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Dive into the research topics where Yasunari Zempo is active.

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Featured researches published by Yasunari Zempo.


Journal of Physics: Conference Series | 2016

High performance FDTD algorithm for GPGPU supercomputers

Andrey Zakirov; V. D. Levchenko; Anastasia Perepelkina; Yasunari Zempo

An implementation of FDTD method for solution of optical and other electrodynamic problems of high computational cost is described. The implementation is based on the LRnLA algorithm DiamondTorre, which is developed specifically for GPGPU hardware. The specifics of the DiamondTorre algorithms for staggered grid (Yee cell) and many-GPU devices are shown. The algorithm is implemented in the software for real physics calculation. The software performance is estimated through algorithms parameters and computer model. The real performance is tested on one GPU device, as well as on the many-GPU cluster. The performance of up to 0.65 • 1012 cell updates per second for 3D domain with 0.3 • 1012 Yee cells total is achieved.


Journal of Physics: Conference Series | 2015

Real-time and real-space program tuned in K-computer

Yasunari Zempo; Nobuhiko Akino; M Ishida; E Tomiyama; H Yamamoto

We have transported the code from Earth Simulator (ES) to K-Computer (KC) keeping a scalability. The base code achieved remarkable linear scalability and significant parallelization efficiency in EC before. The reason of success is that, taking into account the architectural differences between ES and KC, we improve the program step by step. Actually, the program showed a 37% improvement in the total performance with the implementations such as hybrid OpenMP and MPI parallelization, and the optimal rank mapping, and the MPI communication concealment, including removing the unnecessary processes such as zero clear and data copy.


Journal of Physics: Conference Series | 2017

Using memory-efficient algorithm for large-scale time-domain modeling of surface plasmon polaritons propagation in organic light emitting diodes

Andrey Zakirov; Sergei Belousov; Ilya Valuev; V. D. Levchenko; Anastasia Perepelkina; Yasunari Zempo

We demonstrate an efficient approach to numerical modeling of optical properties of large-scale structures with typical dimensions much greater than the wavelength of light. For this purpose, we use the finite-difference time-domain (FDTD) method enhanced with a memory efficient Locally Recursive non-Locally Asynchronous (LRnLA) algorithm called DiamondTorre and implemented for General Purpose Graphical Processing Units (GPGPU) architecture. We apply our approach to simulation of optical properties of organic light emitting diodes (OLEDs), which is an essential step in the process of designing OLEDs with improved efficiency. Specifically, we consider a problem of excitation and propagation of surface plasmon polaritons (SPPs) in a typical OLED, which is a challenging task given that SPP decay length can be about two orders of magnitude greater than the wavelength of excitation. We show that with our approach it is possible to extend the simulated volume size sufficiently so that SPP decay dynamics is accounted for. We further consider an OLED with periodically corrugated metallic cathode and show how the SPP decay length can be greatly reduced due to scattering off the corrugation. Ultimately, we compare the performance of our algorithm to the conventional FDTD and demonstrate that our approach can efficiently be used for large-scale FDTD simulations with the use of only a single GPGPU-powered workstation, which is not practically feasible with the conventional FDTD.


Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy | 2016

Theoretical study of the infrared frequencies of crystalline methyl acetate under interstellar medium conditions.

Radhika Narayanan; Kensuke Inomata; Geetha Gopakumar; Bhalamurugan Sivaraman; Yasunari Zempo; Masahiko Hada

Identification of methyl acetate in the interstellar medium (ISM) and its spectroscopic studies have prompted us to investigate the structure of crystalline methyl acetate using numerical calculations. Here, we present a theoretical study of the structure of crystalline methyl acetate and its isotopologues and compare the calculated infrared (IR) spectra with the available experimental data. The optimized structure and vibrational properties were calculated using SIESTA software at 0 K. In the optimization process, the Perdew-Burke-Ernzerhof functional and conjugate gradient methods were used with double zeta polarization basis functions. After optimization of the periodic structure, the vibrational frequencies and normal modes were calculated within the harmonic approximation. Using the calculated results, we refine the mode assignments of experimental work on crystalline methyl acetate and determine the low frequency modes (below 650 cm(-1)). To investigate the accuracy of the pseudopotential and confirm the IR frequencies, we performed molecular calculations using a periodic model of methyl acetate and its isotopologues using SIESTA and compared them with results obtained from Gaussian 09 (all electron method) calculations. Finally, we assigned the vibrational modes of crystalline CD3-COO-CH3 and CH3-COO-CD3, for which experimental data are not available in the crystalline phase under ISM conditions. For all of the calculation methods, the IR vibrational modes of molecular and crystalline methyl acetate and its isotopologues were in good agreement with the available experimental data and predict the unavailable values.


