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Search Results: 1 - 10 of 149784 matches for " H. Ohsumi "
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Relationship between charge stripe order and structural phase transitions in La$_{1.875}$Ba$_{0.125-x}$Sr$_{x}$CuO$_{4}$
H. Kimura,Y. Noda,H. Goka,M. Fujita,K. Yamada,M. Mizumaki,N. Ikeda,H. Ohsumi
Physics , 2004, DOI: 10.1103/PhysRevB.70.134512
Abstract: The nature of charge stripe order and its relationship with structural phase transitions were studied using synchrotron X-ray diffraction in La$_{1.875}$Ba$_{0.125-x}$Sr$_{x}$CuO$_{4}$ ($0.05\leq x\leq 0.10$). For $x=0.05$, as temperature increased, incommensurate superlattice peaks associated with the charge order disappeared just at the structural phase transition temperature, $T_{\rm d2}$. However, for $x=0.075$ and 0.09, the superlattice peaks still existed as a short range correlation even above $T_{\rm d2}$, indicating a precursor of charge ordering. Furthermore, temperature dependences of the superlattice peak intensity, correlation length, and incommensurability for $x=0.05$ are different from those for $x=0.075$ and 0.09. These results suggest that the transition process into the charge stripe order strongly correlates with the order of the structural phase transitions. A quantitative comparison of the structure factor associated with the charge order have been also made for all the samples.
Relative contributions of lattice distortion and orbital ordering to resonant x-ray scattering in manganites
M. Nakamura,M. Izumi,N. Ogawa,H. Ohsumi,Y. Wakabayashi,K. Miyano
Physics , 2004, DOI: 10.1143/JPSJ.73.2802
Abstract: We investigated the origin of the energy splitting observed in the resonant x-ray scattering (RXS) in manganites. Using thin film samples with controlled lattice parameters and orbital states at a fixed orbital filling, we estimated that the contribution of the interatomic Coulomb interaction relative to the Jahn-Teller mechanism is insignificant and at most 0.27. This indicates that RXS probes mainly Jahn-Teller distortion in manganites.
Two-dimensional Heisenberg behavior of Jeff = 1/2 isospins in the paramagnetic state of spin-orbital Mott insulator Sr2IrO4
S. Fujiyama,H. Ohsumi,T. Komesu,J. Matsuno,B. J. Kim,M. Takata,T. Arima,H. Takagi
Physics , 2012, DOI: 10.1103/PhysRevLett.108.247212
Abstract: Dynamical correlations of Jeff = 1/2 isospins in the paramagnetic state of spin-orbital Mott insu- lator Sr2IrO4 was revealed by resonant magnetic x-ray diffuse scattering. We found two-dimensional antiferromagnetic fluctuation with a large in-plane correlation length exceeding 100 lattice spacings at even 20 K above the mangnetic ordering temperature. In marked contrast to the naive expecta- tion of strong magnetic anisotropy associated with an enhanced spin-orbit coupling, we discovered isotropic isospin correlation that is well described by the two-dimensional S = 1/2 quantum Heisen- berg model. The estimated antiferromagnetic coupling constant as large as J ~ 0.1 eV that is comparable to the small Mott gap (< 0.5 eV) points the weak and marginal Mott character of this spin-orbital entangled system.
Weak antiferromagnetism of J_eff=1/2 band in bilayer iridate Sr3Ir2O7
Shigeki Fujiyama,K. Ohashi,H. Ohsumi,K. Sugimoto,T. Takayama,T. Komesu,M. Takata,T. Arima,H. Takagi
Physics , 2012, DOI: 10.1103/PhysRevB.86.174414
Abstract: The antiferromagnetic structure of Sr3Ir2O7, the bilayer analogue of a spin-orbital Mott insulator Sr2IrO4, was revealed by resonant magnetic x-ray diffraction. Contrasting intensities of the magnetic diffraction at the Ir LIII and LII edges show a Jeff=1/2 character of the magnetic moment as is argued in Sr2IrO4. The magnitude of moment, however, was found to be smaller than that of Sr2IrO4 by a factor of 5-6, implying that Sr3Ir2O7 is no longer a Mott insulator but a weak antiferromagnet. An evident change of the temperature dependence of the resistivity at TN, from almost temperature-independent resistivity to insulating, strongly suggests that the emergent weak magnetism controls the charge gap. The magnetic structure was found to be an out-of-plane collinear antiferromagnetic ordering in contrast to the inplane canted antiferromagnetism in Sr2IrO4, originating from the strong bilayer antiferromagnetic coupling.
