arXiv subjects

Satoshi Takada

Publications and source records attributed to Satoshi Takada.

51 records · Page 3Linked to original sources

arXiv2018

Rheology of cohesive granular particles under constant pressure

The rheology of cohesive granular materials, under a constant pressure condition, is studied using molecular dynamics simulations. Depending on the shear rate, pressure, and interparticle cohesiveness, the system exhibits four distinctive phases: uniform shear, oscillation, shear-banding, and clustering. The friction coefficient is found to increase with the inertial number, irrespective of the cohesiveness. The friction coefficient becomes larger for strong cohesion. This trend is explained by the anisotropies of the coordination number and angular distribution of the interparticle forces. In particular, we demonstrate that the second-nearest neighbors play a role in the rheology of cohesive systems.

cond-mat.soft↗

arXiv2018

Impact cratering mechanics: A forward approach to predicting ejecta velocity distribution and transient crater radii

Impact craters are among the most prominent topographic features on planetary bodies. Crater scaling laws allow us to extract information about the impact histories on the host bodies. The pi-group scaling laws have been constructed based on the point-source approximation, dimensional analysis, and the results from impact experiments. Recent impact experiments, however, demonstrated that the scaling parameters themselves exhibits complex behavior against the change in the impact conditions and target properties. Here, we propose an alternative, fully analytical method to predict impact outcomes based on impact cratering mechanics. This approach is based on the Maxwell Z-model and the residual velocity. We present analytical expressions of (1) the proportionality relation between the ejection velocity and the ejection position, (2) the radius of a growing crater as a function of time, and (3) the transient crater radii. Since we focused on obtaining analytical solutions, a number of simplifications are employed. Due to the simplifications in the strength model, the accuracy of the prediction in the strength-dominated regime is relatively low. Our model reproduces the power-law behavior of the ejecta velocity distribution and the approximate time variation of a growing crater. The predicted radii under typical impact conditions mostly converge to a region between the two typical scaling lines by the pi-group scaling laws, strongly supporting the notion that the new method is one of the simplest ways to predict impact outcomes, as it provides analytical solutions. Our model could serve as a quick-look tool to estimate the impact outcome under a given set of conditions, and it might provide new insights into the nature of impact excavation processes.

astro-ph.EP↗

arXiv2018

Rheology of dilute cohesive granular gases

Rheology of a dilute cohesive granular gas is theoretically and numerically studied. The flow curve between the shear viscosity and the shear rate is derived from the inelastic Boltzmann equation for particles having square-well potentials in a simple shear flow. It is found that (i) the stable uniformly sheared state only exists above a critical shear rate and (ii) the viscosity in the uniformly sheared flow is almost identical to that for uniformly sheared flow of hard core granular particles. Below the critical shear rate, clusters grow with time, in which the viscosity can be approximated by that for the hard-core fluids if we replace the diameter of the particle by the mean diameter of clusters.

cond-mat.soft↗

arXiv2017

Sign Flip in the Casimir Force for Interacting Fermion Systems

In this work we consider a fermionic chain of finite length $\ell$. Fermions are allowed to interact and are forced to obey boundary conditions, thus altering the process of condensation. Our goal is to explore how this affects the quantum vacuum energy for this system. We approach this problem by using a self-consistent method and observe a nontrivial behavior in the Casimir force, displaying a switch from an attractive to a repulsive regime. This flip stems from the competition between the attractive contribution from the usual fermionic Casimir effect and a repulsive one coming from the condensate.

hep-th↗

arXiv2017

Kinetic theory of discontinuous shear thickening

A simple kinetic theory to exhibit a discontinuous shear thickening (DST) is proposed. The model includes the collision integral and the friction from environment as well as a thermostat term characterized by $T_{\rm ex}$. The viscosity of this model is proportional to $\dotγ^2$ for large shear rate $\dotγ$, while it is Newtonian for low $\dotγ$. The emergence of the DST is enhanced for lower density and lower nonzero $T_{\rm ex}$.

cond-mat.soft↗

arXiv2016

Kinetic theory for dilute cohesive granular gases with a square well potential

We develop the kinetic theory of dilute cohesive granular gases in which the attractive part is described by a square well potential. We derive the hydrodynamic equations from the kinetic theory with the microscopic expressions for the dissipation rate and the transport coefficients. We check the validity of our theory by performing the direct simulation Monte Carlo.

cond-mat.stat-mech↗

arXiv2016

Fulde-Ferrell-Larkin-Ovchinnikov states in a superconducting ring with magnetic fields: Phase diagram and the first-order phase transitions

We find the angular Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states (or the twisted kink crystals) in which a phase and an amplitude of a pair potential modulate simultaneously in a quasi-onedimensional superconducting ring with a static Zeeman magnetic field applied on the ring and static Aharonov-Bohm magnetic flux penetrating the ring. The superconducting ring with magnetic flux produces a persistent current, whereas the Zeeman split of Fermi energy results in the spatial modulation of the pair potential. We show that these two magnetic fields stabilize the FFLO phase in a large parameter region of the magnetic fields. We further draw the phase diagram with the two kinds of first-order phase transitions; one corresponds to phase slips separating the Aharonov-Bohm magnetic flux, and the other separates the number of peaks of the pair amplitude for the Zeeman magnetic field.

