Interface Computational Science Group

【Date & Time】18 December 2008 (Thur), 3:30 pm - 5:00 pm
【Place】8F large seminar room, Sengen site
【Speaker】Dr. Zlata Pchelkina
【Affiliation】Institute for Metal Physics, Ural Division, Russian Academy of Sciences
【Title】Electronic Structure of New Iron Arsenide Superconductors
【Abstract】
In the beginning of 2008 the iron oxypnictides, a new family of superconductors, have been discovered starting with the famous work of H. Hosono group [1]. At present the several classes of these compounds are known: Re111 (Re=La,Ce,Pr,Nd,Sm) with parent compound LaO1-xFxFeAs and Tc about 40-55K; A122 (A=Ba,Sr) with parent system Ba1-xKxFe2As2 and Tc about 38K; Li1-xFeAs system with Tc=18K and Sr1-xLaxFFeAs compound with Tc=36K. It was also found that FeSe undergoes superconducting transition at temperature Tc=8K which can be increased to Tc=27K by applying high pressure.
In this talk an overview of different classes of FeAs based superconductors will be given, the basic physical properties and phase diagrams will be described. The results of electronic structure calculations for the parent compounds Re111 [2], Ba(Sr)122 [3], LiFeAs [4] and Ca(Sr)111 [5] will be presented.
[1] Y. Kamihara et al., J. Am. Chem. Soc. 130, 3296 (2008).
[2] I. A. Nekrasov, Z. V. Pchelkina, M. V. Sadovskii,Pis'ma Zh. Eksp. Teor. Fiz. 87, 647 (2008).
[3] I. A. Nekrasov, Z. V. Pchelkina, M. V. Sadovskii,Pis'ma Zh. Eksp. Teor. Fiz. 88, 155 (2008).
[4] I. A. Nekrasov, Z. V. Pchelkina, M. V. Sadovskii,Pis'ma Zh. Eksp. Teor. Fiz. 88, 621 (2008).
[5] I. A. Nekrasov, Z. V. Pchelkina, M. V. Sadovskii,Pis'ma Zh. Eksp. Teor. Fiz. 88, 777 (2008).
【Contact】Dr. Igor Solovyev

【Date & Time】8 December 2008 (Mon), 3:30 pm - 5:00 pm
【Place】4F seminar room, Collaborative Res. Bldg., Namiki site
【Speaker】Dr. Satoshi Okamoto
【Affiliation】Materials Science and Technology Division, Oak Ridge National Lab., USA
【Title】Strongly correlated electrons in heterostructures:transport, magnetism, and superconductivity
【Abstract】
Study of artificial heterostructures involving transition-metal oxides is one of the main topics of current materials science [1]. Among various oxides, of particular interest are correlated-electron systems which exhibit a variety of phenomena such as high-Tc superconductivity in cuprates. Correlated heterostructures are expected to become fundamental building blocks of future oxide electronics utilizing the novel properties of the oxides. Thus, understanding of the correlated electron behaviors at surfaces and interfaces is of crucial importance. In this talk, I will present the recent theoretical development in this field, including the following three topics.
1. Mott-insulator/metal interface [2,3]. It is shown that the spectral function in the interacting region is modified by the coupling with the metallic region, resulting in rather non-linear transport behavior.
2. Surface magnetic behavior of double-exchange ferromagnet for manganites [4]. The magnetic moment in abut-three-unit-cell-wide surface layers is found to decrease rapidly with increasing temperature. Possible improvement of the tunneling magnetoresistance effect is expected by supressing this behavior.
3. Superlattices of high-Tc cuprates [5]. We examine the possibility of enhanced superconductivity in heterostructures involving both the underdoped cuprates and overdoped cuprates by applying the cluster dynamical-mean-field methods.
Work in ORNL is supported by the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, U.S. Department of Energy.
[1] For example, A. Ohtomo et al., Nature (London) 419, 378 (2002).
[2] S. Okamoto (unpublished).
[3] S. Okamoto, Phys. Rev. Lett. 101, 116807 (2008).
[4] S. Okamoto (unpublished).
[5] S. Okamoto and T. A. Maier, Phys. Rev. Lett. 101, 156401 (2008).
【Contact】Dr. Masao Arai

