arXiv subjects

Hiroshi Watanabe

Publications and source records attributed to Hiroshi Watanabe.

At least 91 records · Page 5Linked to original sources

arXiv2019

Mechanism of superconductivity and electron-hole doping asymmetry in $κ$-type molecular conductors

Unconventional superconductivity in molecular conductors is observed at the border of metal-insulator transitions in correlated electrons under the influence of geometrical frustration. The symmetry as well as the mechanism of the superconductivity (SC) is highly controversial. To address this issue, we theoretically explore the electronic properties of carrier-doped molecular Mott system $κ$-(BEDT-TTF)$_2$X. We find significant electron-hole doping asymmetry in the phase diagram where antiferromagnetic (AF) spin order, different patterns of charge order, and SC compete with each other. Hole-doping stabilizes AF phase and promotes SC with "$d_{xy}$"-wave symmetry, which has similarities with high-$T_c$ cuprates. In contrast, in the electron-doped side, geometrical frustration destabilizes the AF phase and the enhanced charge correlation induces another SC with "extended-$s$+$d_{x^2-y^2}$"-wave symmetry. Our results disclose the mechanism of each phase appearing in filling-control molecular Mott systems, and elucidate how physics of different strongly-correlated electrons are connected, namely, molecular conductors and high-$T_c$ cuprates.

cond-mat.supr-con↗

arXiv2019

Finite-Size Effects on Karman Vortex in Molecular Dynamics Simulation

The characteristics of the Karman vortex generated by a molecular dynamics (MD) simulation exhibit strong finite-size effects, and MD can only reproduce the experimental results qualitatively. Here, we seek the simulation conditions for quantitatively reproduce the vortex shedding frequency. We found that the finite-size effects are mainly caused by the interference of vortices and the nonuniformity of the fluid viscosity coefficient.

physics.flu-dyn↗

arXiv2019

Possible superconductivity induced by a large spin-orbit coupling in carrier doped iridium oxide insulators: A weak coupling approach

We study possible superconductivity in a carrier-doped iridium oxide insulator Sr$_{2}$IrO$_{4}$ based on an effective $t_{2g}$ three-orbital Hubbard model on the square lattice with a large spin-orbit coupling (SOC). Numerically solving the linearized Eliashberg equation for the superconducting (SC) gap function with the random phase approximation, we systematically examine both singlet and triplet SC gap functions with possible pairing symmetries and the parameter dependence of the superconductivity. For the realistic SOC $λ$ and Hund's coupling $J/U$ relevant to Sr$_{2}$IrO$_{4}$, namely, for a large $λ$ and small $J/U$ region, we find that the intra-band antiferromagnetic (AF) pseudospin $\bm{j}_{\text{eff}} = -\bm{l}+\bm{s}$ fluctuations favor a $d_{x^{2}-y^{2}}$-wave pseudospin $j_{\text{eff}}=1/2$ singlet pairing in the electron-doping. We also find that the $d_{x^{2}-y^{2}}$-wave pairing is more stabilized with increasing the SOC and decreasing the Hund's coupling. Furthermore, we show for a small $λ$ and large $J/U$ region that an $s_{\pm}$-wave singlet pairing is favored in the hole-doped region. The origin of the $s_{\pm}$-wave pairing is due to the inter-band pair scattering arising from the intra-orbital AF spin $\bm s$ fluctuations. Although the possibility of a pseudospin triplet pairing is considered, we find it always unfavorable for all parameters studied here. The experimental consequences for other strongly correlated materials with a large SOC are also discussed.

cond-mat.supr-con↗

arXiv2018

Fast algorithm for generating random bit strings and multispin coding for directed percolation

We present efficient algorithms to generate a bit string in which each bit is set with arbitrary probability. By adopting a hybrid algorithm, i.e., a finite-bit density approximation with correction techniques, we achieve 3.8 times faster random bit generation than the simple algorithm for the 32-bit case and 6.8 times faster for the 64-bit case. Employing the developed algorithm, we apply the multispin coding technique to one-dimensional bond-directed percolation. The simulations are accelerated by up to a factor of 14 compared with an optimized scalar implementation. The random bit string generation algorithm proposed here is applicable to general Monte Carlo methods.

cond-mat.stat-mech↗

arXiv2018

SIMD Vectorization for the Lennard-Jones Potential with AVX2 and AVX-512 instructions

