Initialising ...
Initialising ...
Initialising ...
Initialising ...
Initialising ...
Initialising ...
Initialising ...
Yamada, Susumu; Okumura, Masahiko; Imamura, Toshiyuki*; Machida, Masahiko
Japan Journal of Industrial and Applied Mathematics, 28(1), p.141 - 151, 2011/04
Times Cited Count：2 Percentile：19.53(Mathematics, Applied)The density-matrix renormalization group (DMRG) method is widely used by computational physicists as a high accuracy tool to explore the ground state in large quantum lattice models, e.g., Heisenberg and Hubbard models, which are well-known standard models describing interacting spins and electrons, respectively, in solid states. After the DMRG method was originally developed for 1-D lattice/chain models, some specific extensions toward 2-D lattice (n-leg ladder) models have been proposed. However, high accuracy as obtained in 1-D models is not always guaranteed in their extended versions because the original exquisite algorithm is partly lost. Thus, we choose an alternative way. It is a direct 2-D extension of DMRG method which instead demands an enormously large memory space, but the memory explosion is resolved by parallelizing the DMRG code with performance tuning. The parallelized direct extended DMRG shows a good accuracy like 1-D models and an excellent parallel efficiency as the number of states kept increases. This success promises accurate analysis on large 2-D (n-leg ladder) quantum lattice models in the near future when peta-flops parallel supercomputers are available.
Ota, Yukihiro; Machida, Masahiko; Koyama, Tomio*
Physica C, 470(Suppl.1), p.S489 - S490, 2010/12
Times Cited Count：0 Percentile：0.00(Physics, Applied)We microscopically study inter-grain Josephson current in iron-pnictide superconductors. The theoretical examination predicts that the inter-grain Josephson current is significantly reduced by 
-wave symmetry as the incoherent tunneling process becomes dominant and the density of states and the gap amplitude between two bands are identical. In addition, we suggest important points to improve the inter-grain critical current density.
Yamada, Susumu; Okumura, Masahiko; Machida, Masahiko
Journal of the Physical Society of Japan, 78(9), p.094004_1 - 094004_5, 2009/09
Times Cited Count：11 Percentile：55.39(Physics, Multidisciplinary)We parallelize the density-matrix renormalization group method to directly extend it to 2-dimensional (n-leg) quantum lattice models. The parallelization is made mainly on the diagonalization for the superblock Hamiltonian since the part requires enormous memory space as the leg number n increases. The superblock Hamiltonian is divided into three parts, and the correspondent superblock vectors are transformed into matrices, whose elements are uniformly distributed into processors. The parallel efficiency shows a high rate as the number of the states kept m increases, and the obtained ground-state energy rapidly converges within a few sweeps. Furthermore, the present algorithm reaches the ground-state which satisfies the Lieb-Mattis theorem with much less m than a different indirect algorithm.
Yamada, Susumu; Imamura, Toshiyuki*; Machida, Masahiko
Nihon Keisan Kogakkai Rombunshu (Internet), 2009(15), 12 Pages, 2009/09
We parallelize the density matrix renormalization group (DMRG) method, which is a groundstate solver for one-dimensional quantum lattice systems. The parallelization allows us to extend the applicable range of the DMRG to multiple leg ladders or quasi two-dimension cases. Such an extension is regarded to bring about several breakthroughs in quantum physics, chemistry, nano-device engineering, and so on. The parallelization requires an all-to-all communication which involves all processes. Such a communication is not suitable for multi-core clusters, which are presently the main stream of the parallel computer, because the network bandwidth should be shared by a large number of processes. Therefore, we propose a new communication strategy to eliminate the all-to-all communication by rearranging a data distribution. We evaluate performance of the new strategy on "SGI Altix 3700Bx2" (single-core system) and "T2K Open Supercomputer (Todai Combined Cluster)" (quad-core system) and confirm that the strategy is quite crucial for typical multi-core systems like T2K.
Ota, Yukihiro; Machida, Masahiko; Koyama, Tomio*; Matsumoto, Hideki*
Physical Review Letters, 102(23), p.237003_1 - 237003_4, 2009/06
Times Cited Count：64 Percentile：88.52(Physics, Multidisciplinary)Using the functional integral method, we construct a theory of heterotic superconductor-insulator-superconductor Josephson junctions between one- and two-gap superconductors. The theory predicts the presence of in-phase and out-of-phase collective oscillation modes of superconducting phases. The former corresponds to the Josephson plasma mode whose frequency is drastically reduced for 
-wave symmetry, and the latter is a counterpart of Leggett's mode in Josephson junctions. We also reveal that the critical current and the Fraunhofer pattern strongly depend on the symmetry type of the two-gap superconductor.
Okumura, Masahiko; Yamada, Susumu; Taniguchi, Nobuhiko*; Machida, Masahiko
Physical Review B, 79(18), p.184417_1 - 184417_5, 2009/05
Times Cited Count：2 Percentile：10.83(Materials Science, Multidisciplinary)In order to study an interplay of disorder, correlation, and spin imbalance on antiferromagnetism, we systematically explore the ground state of one-dimensional spin-imbalanced Anderson-Hubbard model by using the density-matrix renormalization-group method. We find that disorders localize the antiferromagnetic spin-density wave induced by imbalanced fermions and the increase in the disorder magnitude shrinks the areas of the localized antiferromagnetized regions. Moreover, the antiferromagnetism finally disappears above a large disorder. The localization behaviors are observable in atomic Fermi gases loaded on optical lattices as broadening of the momentum distribution of the spin density by using the Stern-Gerlach type of time-of-fight imaging.