【Date & Time】18 November 2008 (Tue), 1:30 pm - 3:00 pm
【Place】4F seminar room, MANA Bldg., Namiki Site
【Speaker】Prof. Martin J. Field
【Affiliation】Laboratoire de Dynamique Moleculaire, Institut de Biologie Structurale, France
【Title】Algorithms and applications of QM/MM hybrid simulations for elucidation of functional mechanisms and structural-based rational design of biological molecules
【Abstract】
Interactions of biological macromolecules such as proteins, nucleic acids are the source of biological functions. However, it is difficult to perform theoretical investigations of such interactions based on their electronic structures, because of huge number of atoms and thermal fluctuations. To investigate these systems, quantum mechanics/molecular mechanics (QM/MM) calculations are utilized, in which the system is divided into QM and MM atoms. Here, QM regions correspond to active sites to be investigated, and are described on the basis of the quantum mechanics theory. MM regions correspond to the remainder of the system, and are described at the molecular mechanics level.
In this seminar, recent progresses in QM/MM hybrid calculations, in particular, of biological macromolecules, are presented, including pharmaceutical issues. Our original calculation methods and their applications into proteins and structure-based rational drug design of therapeutic regents on the basis of our original algorithms and systems for QM/MM hybrid simulations are also discussed.
【Contact】Dr. Hiori Kino

【Date & Time】10 November 2008 (Mon), 3:30 pm - 5:00 pm
【Place】4F seminar room, Collaborative Res. Bldg., Namiki site
【Speaker】Prof. Takeo Fujiwara
【Affiliation】University of Tokyo, JST-CREST
【Title】GWA method with LSDA+U (U+ GWA) and applications to transition metal oxides
【Abstract】
We propose a novel GW approximation (GWA) method named U+ GWA, where we start GWA with more localized wave-functions obtained by LSDA+U method. Then U+ GWAtogether with GWA are applied to several transitionmetal oxides, MnO,NiO, V2O3 and LaMO3 (M = Ti ∼ Cu) with eigenvalue-only self-consistency. The band gaps, magnetic moments and energy spectra in these materials show an excellent agreement with the experimentally observed results. The method of the unique choice of U values will be discussed in details. The criterion whether we should adopt GWA or U+ GWA will be discussed and assessed with the help of the off-diagonal elements of the self-energy.
【Contact】Dr. Masao Arai

【Date & Time】6 November 2008 (Thur), 3:00 pm - 4:30 pm
【Place】4F seminar room, Collaborative Res. Bldg., Namiki site
【Speaker】Prof. Shuichi Murakami
【Affiliation】Department of Physics, Tokyo Institute of Technology
【Title】Topological insulators and surface states
【Abstract】
The quantum spin Hall (QSH) system has been intensively studied in recent years, as a topic related with the spin Hall effect. While it is a spin analogue of the quantum Hall system, it occurs under zero magnetic field, and the spin-orbit coupling in the system plays the role of spin-dependent "magnetic field". This system is insulating in the bulk, whereas the edge states (in 2D) or the surface states (in 3D) are gapless, carrying spin currents. These edge or surface states remain gapless, being topologically protected against nonmagnetic impurities and surface roughness. In my talk we present our recent research results together with related theories and experiments.
【Contact】Dr. Yoshitaka Tateyama

【Date & Time】30 October 2008 (Thur), 3:00 pm - 4:30 pm
【Place】8F large seminar room, Sengen site
【Speaker】Dr. Hiroyuki Yamase
【Affiliation】Materials Interdisciplinary G, Innovative Materials Engineering Lab., NIMS
【Title】Pomeranchuk instability in correlated electron systems
【Abstract】
The spontaneous Fermi surface symmetry breaking, the so-called Pomeranchuk instability, is a fundamentally new concept in the physics of correlated electron systems [1,2]. This instability breaks the point-group symmetry of the underlying lattice structure.
In this talk, I discuss the most fundamental aspect of Pomeranchuk instability by performing the exact analysis of a pure forward scattering model. Current understanding of Pomeranchuk instability for high-temperature cuprate superconductors and strontium ruthenates is also reviewed.
[1] H. Yamase and H. Kohno, JPSJ 69, 332 (2000); 69, 2151 (2000).
[2] C. J. Halboth and W. Metzner, PRL 85, 5162 (2000).
【Contact】Dr. Akihiro Tanaka