This work describes the SIMD vectorization of the force calculation of the Lennard-Jones potential with Intel AVX2 and AVX-512 instruction sets. Since the force-calculation kernel of the molecular dynamics method involves indirect access to memory, the data layout is one of the most important factors in vectorization. We find that the Array of Structures (AoS) with padding exhibits better performance than Structure of Arrays (SoA) with appropriate vectorization and optimizations. In particular, AoS with 512-bit width exhibits the best performance among the architectures. While the difference in performance between AoS and SoA is significant for the vectorization with AVX2, that with AVX-512 is minor. The effect of other optimization techniques, such as software pipelining together with vectorization, is also discussed. We present results for benchmarks on three CPU architectures: Intel Haswell (HSW), Knights Landing (KNL), and Skylake (SKL). The performance gains by vectorization are about 42\% on HSW compared with the code optimized without vectorization. On KNL, the hand-vectorized codes exhibit 34\% better performance than the codes vectorized automatically by the Intel compiler. On SKL, the code vectorized with AVX2 exhibits slightly better performance than that with vectorized AVX-512.

cs.MS↗

arXiv2018

Stability of velocity-Verlet- and Liouville-operator-derived algorithms to integrate non-Hamiltonian systems

We investigate the difference between the velocity Verlet and the Liouville-operator-derived (LOD) algorithms by studying two non-Hamiltonian systems, one dissipative and the other conservative, for which the Jacobian of the transformation can be determined exactly. For the two systems, we demonstrate that (1) the velocity Verlet scheme fails to integrate the former system while the first- and second-order LOD schemes succeed, (2) some first-order LOD fails to integrate the latter system while the velocity Verlet and the other first- and second-order schemes succeed. We have shown that the LOD schemes are stable for the former system by determining the explicit forms of the shadow Hamiltonians which are exactly conserved by the schemes. We have shown that Jacobian of the velocity Verlet scheme for the former system and that of the first-order LOD scheme for the latter system are always smaller than the exact values, and therefore, the schemes are unstable. The decomposition-order dependence of LOD schemes is also considered.

physics.comp-ph↗

arXiv2018

Can Blockchain Protect Internet-of-Things?

In the Internet-of-Things, the number of connected devices is expected to be extremely huge, i.e., more than a couple of ten billion. It is however well-known that the security for the Internet-of-Things is still open problem. In particular, it is difficult to certify the identification of connected devices and to prevent the illegal spoofing. It is because the conventional security technologies have advanced for mainly protecting logical network and not for physical network like the Internet-of-Things. In order to protect the Internet-of-Things with advanced security technologies, we propose a new concept (datachain layer) which is a well-designed combination of physical chip identification and blockchain. With a proposed solution of the physical chip identification, the physical addresses of connected devices are uniquely connected to the logical addresses to be protected by blockchain.

cs.CR↗

arXiv2018

Polymer effects on Karman Vortex: Molecular Dynamics Study

We investigated the Karman vortex behind a circular cylinder in a polymer solution by a molecular dynamics simulation. The vortex characteristics are distinctly different for short and long polymers. The solution with the long polymer exhibits a reduction in the vortex shedding frequency and broadening of the lift coefficient spectrum. On the other hand, the characteristics of the short-polymer solution are almost same as those of the Newtonian fluid. These facts are consistent with the experiments. Because the distributions of the gyration radius and the orientational order of the long-polymer solution are highly inhomogeneous in the flow field, we conclude that the extensional property of the polymer plays an important role in changing the flow characteristics.

cond-mat.soft↗

arXiv2017

100 MHz high-speed strain monitor using fiber Bragg grating and optical filter applied for magnetostriction measurements of cobaltite at magnetic fields beyond 100 T

High-speed 100 MHz strain monitor using fiber Bragg grating (FBG) and an optical filter has been devised for the magnetostriction measurements under ultrahigh magnetic fields. The longitudinal magnetostriction of LaCoO$_{3}$ has been measured at room temperature, 115, 7 and 4.2 K up to the maximum magnetic field of 150 T. The field-induced lattice elongations are observed, which are attributed to the spin-state crossover from the low-spin ground state to excited spin-states.

cond-mat.mtrl-sci↗

arXiv2017

Optical signature of Weyl electronic structures in tantalum pnictides Ta$Pn$ ($Pn=$ P, As)