Okumura, Masahiko; Yamada, Susumu; Machida, Masahiko
Soryushiron Kenkyu, 116(6), p.F69 - F71, 2009/02
no abstracts in English
Imamura, Toshiyuki*; Yamada, Susumu; Machida, Masahiko
Joho Shori Gakkai Kenkyu Hokoku 2007-HPC-111, p.167 - 172, 2007/08
no abstracts in English
Owada, Kenji; Namikawa, Kazumichi; Mizuki, Junichiro; Shimomura, Susumu*; Nakao, Hironori*; Ito, Kazuki*; Matsushita, Mitsuyoshi*; Yoneda, Yasuhiro; Murakami, Yoichi*; Hirota, Kazuma*
Transactions of the Materials Research Society of Japan, 32(1), p.7 - 10, 2007/03
An X-ray speckle pattern well reflects a particle or domain alignment on the inside of materials. Detecting the alignment is important for understanding a function of the materials such as ferroelectric, piezoelectric materials, photonic crystals and so on. We have thus constructed the apparatus for coherent X-ray diffraction at BL22XU@SPring-8, high quality slits for obtaining the full coherent X-ray beam, high resolution X-ray CCD camera for obtaining the well-resolved speckle pattern, and so on. By using the devices, we have successfully observed the speckle pattern from Cu
Au, the ferroelectric material PZN-9%PT (91%Pb(Zn
Nb
)O-9%PbTiO) and Sr-doped BaTiO. The 2DFFT image gives us a spatial autocorrelation function which includes the information of the arrangement of the domains in the crystal within micrometers scale.
Yamada, Susumu; Imamura, Toshiyuki*; Okumura, Masahiko; Igarashi, Ryo; Machida, Masahiko
no journal, ,
no abstracts in English
Yamada, Susumu; Okumura, Masahiko; Igarashi, Ryo; Machida, Masahiko
no journal, ,
no abstracts in English
Ota, Yukihiro; Machida, Masahiko; Koyama, Tomio*
no journal, ,
Recent discovery of iron-based superconductors has triggered the study of multi-gap superfluidity/superconductivity. We derive the Ginzburg-Landau (GL) equation in a 
-gap system, which is a simultaneous non-linear partial differential equation. The system has not only a usual non-linear term but also coupling between different gaps (i.e., inter-band Josephson coupling). We show that this equation has non-trivial class of the solutions based on the linearized GL equation when 
. We stress that such a class never exist in the case of 
. We will also discuss generalization of the present result.
Okumura, Masahiko; Onishi, Hiroaki; Yamada, Susumu; Sakai, Toru; Machida, Masahiko
no journal, ,
no abstracts in English
Ota, Yukihiro; Machida, Masahiko; Koyama, Tomio*
no journal, ,
Superconductors and the related devices are promising systems for physical realization of quantum information processing (QIP). In this paper, we focus on a novel superconducting material, iron-pnictide superconductor, and examine its application potential to QIP. A notable character of this material is multi-band superconductivity. It indicates that multiple superconducting gaps exist below the superconducting transition temperature. Hence, Josephson junctions with these materials have multiple tunneling channels. We show the first theoretical derivation of a basic description of an array of intrinsic Josephson junctions with multi-gap superconductors. Then, we reveal how multiple gap characters provide an advantage for physical realization of QIP compared to conventional single-gap superconductors.
Ota, Yukihiro; Machida, Masahiko; Koyama, Tomio*; Matsumoto, Hideki*
no journal, ,
In order to examine pairing symmetry and application potential of iron-pnictide superconductors, Josephson junctions including such novel superconductors have been intensively examined. In this paper, we propose a heterotic SIS Josephson junction between multigap and conventional s-wave superconductors and examine ac Josephson effects. We investigate the effects of relative phase fluctuations on the ac Josephson effects. We reveal that the presence of Leggett's collective modes drastically changes the Shapiro steps.
Machida, Masahiko; Sasa, Narimasa; Ota, Yukihiro; Koyama, Tomio*; Matsumoto, Hideki*
no journal, ,
Yamada, Susumu; Igarashi, Ryo; Okumura, Masahiko*; Yamamoto, Atsushi; Machida, Masahiko
no journal, ,
no abstracts in English
Yamada, Susumu; Igarashi, Ryo; Okumura, Masahiko*; Yamamoto, Atsushi; Machida, Masahiko
no journal, ,
no abstracts in English
Yamamoto, Atsushi; Yamada, Susumu; Machida, Masahiko
no journal, ,
no abstracts in English
Sasa, Narimasa; Machida, Masahiko
no journal, ,
Numerical simulations of homogeneous isotropic superfluid turbulence are performed to investigate vortex structure and statistical properties. Large scale structure of quantized vortices is similar to that of Navier-Stokes turbulence. However, inertial range becomes narrower by bottle neck effect of cascade process. We compare the finite difference scheme with the spectral scheme whether the schemes are suitable for a large scale parallel simulation.