Spin and Orbital Contributions to Magnetically Ordered Moments in 5d Layered Perovskite Sr2IrO4
S. Fujiyama,H. Ohsumi,K. Ohashi,D. Hirai,B. J. Kim,T. Arima,M. Takata,H. Takagi
Physics , 2013, DOI: 10.1103/PhysRevLett.112.016405
Abstract: The ratio of orbital (L) and spin (S) contributions to the magnetically ordered moments of a 5d transition metal oxide, Sr2IrO4 was evaluated by non-resonant magnetic x-ray diffraction. We applied a new experimental setting to minimize the error in which we varied only the linear-polarization of incident x-ray at a fixed scattering angle. Strong polarization dependence of the intensity of magnetic diffraction was observed, from which we conclude that the ordered moments contain substantial contribution from the orbital degree of freedom with the ratio of / ~5.0, evidencing the pronounce effect of spin-orbit coupling. The obtained ratio is close to but slightly larger than the expected value for the ideal J_eff = 1/2 moment of a spin-orbital Mott insulator, ||/|| = 4, which cannot be accounted by the redistribution of orbital components within the t_2g bands associated with the elongation of the IrO6 octahedra.
Atg14: A Key Player in Orchestrating Autophagy
Keisuke Obara,Yoshinori Ohsumi
International Journal of Cell Biology , 2011, DOI: 10.1155/2011/713435
Abstract: Phosphorylation of phosphatidylinositol (PtdIns) by a PtdIns 3-kinase is an essential process in autophagy. Atg14, a specific subunit of one of the PtdIns 3-kinase complexes, targets the complex to the probable site of autophagosome formation, thereby, sorting the complex to function specifically in autophagy. The N-terminal half of Atg14, containing coiled-coil domains, is required to form the PtdIns 3-kinase complex and target it to the proper site. The C-terminal half of yeast Atg14 is suggested to be involved in the formation of a normal-sized autophagosome. The C-terminal half of mammalian Atg14 contains the Barkor/Atg14(L) autophagosome-targeting sequence (BATS) domain that preferentially binds to the highly curved membranes containing PtdIns(3)P and is proposed to target the PtdIns 3-kinase complex efficiently to the isolation membrane. Thus, the N- and C-terminal halves of Atg14 are likely to have an essential core function and a regulatory role, respectively. 1. PtdIns 3-Kinase in Autophagy Eukaryotic cells can enclose their own cytoplasmic components in a double-membrane structure, the autophagosome, and deliver it to a lytic compartment, the vacuole/lysosome, where the contents are then degraded. This conserved system is involved not only in the recycling of proteins under starvation conditions but also in the clearance of organelles and aberrant aggregate-prone proteins, digestion of invading pathogens, and so on [1–4]. Genes involved in autophagy were first identified by yeast genetic screenings [5–7]. At present, more than 30 autophagy-related (ATG) genes have been identified in yeast, and of them at least 18 genes are essential for autophagosome formation, a crucial process in autophagy. Most of these 18 genes are conserved in mammals, suggesting that the mechanism of autophagosome formation is basically conserved from yeast to mammals. The 18 Atg proteins can be divided into five groups according to their functions [8, 9]. One group consists of subunits of a class III phosphatidylinositol (PtdIns) 3-kinase complex (hereafter, PtdIns 3-kinase indicates the class III PtdIns 3-kinase). Atg14 and Vps30/Atg6 are two such proteins and are included in this group together with Vps34 and Vps15, catalytic and regulatory subunits, respectively, (the functions of Vps34 and Vps15 in the vacuolar protein sorting pathway have been studied in detail, and; thus, they are not designated as Atg proteins although they are essential for autophagy). Atg14 is a key subunit in determining the function of the PtdIns 3-kinase complex and is the focus of this
PtdIns 3-Kinase Orchestrates Autophagosome Formation in Yeast
Keisuke Obara,Yoshinori Ohsumi
Journal of Lipids , 2011, DOI: 10.1155/2011/498768