Science and Technology of Advanced Materials | 2014

Focus on organic electronics

Alexander Wei; Yasunari Zempo

Electronically active materials, once largely the domain of inorganic metals and semiconductors, have become increasingly organic in character. Contemporary electronic materials such as light-emitting device (LED) displays, photovoltaic cells, and thin-film transistors (TFTs) all feature organic molecules with π-conjugated backbones, whose processability and mechanical flexibility present opportunities not available to traditional inorganic materials. For example, the device structure of organic LEDs is robust yet simple in design, comprised of a light-emitting layer of conjugated polymers between cathode and anode, while producing emissions at low voltages and energy consumption. Simply by manipulating the substituents along the molecular backbone, π-conjugated systems can also be tuned to emit at specific wavelengths, and can even be designed for white light emission. The commercial development of these applications has been quite rapid: at the time of this writing, organic LED displays are now available with areal dimensions of 1–2 m2 and resolutions up to 400 ppi. The successful rise of organic electronics has been driven by several seminal contributions, two of which have been recognized with awards of the Nobel Prize in Chemistry: one in 2000 to Heeger, MacDiarmid, and Shirakawa for the discovery of conducting polymers, the other in 2010 to Heck, Negishi, and Suzuki for the development of transition-metal mediated cross-coupling, the synthetic basis for many of today’s π-conjugated polymers. Nevertheless, there are many opportunities for significant advances in scope as well as in performance, and for the scalable integration of organic electronic materials with inorganic substrates. Solid-state lighting technology represents one such area; electroluminescent organic materials are now widely available, and recent developments have produced organic materials with novel actuation properties such as thermally activated (delayed) fluorescence and polarized light emission. Many of these discoveries began with the design of small molecules, then developed into polymers with pliable backbones or extended π-conjugation, to facilitate their processing into organic LEDs and photovoltaic materials. The understanding and design of materials properties is aided by intensive computational studies, powered by ongoing developments in information technology. From these perspectives, organic structures will continue to play important roles in the design of next-generation electronic materials. Efficient organic electronic devices depend on more than molecular structure. For example, mechanisms of photoactivated transport need to be elucidated in order to improve the efficiency of organic TFTs and photodiodes. Device optimization also requires us to understand the fundamental physics of charge injection, transport, and recombination between layers, and also between layers and substrate. Effective engineering of these interfaces requires intensive investigations in both materials design and layer integration—topics of significant interest in this issue of Science and Technology of Advanced Materials. We would like to express our thanks to all authors who contributed to this focus issue on organic electronics, and hope that its contents will serve both as a digest and a catalyst for exciting research activities in this direction.


MRS Proceedings | 2004

Optical Responses of Conjugated Polymers by TDDFT in Real-Space and Real-Time Approach

Nobuhiko Akino; Yasunari Zempo

The time dependent density functional theory (TDDFT) has applied to study the optical responses of the conjugated polymers such as poly( p -phenylenevinylene) and poly(9, 9-dialkyl-fluorene). In our study, the real-space grid representation is used for the electron wavefunctions in contrast to a conventional basis set on each atom. In the calculations of the optical responses, the real-time approach is employed, where we follow the linear responses of the systems under externally applied perturbations in the real time. Since a real polymer is too large to handle, we have calculated the oligomers with different length and observed the spectrum peak is redshifted as the length of oligomer increases. The property of the polymer is extrapolated as the infinitely long oligomer. The estimated polymer spectra agree with the experiments reasonably well.


Keldysh Institute Preprints | 2016

High performance FDTD code implementation for GPGPU supercomputers

Andrey Zakirov; V. D. Levchenko; Anastasia Perepelkina; Yasunari Zempo


MRS Proceedings | 2013

Automatic Determination of Tight-Binding Parameters in Bulk Systems

Yasuaki Ohtani; Takeo Fujiwara; Shinya Nishino; Takashi Suzuki; Susumu Yamamoto; Yasunari Zempo


Journal of Computer Chemistry, Japan | 2014

Improved Technique of Optical Spectrum Analysis of Real-Time TDDFT Using Maximum Entropy Method

Mitsuki Toogoshi; Mai Kato; Satoru S. Kano; Yasunari Zempo


Bulletin of the American Physical Society | 2017

Maximum Entropy Method applied to Real-time Time-Dependent Density Functional Theory

Yasunari Zempo; Mitsuki Toogoshi; Satoru S. Kano

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Anastasia Perepelkina

Keldysh Institute of Applied Mathematics

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Andrey Zakirov

Keldysh Institute of Applied Mathematics

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V. D. Levchenko

Keldysh Institute of Applied Mathematics

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Geetha Gopakumar

Tokyo Metropolitan University

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Kensuke Inomata

Tokyo Metropolitan University

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