【Date & Time】31 July 2008 (Thur), 10:30 am - 12:00 am
【Place】6F seminar room, Sengen site
【Speaker】Prof. Renat Sabirianov
【Affiliation】Dept. of Physics, Univ. of Nebraska / ISSP, Univ. of Tokyo
【Title】Novel effects in spin-dependent transport
【Abstract】
Spin dependent transport in a synergistic combination with novel nanoscale magnetic and ferroelectric structures can be used to manipulate spin-dependent currents yielding new paradigms for device operation. I will discuss recent advances in spintronics concentrating on the discovery of new magnetoresistive phenomena: (1) ballistic anisotropic magnetoresistance (BAMR), a quantized change in the ballistic conductance of the nanowires and nanocontact with magnetization direction, (2) tunneling anisotropic magnetoresistance (TAMR) in magnetic break junctions, and (3) voltage induced magnetoresistance (MR) sign reversal. Transition metal oxides exhibit large changes in conductivity when an appropriate bias is applied, which make them promising for memory devices. The change in the MR associated with this transition has not been, however, explored, but may uncover fascinating physics. We have recently observed a MR sign reversal associated with the bias dependent on/off state in a Ni/NiO/CoNi/NiO/Co magnetic tunnel junction (MTJ) in nanowire geometry at T=1.5K. This behavior may be explained by a metal-insulator transition driven by a charge accumulation controlled by applied bias voltage.
【Contact】Dr. Igor Solovyev

【Date & Time】14 July 2008 (Mon), 3:30 pm - 5:00 pm
【Place】8F larger seminar room, Sengen site
【Speaker】Dr. Motoyuki Shiga
【Affiliation】Japan Atomic Energy Agency (JAEA)
【Title】Quantum simulations by ab initio path integral molecular dynamics

【Abstract】
Nuclear quantum effect such as zero point vibration or tunneling is important in molecular systems containing light atoms like hydrogen, and it is considered to have influence on the chemical and physical properties. In this presentation, I would like to introduce the methodology of ab initio calculations including nuclear quantum effect by path integral molecular dynamics, and report some application studies on the isotope effect of hydrogen bonds.
【Contact】Dr. Yoshitaka Tateyama

【Date & Time】2 July 2008 (Wed), 3:30 pm - 5:00 pm
【Place】8F large seminar room, Sengen site
【Speaker】Dr. Takashi Oka
【Affiliation】Department of Physics, University of Tokyo
【Title】Photo-induced Tomonaga-Luttinger-like liquid in a one-dimensional Mott insulator
【Abstract】
We theoretically study the non-equilibrium quantum phase transition in strongly correlated electron systemswhen strong AC electric fields are applied tothe Mott insulator.
The time-dependent density matrix renormalization group method is used to calculate the nonlinear optical conductivity for the half-filled one-dimensional Hubbard model, where we have found an emergence ofmetallic states that have a linear dispersion within the energy gap. This indicates that the photo-doped carriers behave collectively as in the Tomonaga-Luttinger liquid. A non-equilibrium phase diagram for the photo-induced insulator-to-metal transition is then proposed.
We further show that the numerical result is similar to an analytic result for an effective Dirac model, where the Floquet method is employed to incorporate the effect of strong electric fields.
【Contact】Dr. Akihiro Tanaka

【Date & Time】1 July 2008 (Tue), 1:30 pm - 3:00 pm
【Place】5F seminar room, Sengen site
【Speaker】Dr. Takahiro Yamasaki
【Affiliation】FUJITSU Laboratories Ltd.
【Title】A first-principles molecular dynamics program PHASE - program structure and parallelization methods -
【Abstract】
PHASE is a first-principles molecular dynamics program that optimizes geometories and calculates electronic structures of materials using pseudopotentials and plane wave basis based on the density functional theory. It has had been developed in a MITI project, then successivey in a MEXT project. It has calculational functions of XPS, positoron life time evaluation, STM images, lattice vibrations, and reaction path searchings. PHASE runs on personal computers, cluster workstations, and large scale computers like the Earth Simulator. It is effectively parallelized in a MPI-SMP hybrid way, and can run very large scale calculations of 10,000 atom order on the Earth Simulator. It is tuned not only for vector processors but also for scalar ones. The program is consist of almost fifty hierarchical modules. In this seminar, we would explain roles of the principal modules and the methodology of parallelization. Parallelization problems for peta-scale computers would also be discussed.
【Contact】Dr. Hiori Kino