To investigate the electronic structure of Weyl semimetals Ta$Pn$ ($Pn=$P, As), optical conductivity [$σ(ω)$] spectra are measured over a wide range of photon energies and temperatures, and these measured values are compared with band calculations. Two significant structures can be observed: a bending structure at $\hbarω\sim$85 meV in TaAs, and peaks at $\hbarω\sim$ 50 meV (TaP) and $\sim$30 meV (TaAs). The bending structure can be explained by the interband transition between saddle points connecting a set of $W_2$ Weyl points. The temperature dependence of the peak intensity can be fitted by assuming the interband transition between saddle points connecting a set of $W_1$ Weyl points. Owing to the different temperature dependence of the Drude weight in both materials, it is found that the Weyl points of TaAs are located near the Fermi level, whereas those of TaP are further away.

cond-mat.mtrl-sci↗

arXiv2017

Failure of deterministic and stochastic thermostats to control temperature of molecular systems

We investigate the ergodicity and "hot solvent/cold solute" problems in molecular dynamics simulations. While the kinetic moments and the stimulated Nosé--Hoover methods improve the ergodicity of a harmonic-oscillator system, both methods exhibit the "hot solvent/cold solute" problem in a binary liquid system. These results show that the devices to improve the ergodicity do not resolve the "hot solvent/cold solute" problem.

cond-mat.stat-mech↗

arXiv2017

High-speed 100 MHz strain monitor using fiber Bragg grating and optical filter for magnetostriction measurements under ultrahigh magnetic fields

A high-speed 100 MHz strain monitor using a fiber Bragg grating, an optical filter, and a mode-locked optical fiber laser has been devised, which has a resolution of $ΔL/L\sim10^{-4}$. The strain monitor is sufficiently fast and robust for the magnetostriction measurements of magnetic materials under ultrahigh magnetic fields generated with destructive pulse magnets, where the sweep rate is in the range of 10-100 T/$μ$s. As a working example, the magnetostriction of LaCoO$_{3}$ was measured at room temperature, 115 K, and 7$\sim$4.2 K up to a maximum magnetic field of 150 T. The smooth $B^{2}$ dependence and the first-order transition were observed at 115 K and 7$\sim$4.2 K, respectively, reflecting the field-induced spin-state evolution.

cond-mat.mtrl-sci↗

arXiv2017

Energy dissipative solutions to the Kobayashi-Warren-Carter system

In this paper we study a variational system of two parabolic PDEs, called the Kobayashi-Warren-Carter system, which models the grain boundary motion in a polycrystal. The focus of the study is the existence of solutions to this system which dissipate the associated energy functional. We obtain existence of this type of solutions via a suitable approximation of the energy functional with Laplacians and an extra regularization of the weighted total variation term of the energy. As a byproduct of this result, we also prove some $Γ$-convergence results concerning weighted total variations and the corresponding time-dependent cases. Finally, the regularity obtained for the solutions together with the energy dissipation property, permits us to completely characterize the $ω$-limit set of the solutions.

math.AP↗

arXiv2017

Phase-field systems for grain boundary motions under isothermal solidifications

Two main existence theorems are proved for two nonstandard systems of parabolic initial-boundary value problems. The systems are based on the "$ ϕ$-$ η$-$ θ$ model" proposed by Kobayashi [RIMS Kokyuroku, 1210 (2001), 68-77] as a phase-field model of planar grain boundary motion under isothermal solidification. Although each of the systems has specific characteristics and mathematical difficulties, the proofs of the main theorems are based on the time discretization method by means of a common approximating problem. As a consequence, we provide a uniform solution method for a wide scope of parabolic systems associated with the $ ϕ$-$ η$-$ θ$ model.

math.AP↗

arXiv2017

Phase Competition and Superconductivity in $κ$-(BEDT-TTF)$_2$X: Importance of Intermolecular Coulomb Interactions

We theoretically study the competition among different electronic phases in molecular conductors $κ$-(BEDT-TTF)$_2$X. The ground-state properties of a 3/4-filled extended Hubbard model with the $κ$-type geometry are investigated by a variational Monte Carlo method. We find various competing phases: dimer-Mott insulator, polar charge-ordered insulator, 3-fold charge-ordered metal, and superconductivity, whose pairing symmetry is an "extended-$s$+$d_{x^2-y^2}$"-wave type. Our results show that the superconducting phase is stabilized not on the verge of the Mott metal-insulator transition but near charge order instabilities, clearly indicating the importance of the intradimer charge degree of freedom and the intermolecular Coulomb interactions, beyond the simple description of the half-filled Hubbard model.

