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Kunimasa Miyazaki

Publications and source records attributed to Kunimasa Miyazaki.

At least 19 recordsLinked to original sources
arXiv2026

Hyperuniformity near jamming transition over a wide range of bidispersity

We numerically investigate hyperuniformity in two-dimensional frictionless jammed packings of bidisperse systems. Hyperuniformity is characterized by the suppression of density fluctuations at large length scales, and the structure factor asymptotically vanishes in the small-wavenumber limit as $S(q) \propto q^α$, where $α> 0$. It is well known that jammed configurations exhibit hyperuniformity over a wide range of wavenumbers windows, down to $q^{\ast}σ\approx 0.2$, where $σ$ is the particle diameter. In two dimensions, we find that the exponent $α$ is approximately $0.6\text{--}0.7$. This contrasts with the reported value of $α= 1$ for three-dimensional systems. We employ an advanced method recently introduced by Rissone \textit{et al.} \href{https://link.aps.org/doi/10.1103/PhysRevLett.127.038001}{[Phys. Rev. Lett. {\bf 127}, 038001 (2021)]}, originally developed for monodisperse and three-dimensional systems, to determine $α$ with high precision. This exponent is found to be unchanged for all size ratios between small and large particles, except in the monodisperse case, where the system crystallizes.

cond-mat.soft↗
arXiv2026

Pre-yielding mechanical response near the jamming transition

The mechanical and rheological properties of jammed packings of frictionless particles under shear strain remain not fully understood, even when the strain amplitude is very small and well below the yielding threshold. Systems above the jamming transition point $ϕ_J$ are known to display two anomalous mechanical behaviors with respect to the driving frequency $ω$ (or time $t$) and the strain amplitude $γ$. In the linear-response regime ($γ\to 0$), the complex modulus exhibits an algebraic scaling, $G(ω)\simω^{1/2}$ (or $G(t)\sim t^{-1/2}$ in the time representation). In contrast, in the quasi-static limit ($ω\to 0$), the modulus shows the nonlinear behavior, $G(γ)\simγ^{-1/2}$, a phenomenon referred to as softening. The ranges of $ω$ and $γ$ over which these algebraic scalings hold broaden as $ϕ_J$ is approached from above, whereas both $G(ω)$ and $G(γ)$ vanish for $ϕ< ϕ_J$. In this study, we investigate the mechanical response in the regime where these two anomalies coexist in the vicinity of $ϕ_J$. To this end, we perform numerical analyses using two rheological protocols: oscillatory shear and transient stress relaxation. Our results demonstrate that the mechanical responses are not simply described as a superposition of the two algebraic relaxations and instead exhibit rich nonlinear viscoelastic behavior both above and even below $ϕ_J$.

cond-mat.soft↗
arXiv2025

Crystallization of Chiral Active Brownian Particles at Low Densities

Chiral active matter is a variant of active matter systems in which the motion of the constituent particles violates mirror symmetry. In this letter, we simulate two-dimensional chiral Active Brownian Particles, the simplest chiral model in which each particle undergoes circular motion, and show that the system crystallizes at low densities well below the melting point of the equilibrium counterpart. Crystallization is only possible if the orbital radius is long enough to align the circulating particles, but short enough for neighboring particles to avoid collisions. Of course, the system must be driven sufficiently far from equilibrium, since chirality cannot affect thermodynamic properties in classical equilibrium systems. The fluid-crystal phase diagram shows a re-entrant melting transition as a function of the radius of the circles. We show that at least one of the two transitions follows the same two-step melting scenario as in equilibrium systems.