【Date & Time】26 June 2008 (Thur), 1:00 pm - 2:30 pm
【Place】6F seminar room, Sengen site
【Speaker】Dr. Kazuma Nakamura
【Affiliation】Department of Applied Physics, University of Tokyo
【Title】Low-energy models for Molecular Solids: θ- and κ-(BEDT-TTF)2X
【Abstract】
Aiming at a deeper understanding of physics and chemistry of organic molecular solids, we perform first-principles analyses for two different types of (BEDT-TTF)2X; θ- and κ-type compounds. These compounds have strong two-dimensional anisotropy and the conducting layer relevant to the low-energy physics near the Fermi energy consists of aligned BEDT-TTF molecules. In spite of the structural similarity, the physics of the θ and κ compounds is completely different; in the θ type, puzzling coexistence of two different charge-ordering fluctuations in metallic phase is observed near charge-ordered and spin-Peierls phases. On the other hand, the κ compound shows typical Mott physics exhibiting a phase diagram involving antiferromagnetic Mott insulator, genuine Mott insulator with spin liquid nature, paramagnetic metal, and superconductor phases. The origin of such a diversity and mechanisms of stabilizing each phase themselves are intriguing questions to be answered.
I will present ab initio downfolded low-energy effective models of the θ and κ compounds. By the downfolding procedure, we derive model parameters such as transfer integrals and interaction terms, with the help of the maximally-localized-Wannier-function technique and the constrained random-phase-approximation methodology. Novel expressions for entire three-dimensional data of the screened Coulomb and exchange interactions are first derived. The difference between the resulting models for the θ and κ compounds is discussed with particular focus on a substantial difference in the range of the screened Coulomb interaction, together with their comparisons with the screened interactions of several types of transition metal oxides.
【Contact】Dr. Hiori Kino

【Date & Time】9 May 2008 (Fri), 10:30 am - 12:00 am
【Place】8F large seminar room, Sengen site
【Speaker】Prof. Ryotaro Arita
【Affiliation】Department of Applied Physics, University of Tokyo
【Title】Pudding-mold type band as the origin of large thermopower in transition metal oxides
【Abstract】
Conversion of temperature differences directly into electricity is one of the most fascinating phenomena in condensed matter physics. Designing and searching for good thermoelectric materials indeed have a long history of extensive studies due to the scientific interest and potential technological importance.
Hitherto, it has been believed that the coexistence of low resistivity and large thermopower is difficult to realize, so that the main target thermoelectric materials have been various insulators or semiconductors. More recently, the focus of interest shifted however to thermoelectrical metals, where one is searching for exceptionally large power factors. NaxCoO2 and LiRh2O4 are in fact good examples of such interesting thermoelectric metals.
Here we propose that a peculiar shape of conduction bands plays an important role to realize the coexistence of large conductivity and thermopower.
We first examine this idea for NaxCoO2 and LiRh2O4 by the Boltzmann equation approach based on the local density approximation (LDA). We then step forward to the combination of LDA and the dynamical mean field theory (LDA+DMFT) for LiRh2O4, a 3D frustrated paramagnetic metal for which DMFT is expected to be a good approximation. We find that the experimental values of the large thermopower are successfully reproduced. We also simulate how the power factor can be much further enhanced through doping.
【Contact】Dr. Igor Solovyev

