providing promising design candidates for mechanical devices. However, their applications are usually limited to the objects which do not move significantly due to the difficulty in computationally efficient handling of the contact and interactions among multiple structures or with boundaries by conventionally used simulation techniques. In the present study, we propose a topology optimization method for moving objects incorporating the material point method, which is often used to simulate the motion of objects in the field of computer graphics. Several numerical experiments demonstrate the effectiveness and the utility of the proposed method.

cs.CE↗

arXiv2023

Thermodynamic Properties of the Mott Insulator-Metal Transition in a Triangular Lattice System Without Magnetic Order

The organic system, $κ$-[(BEDT-TTF)$_{1-x}$(BEDT-STF)$_x$]$_2$Cu$_2$(CN)$_3$, showing the Mott transition between a nonmagnetic Mott insulating (NMI) state and a Fermi liquid (FL), is systematically studied by calorimetric measurements. An increase of the electronic heat capacity at the transition from the NMI state to the FL state which keeps the triangular dimer lattice demonstrates that the charge sector lost in the Mott insulating state is recovered in the FL state. We observed that the remaining low-energy spin excitations in the Mott insulating state show unique temperature dependence, and that the NMI state has a larger lattice entropy originating from the frustrated lattice, which leads to the Pomeranchuk-like effect on the electron localization. Near the Mott boundary, an unexpected enhancement and magnetic-field dependence of heat capacity are observed. This anomalous heat capacity is different from the behavior in the typical first-order Mott transition and shows similarities with quantum critical behavior. To reconcile our results with previously reported scenarios about a spin gap and the first-order Mott transition, further studies are desired.

cond-mat.str-el↗

arXiv2022

Cellular Topology Optimization on Differentiable Voronoi Diagrams

Cellular structures manifest their outstanding mechanical properties in many biological systems. One key challenge for designing and optimizing these geometrically complicated structures lies in devising an effective geometric representation to characterize the system's spatially varying cellular evolution driven by objective sensitivities. A conventional discrete cellular structure, e.g., a Voronoi diagram, whose representation relies on discrete Voronoi cells and faces, lacks its differentiability to facilitate large-scale, gradient-based topology optimizations. We propose a topology optimization algorithm based on a differentiable and generalized Voronoi representation that can evolve the cellular structure as a continuous field. The central piece of our method is a hybrid particle-grid representation to encode the previously discrete Voronoi diagram into a continuous density field defined in a Euclidean space. Based on this differentiable representation, we further extend it to tackle anisotropic cells, free boundaries, and functionally-graded cellular structures. Our differentiable Voronoi diagram enables the integration of an effective cellular representation into the state-of-the-art topology optimization pipelines, which defines a novel design space for cellular structures to explore design options effectively that were impractical for previous approaches. We showcase the efficacy of our approach by optimizing cellular structures with up to thousands of anisotropic cells, including femur bone and Odonata wing.

cs.GR↗

arXiv2022

Free material optimization of thermal conductivity tensors with asymmetric components

Free Material Optimization (FMO), a branch of topology optimization, in which the design variables are the full constitutive tensors, can provide the most general form of the design problems. Considering the microstructure composed of isotropic materials, the constitutive tensors are yet positive definite and symmetric. On the other hand, it has been reported that the symmetry of this constitutive tensor can be broken in appearance by considering other physical phenomena. In the present study, we focus on the thermal Hall effect, which is explained as the phenomena that induces the temperature gradient orthogonal to a given temperature gradient across a solid when a magnetic field is applied to the solid. This effect makes the thermal conductivity tensor asymmetric and justifies extending the space of the constitutive tensors to be an asymmetric domain. We propose the FMO for asymmetric constitutive tensors, parameterizing the design space so that the physically available property could be naturally satisfied. Several numerical experiments are provided to show the validity and the utility of the proposed method.

cs.CE↗

arXiv2022

Charge imbalance in $λ$-(BETS)$_2$GaCl$_4$ and their interplay with superconductivity

The two-dimensional organic superconductor $λ$-(BETS)$_2$GaCl$_4$ exhibits pronounced charge fluctuations below $T \approx 150$~K, in contrast to the sibling compound $κ$-(BETS)$_2$GaCl$_4$ that remains metallic down to milli-Kelvin. Infrared spectroscopy reveals only minor splitting in the vibrational features of the latter compound, common to other strongly dimerized $κ$-salts. When the organic molecules are arranged in the $λ$-pattern, however, a strong vibrational $ν_{27}(b_{1u})$ mode is present, that forms a narrow doublet. %indicating static charge imbalance of about 2\%. Most important, when cooling $λ$-(BETS)$_2$GaCl$_4$ below 150~K, two weak side modes appear due to charge disproportionation that amounts to $2δ=0.14e$. In analogy to the $β^{\prime\prime}$-type organic conductors, we propose that charge fluctuations play an important role in emerging of unconventional superconductivity in ł at $T_c=4.7$~K. We discuss the possibility of a charge-density-wave that coexists with the proposed spin-density-wave state.

cond-mat.str-el↗