cond-mat.soft↗
arXiv2025

Singular density correlations in chiral active fluids in three dimensions

We investigate density fluctuations in three-dimensional chiral active fluids by using a simple model of helical self-propelled particles. Helical motion is generated by a constant angular velocity (or chiral torque) acting on the self-propelled force. The chiral torque is assumed to have the same direction and magnitude for all particles. Due to the helical nature of the particle motion, the system is generically anisotropic even when it is spatially homogeneous. Numerical simulations demonstrate that the helicity induces an anisotropic pattern and a singularity in the static structure factor (the density correlation function in Fourier space) in the low-wavenumber limit. Moreover, the system in the limit of infinite persistence time exhibits hyperuniformity in the direction perpendicular to the chiral torque, while giant density fluctuations emerge along the parallel direction. We then construct a fluctuating hydrodynamic theory for the system to describe the singular behavior. A linear analysis of the resulting equations yields an analytical expression for the static structure factor, which qualitatively agrees with our numerical findings.

cond-mat.soft↗
arXiv2025

Long-range translational order and hyperuniformity in two-dimensional chiral active crystal

We numerically study two-dimensional athermal chiral active particles at high densities. The particles in this system perform the circular motion with frequency $Ω$. We show that the system crystallizes at high densities even in two dimensions, accompanied by the true long-range translational order. This is due to the anomalous suppression of displacement fluctuations associated with hyperuniformity. These findings can be explained using an active elastic theory quantitatively. Surprisingly, the crystals become unstable and melt in the limit of $Ω=0$, for the spatial dimension of four or less. This result can be explained by a mechanism akin to quenched random systems for which the lower critical dimension is four.

cond-mat.soft↗
arXiv2025

Frequency-dependent specific heat in quantum supercooled liquids: A mode-coupling study

Frequency-dependence of specific heat in supercooled hard sphere liquid is computed using quantum mode-coupling theory (QMCT). Mode-coupling equations are solved using recently proposed perturbative method that allows to study relaxation in the moderate quantum regime where quantum effects assist liquid to glass transition. Zwanzig's formulation is used to compute the frequency-dependent specific heat in supercooled state using dynamical information from QMCT. Specific heat shows strong variation as the quantumness of the liquid is changed, which becomes more significant as density is increased. It is found that, near the transition point, different dynamical modes contribute to the specific heat in the classical and the quantum liquids.

cond-mat.stat-mech↗
arXiv2024

Universal mechanism of shear thinning in supercooled liquids

Soft glassy materials experience a significant reduction in viscosity $η$ when subjected to shear flow, known as shear thinning. This phenomenon is characterized by a power-law scaling of $η$ with the shear rate $\dotγ$, $η\propto \dotγ^{-ν}$, where the exponent $ν$ is typically around $0.7$ to $0.8$ across different materials. Two decades ago, the mode coupling theory (MCT) suggested that shear thinning occurs due to the advection. However, it predicts too large $ν= 1$ (> $0.7$ to $0.8$) and overestimates the onset shear rate by orders of magnitude. Recently, it was claimed that a minute distortion of the particle configuration is responsible for shear thinning. Here we extend the MCT to include the distortion, and find that both advection and distortion contribute to shear thinning, but the latter is dominant. Our formulation works quantitatively for several different glass formers. We explain why shear thinning is universal for many glassy materials.

cond-mat.soft↗
arXiv2023

Microscopic theory for hyperuniformity in two-dimensional chiral active fluid

Some nonequilibrium systems exhibit anomalous suppression of the large-scale density fluctuations, so-called hyperuniformity. Recently, hyperuniformity was found numerically in a simple model of chiral active fluids [Q.-L. Lei et al., Sci. Adv. 5, eaau7423 (2019)]. We revisit this phenomenon and put forward a microscopic theory to explain it. An effective fluctuating hydrodynamic equation is derived for a simple particle model of chiral active matter. We show that the linear analysis of the obtained hydrodynamic equation captures hyperuniformity. Our theory yields hyperuniformity characterized by the same exponents as the numerical observation, but the agreement with the numerical data is qualitative. We also argue that the hydrodynamic equation for the effective particle representation, in which each rotating trajectory is regarded as an effective particle, has the same form as the macroscopic description of the random organization model with the center of mass conservation.