【Date & Time】24 April 2008 (Thur), 3:30 pm - 5:00 pm
【Place】8F larger seminar room, Sengen site
【Speaker】Prof. Naoki Kawashima
【Affiliation】Institute for Solid State Physics (ISSP), University of Tokyo
【Title】Quantum Monte Carlo Simulation of VBS States
【Abstract】
We consider SU(N) generalization of the standard Heisenberg model. In other words, we regard operators S in the ordinary SU(2) Heisenberg Hamiltonian as the generators of SU(N) rotation rather than SU(2). We define the model on a bipartite lattice and use different representation for two sublattices: some representation for the sublattice A and its conjugate for the sublattice B. There is an interesting conjecture [1] for this very simple model, i.e., there is a transition from the Neel state to the VBS state as N is increased, and the nature of the VBS state changes periodically depending on the rank of the representation (=n). A similar conjecture for the one dimensional case is wellknown as Haldane's conjecture and has been confirmed by numerical calculations and other techniques. For the fundamental representation (n=1), we confirmed that there is a transition to the VBS state. [2]
Our recent calculations [3] further revealed that (1) in the fundamental representation, the plaquette type VBS pattern is as likely as the columnar VBS pattern, and U(1)-like behavior is observed, and that (2) the VBS pattern n=2 or above is vanishing or very small. The U(1)-like behavior is later observed in other models. [4,5] A related calculation on the quasi-one-dimensional biquadratic Heisenberg model [6] will also be discussed.
[1] N. Read and S. Sachdev, Nucl. Phys. B316, 609 (1989).
[2] K. Harada, N. Kawashima, and M. Troyer, Phys. Rev. Lett. 90, 117203 (2003).
[3] N. Kawashima and Y. Tanabe, Phys. Rev. Lett. 98, 057202 (2007).
[4] K. S. Beach and A. W. Sandvik, Phys. Rev. Lett. 99, 047202 (2007).
[5] M. Tsukamoto, K. Harada, and N. Kawashima, unpublished.
[6] K. Harada, N. Kawashima, and M. Troyer, J. Phys. Soc. Jpn. 76, 013703 (2007).
【Contact】Dr. Yoshihiko Nonomura

【Date & Time】16 April 2008 (Wed), 3:30 pm - 5:00 pm
【Place】8F larger seminar room, Sengen site
【Speaker】Dr. Eric J. Cockayne
【Affiliation】Ceramics Division, National Institute of Standards and Technology (NIST), USA
【Title】Effect of defects on the structure, electronic structure, and dielectric properties of HfO2
【Abstract】
This talk details the results of first-principles density functional theory studies of the effect of various defects on the structure, electronic structure, and dielectric properties of HfO2. The defects studied include oxygen vacancies, ionic substitutions, interstitials, and combinations of defects. Most defects cause a larger permittivity due to a variety of factors; the most significant enhancement occurs if the tetragonal or cubic phase of HfO2 is stabilized with respect to the monoclinic phase. The effect of Al substitution on Si-SiO2-HfO2 gate stacks is also investigated, and discussed in terms of a dipole model.
【Contact】Dr. Naoto Umezawa

【Date & Time】17 March 2008 (Mon), 10:30 am - 12:00 am
【Place】8F larger seminar room, Sengen site
【Speaker】Prof. Stefan Goedecker
【Affiliation】Department of Physics and Astronomy, University of Basel
【Title】Wavelets as basis functions for electronic structure calculations and their implementation in the ABINIT code
【Abstract】
Wavelets are a relatively new class of mathematical functions and I will describe how they can be used as a basis set in density functional electronic structure calculations. Wavelets offer many advantages since they are adaptive, orthogonal and localized both in real and Fourier space. I will present performance comparisons with plane wave codes and some applications.
【Contact】Dr. Yoshitaka Tateyama

【Date & Time】13 March 2008 (Thur), 3:30 pm - 5:00 pm
【Place】6F seminar room, Sengen site
【Speaker】Dr. Tetsuro Suzuki
【Affiliation】Particle Simulation & Thermodynamics group, NIMS-CMSC
【Title】Role of randomness in martensitic transformation
【Abstract】
Critical role of randomness in the martensitic transformation is examined by use of molecular dynamics and random field Ising spin model (RFIM). Sarkar, Ren and Otsuka have recently reported that the usual abrupt onset of the martensitic transformation in Ti-Ni alloy is replaced by the continuous one by introducing deviation from 50-50 stoichiometry. We have shown by molecular dynamics that this is caused by the randomness associated the deviation from the stoichiometry. RFIM shows that such replacement of abrupt onset by continuous one as reported above is one example of the critical role of randomness in a wide range of first order phase transition.
【Contact】Dr. Masato Shimono