cond-mat.supr-con↗

arXiv2015

Drag Law of Two Dimensional Granular Fluids

The drag force law acting on a moving circular disk in a two-dimensional granular medium is analyzed based on the discrete element method (DEM). It is remarkable that the drag force on the moving disk in moderate dense and pure two-dimensional granular medium can be well reproduced by a perfect fluid with separation from the surface of the tracer. A yield force, being independent of the moving speed of the disk, appears if a dry friction between the granular disks and the bottom plate exists. The perfect fluidity is violated in this case. The yield force and the drag force diverge at the jamming point.

cond-mat.soft↗

arXiv2015

Hydrodynamic instabilities in shear flows of cohesive granular particles

We extend the dynamic van der Waals model introduced by A. Onuki [Phys. Rev. Lett. 94, 054501 (2005)] to the description of cohesive granular flows under a plane shear to study their hydrodynamic instabilities. Numerically solving the dynamic van der Waals model, we observe various heterogeneous structures of the density in steady states, where the viscous heating is balanced with the energy dissipation caused by inelastic collisions. Based on the linear stability analysis, we find that the spatial structures are determined by the mean volume fraction, the applied shear rate, and the inelasticity, where the instability is triggered if the system is thermodynamically unstable, i.e. the pressure, $p$, and the volume fraction, $ϕ$, satisfy $\partial p/\partialϕ<0$.

cond-mat.soft↗

arXiv2014

Simulation of cohesive fine powders under a plane shear

Three dimensional molecular dynamics simulations of cohesive dissipative powders under a plane shear are performed. We find the various phases depending on the dimensionless shear rate and the dissipation rate as well as the density. We also find that the shape of clusters depends on the initial condition of velocities of particles when the dissipation is large. Our simple stochastic model reproduces the non-Gaussian velocity distribution function appearing in the coexistence phase of a gas and a plate.

cond-mat.soft↗

arXiv2012

Simulation of pattern dynamics of cohesive granular particles under a plane shear

We have performed three-dimensional molecular dynamics simulation of cohesive granular particles under a plane shear. From the simulation, we found the existence of three distinct phases in steady states: (I) a uniform shear phase, (II) a coexistent phase of a shear band and a gas region and (III) a crystal phase. We also found that the critical line between (II) and (III) is approximately represented by $ζ\propto \exp(β\dotγL_y)$, where $ζ$, $β$, $\dotγ$, $L_y$ are the dissipation rate, an unimportant constant, the shear rate and the system size of the velocity gradient direction, respectively.

cond-mat.soft↗

arXiv2012

Pattern dynamics of cohesive granular particles under a plane shear

We perform three dimensional molecular dynamics simulations of cohesive granular particles under a plane shear. From the simulations, we found that the granular temperature of the system abruptly decreases to zero after reaching the critical temperature, where the characteristic time $t_{\rm cl}$ is approximately represented by $t_{\rm cl} \propto (ζ-ζ_{\rm cr})^{-β}$ with the dissipation rate $ζ$, the critical dissipation rate $ζ_{\rm cr}$ and the exponent $β\simeq 0.8$. We also found that there exist a variety types of clusters depending on the initial density and the dissipation rate.

cond-mat.soft↗

arXiv1997

Haldane and dimer gap in general double spin-chain models

We have obtained an analytic expression for the $k$ dependence of excitation energy gap for an arbitrary double $S=1/2$ spin-chain by using the nonlocal unitary transformation and the variational method. It is checked to explain the gap behavior of various systems, which include the Haldane system and the dimer system in both extreme limits, and also the ladder model and the Majumdar-Ghosh model. The string order parameter, the dimer order parameter, and the local spin value are also calculated in the ground state. The ground-state energy exhibits a great stabilization by an antiferromagnetic bond dimerization, which might be realized in various new compounds. We also mention the relation of the convergence to the Haldane state with the spin-exchange symmetry of the model. The excited state has one domain wall of a local triplet type except in the vicinity of the Majumdar-Ghosh point, where a local triplet is decomposed to two $S=1/2$ free spins moving among the singlet dimers.

cond-mat↗

arXiv1996

Behavior of a frustrated quantum spin chain with bond dimerization

We clarified behavior of the excitation gap in a frustrated S=1/2 quantum spin chain with bond dimerization by using the numerical diagonalization of finite systems and a variational approach. The model interpolates between the independent dimer model and the S=1 spin chain by changing a strength of the dimerization. The energy gap is minimum at the fully-frustrated point, where a localized kink and a freely mobile anti-kink govern the low-lying excitations. Away from the point, a kink and an antikink form a bound state by an effective triangular potential between them. The consequential gap enhancement and the localization length of the bound state is obtained exactly in the continuous limit. The gap enhancement obeys a power law with exponent 2/3. The method and the obtained results are common to other frustrated double spin-chain systems, such as the one-dimensional J_1 - J_2 model, or the frustrated ladder model.

cond-mat↗

arXiv1996

Relaxation of frustration and gap enhancement by the lattice distortion in the $Δ$ chain

We clarify an instability of the ground state of the $Δ$ chain against the lattice distortion that increases a strength $(λ)$ of a bond in each triangle. It relaxes the frustration and causes a remarkable gap enhancement; only a $6\%$ increase of $λ$ causes the gap doubled from the fully-frustrated case $(λ=1)$. The lowest excitation is revealed to be a kink-antikink bound state whose correlation length decreases drastically with $λ$ increase. The enhancement follows a power law, $ΔE_{\rm gap}\sim (λ-1) + 1.44 (λ-1)^{\frac{2}{3}}$, which can be obtained from the exact result of the continuous model. This model describes a spin gap behavior of the delafossite YCuO$_{2.5}$.

cond-mat↗