cond-mat.soft↗
arXiv2023

Anomalous fluctuations in homogeneous fluid phase of active Brownian particles

Giant number fluctuations (GNF) are an anomaly universally observed in active fluids with polar or nematic order. In this paper, we show that GNF arise in the fluid phase of active Brownian particles (ABP), where the polar order is absent. GNF in ABP extends over a large but finite length which characterizes the growing velocity correlations. To suppress unwanted phase separation and allow ones to explore the disordered fluid phase at large activities, we impart the inertia, or the mass, to the ABP. A linearized hydrodynamic theory captures our findings, but only qualitatively. We find numerically a nontrivial scaling relation for the density correlation function, which the linearized theory cannot explain. The results suggest ubiquitousness of the anomalous fluctuations even in the disordered homogeneous fluid phase in the absence of the directional order.

cond-mat.soft↗
arXiv2022

Dynamical Susceptibilities Near Ideal Glass Transitions

Building on the recently derived inhomogeneous mode-coupling theory, we extend the generalised mode-coupling theory of supercooled liquids to inhomogeneous environments. This provides a first-principles-based, systematic and rigorous way of deriving high-point dynamical susceptibilities from variations of the many-body dynamic structure factors with respect to their conjugate field. This new framework allows for a novel and fully microscopic possibility to probe for collective relaxation mechanisms in supercooled liquids near the mode-coupling glass transition.

cond-mat.stat-mech↗
arXiv2021

Anomalous transport phenomenon of a charged Brownian particle under the thermal gradient and the magnetic field

There is a growing interest in the stochastic processes of nonequilibrium systems subject to non-conserved forces, such as the magnetic forces acting on charged particles and the chiral self-propelled force acting on active particles. In this paper, we consider the stationary transport of non-interacting Brownian particles under a constant magnetic field in a position-dependent temperature background. We demonstrate the existence of the Nernst-like stationary density current perpendicular to both the temperature gradient and magnetic field, induced by the intricate coupling between the non-conserved force and the multiplicative noises due to the position-dependent temperature.

cond-mat.stat-mech↗
arXiv2021

Geometrical properties of mechanically annealed systems near the jamming transition

Geometrical properties of two-dimensional mixtures near the jamming transition point are numerically investigated using harmonic particles under mechanical training. The configurations generated by the quasi-static compression and oscillatory shear deformations exhibit anomalous suppression of the density fluctuations, known as hyperuniformity, below and above the jamming transition. For the jammed system trained by compression above the transition point, the hyperuniformity exponent increases. For the system below the transition point under oscillatory shear, the hyperuniformity exponent also increases until the shear amplitude reaches the threshold value. The threshold value matches with the transition point from the point-reversible phase where the particles experience no collision to the loop-reversible phase where the particles' displacements are non-affine during a shear-cycle before coming back to an original position. The results demonstrated in this paper are explained in terms of neither of universal criticality of the jamming transition nor the nonequilibrium phase transitions.

cond-mat.soft↗
arXiv2021

Multiple glass transitions and higher order replica symmetry breaking of binary mixtures

We extend the replica liquid theory in order to describe the multiple glass transitions of binary mixtures with large size disparities, by taking into account the two-step replica symmetry breaking (2RSB). We determine the glass phase diagram of the mixture of large and small particles in the large-dimension limit where the mean-field theory becomes exact. When the size ratio of particles is beyond a critical value, the theory predicts three distinct glass phases; (i) the 1RSB double glass where both components vitrify simultaneously, (ii) the 1RSB single glass where only large particles are frozen while small particles remain mobile, and (iii) a new glass phase called the 2RSB double glass where both components vitrify simultaneously but with an energy landscape topography distinct from the 1RSB double glass.