【Date & Time】6 March 2008 (Thur), 1:30 pm - 3:00 pm
【Place】6F seminar room, Sengen site
【Speaker】Prof. Shigenobu Ogata
【Affiliation】Department of Mechanical Science and Bioengineering, Osaka University
【Title】Modeling of Materials Deformation - First Principle Study of Ductility of Solids
【Abstract】
We newly propose ductility parameter D, which is defined as the ratio of energies needed for bond breaking under uniform shear and tensile deformations. So far the ratio of bulk modulus and shear modulus [1] and ideal tensile and shear strength [2] and so on are carried out to estimate ductility of solids. However they cannot clearly demonstrate the actual big ductility gap between ceramics and metals. We estimated our proposed D of 23 materials including different ceramics, alloys and metals by performing shear and tensile testing simulations to these materials using ab-initio density functional theory method [3]. We find that (1) ductility of crystalline solids is mainly controlled by the ideal shear strain [4], (2) the D can clearly demonstrate the big ductility gap between ceramics and metals, and (3) the D has clear linear correlation with widely used experimental toughness value in wide range of different materials from ceramics to metals. Hence we conclude D is a good predictor of ductility and also fracture toughness.
[1] A. Kelly, W. R. Tyson and A. H. Cottrell, Philos. Mag., Vol. 15 (1967), p. 567.
[2] S. S. Hecker, D. L. Rohr and D. F. Stein, Met. Trans., Vol. A 9 (1978), p. 481.
[3] G. Kresse and J. Furthmuller, Phys. Rev. B, Vol. 54 (1996), p. 11169.
[4] S. Ogata, J. Li, N. Hirosaki, Y. Shibutani and S. Yip, Phys. Rev. B, Vol. 70 (2004), 104104.
【Contact】Dr. Taichi Abe

【Date & Time】21 February 2008 (Thur), 3:30 pm - 5:00 pm
【Place】6F seminar room, Sengen site
【Speaker】Dr. Kazuaki Kobayashi
【Affiliation】First-Principles Simulation group (II), NIMS-CMSC
【Title】First-principles study of 5H-BN and related polytypes
【Abstract】
5H-BN has been synthesized by Komatsu et al.[1] 5H-BN is one of polytypes which are sp3 bonded hexagonal layered compounds. We have calculated the electronic and lattice properties of BN,[2] SiC, and AlN polytypes. Calculated polytypes are 2H, 3C(=3H), 4H, 5H, and 6H. We have found that 6H-AlN(ABCBCB) is more favorable than 6H-AlN(ABCACB) [Kobayashi and S. Komatsu: submitted to JPSJ]. This seminar will be talked in Japanese. Some slides are edited in Japanese for JSAP Spring meeting.
[1] S. Komatsu, K. Okada, Y. Shimizu, and Y. Moriyoshi: J. Phys. Chem. B 103 (1999) 3289.
[2] K. Kobayashi and S. Komatsu: J. Phys. Soc. Jpn. 76 (2007) 113707.
【Contact】Dr. Masao Arai

【Date & Time】4 February 2008 (Mon), 3:30 pm - 5:00 pm
【Place】8F larger seminar room, Sengen site
【Speaker】Prof. M. J. Gillan
【Affiliation】Materials Simulation Laboragtory, London Centre for Nanotechnology, Department of Physics and Astronomy, University College London, UK
【Title】Towards chemical accuracy for molecular processes on oxide surfaces
【Abstract】
Density functional theory (DFT) is widely used for studying the energetics of molecular processes on surfaces, but for oxide surfaces its accuracy leaves much to be desired. For example, the adsorption energy of the H2O molecule on the MgO (001) surface calculated with different exchange-correlation functionals varies by over 0.5 eV. It is clear that DFT calculations for such systems need to be calibrated against more systematic and accurate methods, which should themselves be checked against experimental measurements. Taking H2O on MgO (001) as an example, I will show how both quantum Monte Carlo (QMC) calculations and high-level quantum chemistry methods can be used to place constraints on the errors of DFT calculations. I will also outline how statistical-mechanical calculations of the absolute molecular desorption rate, compared with the results of temperature programmed desorption experiments, can provide another way of assessing DFT errors. The future possibility of statistical-mechanical calculations based on QMC or high-level quantum chemistry will be indicated.
【Contact】Dr. Tsuyoshi Miyazaki