cond-mat.supr-con↗

arXiv2016

Ripening Kinetics of Bubbles: A Molecular Dynamics Study

The ripening kinetics of bubbles is studied by performing molecular dynamics simulations. From the time evolution of a system, the growth rates of individual bubbles are determined. At low temperatures, the system exhibits a $t^{1/2}$ law and the growth rate is well described by classical Lifshitz-Slyozov-Wagner (LSW) theory for the reaction-limited case. This is direct evidence that the bubble coarsening at low temperatures is reaction-limited. At high temperatures, although the system exhibits a $t^{1/3}$ law, which suggests that it is diffusion-limited, the accuracy of the growth rate is insufficient to determine whether the form is consistent with the prediction of LSW theory for the diffusion-limited case. The gas volume fraction dependence of the coarsening behavior is also studied. Although the behavior of the system at low temperatures has little sensitivity to the gas volume fraction up to 10%, that at high temperatures deviates from the prediction of LSW theory for the diffusion-limited case as the gas volume fraction increases. These results show that the mean-field-like treatment is valid for a reaction-limited system even with a finite volume fraction, while it becomes inappropriate for a diffusion-limited system since classical LSW theory for the diffusion-limited case is valid at the dilute limit.

cond-mat.stat-mech↗

arXiv2016

Enhanced collectivity of gamma vibration in neutron-rich Dy isotopes with N=108 - 110

Background: The $γ$ vibrational mode of excitation is an acknowledged collective mode in deformed nuclei. The collectivity depends on the details of the shell structure around the Fermi levels, in particular the presence of the orbitals that have the enhanced transition matrix elements of the non-axial quadrupole excitation. Quite recently, a sudden decrease in the excitation energy of the $γ$ vibration was observed at RIKEN RIBF for the neutron-rich Dy isotopes at $N=106$. Purpose: In the present work, by studying systematically the microscopic structure of the $γ$ vibration in the neutron-rich Dy isotopes with $N=98-114$, we try to understand the mechanism of the observed softening. Methods: The low-frequency modes of excitation in the neutron-rich rare-earth nuclei are described based on nuclear density-functional theory. We employ the Skyrme energy-density functionals (EDF) in the Hartree-Fock-Bogoliubov calculation for the ground states and in the Quasiparticle Random-Phase Approximation (QRPA) for the excitations. Results: The lowering of the excitation energy around $N=106$ is reproduced well by employing the SkM* and SLy4 functionals. We find the similar isotopic dependence of the excitation energy in the neutron-rich Er and Yb isotopes as well. Conclusions: The microscopic framework of the Skyrme-EDF based QRPA describes well the isotopic dependence of the energy of the $γ$ vibration in the well-deformed neutron-rich rare-earth nuclei. The strong collectivity at $N=108-110$ is expected as the Fermi level of neutrons lies just among the orbitals that play an important role in generating the collectivity around $N=106$.

nucl-th↗

arXiv2015

Charge-density wave induced by combined electron-electron and electron-phonon interactions in 1$T$-TiSe$_2$: A variational Monte Carlo study

To clarify the origin of a charge-density wave (CDW) phase in 1$T$-TiSe$_2$, we study the ground state property of a half-filled two-band Hubbard model in a triangular lattice including electron-phonon interaction. By using the variational Monte Carlo method, the electronic and lattice degrees of freedom are both treated quantum mechanically on an equal footing beyond the mean-field approximation. We find that the cooperation between Coulomb interaction and electron-phonon interaction is essential to induce the CDW phase. We show that the "pure" exciton condensation without lattice distortion is difficult to realize under the poor nesting condition of the underlying Fermi surface. Furthermore, by systematically calculating the momentum resolved hybridization between the two bands, we examine the character of electron-hole pairing from the viewpoint of BCS-BEC crossover within the CDW phase and find that the strong-coupling BEC-like pairing dominates. We therefore propose that the CDW phase observed in 1$T$-TiSe$_2$ originates from a BEC-like electron-hole pairing.

cond-mat.str-el↗