cond-mat.soft↗
arXiv2020

Structural relaxation in quantum supercooled liquids: A mode-coupling approach

We study supercooled dynamics in quantum hard-sphere liquid using quantum mode-coupling formulation. In the moderate quantum regime, classical cage effects lead to slower dynamics compared to strongly quantum regime, where tunneling overcomes classical caging, leading to faster relaxation. As a result, the glass transition critical density can become significantly higher than for the classical liquids. Perturbative approach is used to solve time dependent quantum mode-coupling equations to study in detail the dynamics of the supercooled liquid in moderate quantum regime. Similar to the classical case, relaxation time shows power-law increase with increasing density in the supercooled regime. However, the power-law exponent is found to be dependent on the quantumness; it increases linearly as the quantumness is increased in the moderate quantum regime.

cond-mat.stat-mech↗
arXiv2020

Glass stability changes the nature of yielding under oscillatory shear

We perform molecular dynamics simulations to investigate the effect of a glass preparation on its yielding transition under oscillatory shear. We use swap Monte Carlo to investigate a broad range of glass stabilities from poorly annealed to highly stable systems. We observe a qualitative change in the nature of yielding, which evolves from ductile to brittle as glass stability increases. Our results disentangle the relative role of mechanical and thermal annealing on the mechanical properties of amorphous solids, which is relevant for various experimental situations from the rheology of soft materials to fatigue failure in metallic glasses.

cond-mat.dis-nn↗
arXiv2020

Shear jamming and shear melting in mechanically trained frictionless particles

We investigate criticality near the jamming transition in both quiescent systems and those under shear by considering the effect of mechanical training on the jamming transition and nonlinear rheology. We simulate frictionless soft particles undergoing athermal quasi-static shear using initial configurations trained with athermal quasi-static cyclic volume deformations. The jamming transition density of the initial configuration $φ_{\rm J0}$ is systematically altered by tuning the ``depth'' of mechanical training. We exert a steady shear on these configurations and observe either shear jamming (gain of stiffness due to shear) or shear melting (loss of stiffness due to shear), depending on the depth of training and proximity to the jamming transition density. We also observe that the characteristic strains, at which shear jamming or melting occur, diverge at a unique density $φ_{\rm JS}$. This is due to the shift of the jamming transition density from $φ_{\rm J0}$ to $φ_{\rm JS}$ under shear, associated with loss of memory of the initial configuration. Finally, we thoroughly investigate nonlinear rheology near the jamming transition density, and contrary to previous works, we find a nonlinear ``softening'' takes place below as well as above the jamming transition density.

cond-mat.soft↗
arXiv2019

Classification of the reversible-irreversible transitions in particle trajectories across the jamming transition point

The reversible-irreversible (RI) transition of particle trajectories in athermal colloidal suspensions under cyclic shear deformation is an archetypal nonequilibrium phase transition which attracts much attention recently. In the low-density limit, the RI transition is predicted to belong to a universality class of the absorbing state transitions, whereas at the high densities well above the jamming transition density, $φ_{\rm J}$, the transition is discontinuous and is closely related to the yielding transition. The transition between the two limiting cases is largely unexplored. In this paper, we study the RI transition of athermal frictionless colloidal particles over a wide range of densities, with emphasis on the region below $φ_{\rm J}$, by using oscillatory sheared molecular dynamics simulation. We reveal that the nature of the RI transitions in the intermediate densities is very rich. As demonstrated by the previous work by Schreck $et\ al.$ [Phys. Rev. E. ${\bf 88}$, 052205 (2013)], there exist the point-reversible and the loop-reversible phases depending on the density and the shear strain amplitude. We find that, between the two reversible phases, a quasi-irreversible phase where the particles' trajectories are highly non-affine and diffusive. The averaged number of contacts of particles is found to characterize the phase boundaries. We also find that the system undergoes the yielding transition below but in the vicinity of $φ_{\rm J}$ when the strain with a small but finite strain rate is applied. This yielding transition line matches with the RI transition line separating the loop-reversible from the irreversible phases. Surprisingly, the nonlinear rheological response called "softening" has been observed even below $φ_{\rm J}$. These findings imply that geometrical properties encoded in the sheared configurations control the dynamical transitions.

cond-mat.soft↗