Abstract: Eukaryotic cells can massively transport their own cytoplasmic contents into a lytic compartment, the vacuole/lysosome, for recycling through a conserved system called autophagy. The key process in autophagy is the sequestration of cytoplasmic contents within a double-membrane structure, the autophagosome. Autophagosome formation requires the elaborate cooperation of Atg (autophagy-related) proteins and lipid molecules. Phosphorylation of phosphatidylinositol (PtdIns) by a PtdIns 3-kinase, Vps34, is a key step in coordinating Atg proteins and lipid molecules. Vps34 forms two distinct protein complexes, only one of which is involved in generating autophagic membranes. Upon induction of autophagy, PtdIns(3)P, the enzymatic product of PtdIns 3-kinase, is massively transported into the lumen of the vacuole via autophagy. PtdIns(3)P is enriched on the inner membrane of the autophagosome. PtdIns(3)P recruits the Atg18?Atg2 complex and presumably other Atg proteins to autophagic membranes, thereby coordinating lipid molecules and Atg proteins. 1. Membrane Dynamics of Autophagy Eukaryotic cells are equipped with a self-digesting system called macroautophagy (hereafter, autophagy). Using this system, cells can degrade a portion of their cytoplasmic contents, occasionally including organelles, within a lytic compartment called the vacuole or lysosome. Autophagy is considered a primarily cell survival mechanism that enables macromolecules to be recycled under nutrient-limited conditions. However, recent studies have shown that autophagy is a versatile system that is also involved in the clearance of protein aggregate precursors, defense against invading pathogens, cell differentiation, and so forth [1–4]. Autophagy involves unique membrane dynamics, which has been predominantly shown through detailed electron microscopic analyses of starved yeast cells [5, 6]. Upon the induction of autophagy, a cup-shaped isolation membrane emerges in the cytoplasm and elongates to enclose a portion of the cytoplasm. The isolation membrane fuses at its ends to become a closed double-membrane structure called the autophagosome. The autophagosome then fuses with a vacuole or lysosome, where the inner membrane structure is released into the lumen as the autophagic body and is then degraded (Figure 1) [5, 7]. Autophagosome formation is a fundamental process in autophagy that occurs in a variety of situations. Despite its importance, the molecular mechanism of autophagosome formation is still largely unknown. There are unsolved problems including the source of the autophagosome
Entanglement of Coarse Grained Quantum Field in the Expanding Universe
Yasusada Nambu,Yuji Ohsumi
Physics , 2009, DOI: 10.1103/PhysRevD.80.124031
Abstract: We investigate the entanglement of a quantum field in the expanding universe. By introducing a bipartite system using a coarse-grained scalar field, we apply the separability criterion based on the partial transpose operation and numerically calculate the bipartite entanglement between separate spatial regions. We find that the initial entangled state becomes separable or dis-entangled after the spatial separation of two points exceed the Hubble horizon. This provides the necessary condition for the appearance of classicality of the quantum fluctuation. We also investigate the condition of classicality that the quantum field can be treated as the classical stochastic variables.
Classical and Quantum Correlations of Scalar Field in the Inflationary Universe
Yasusada Nambu,Yuji Ohsumi
Physics , 2011, DOI: 10.1103/PhysRevD.84.044028
Abstract: We investigate classical and quantum correlations of a quantum field in the inflationary universe using a particle detector model. By considering the entanglement and correlations between two comoving detectors interacting with a scalar field, we find that the entanglement between the detectors becomes zero after their physical separation exceeds the Hubble horizon. Furthermore, the quantum discord, which is defined as the quantum part of total correlation, approaches zero on super horizon scale. These behaviors support appearance of classical nature of the quantum fluctuation generated during the inflationary era.
Tetrahedral Magnetic Order and the Metal-Insulator Transition in the Pyrochlore Lattice of Cd2Os2O7
J. Yamaura,K. Ohgushi,H. Ohsumi,T. Hasegawa,I. Yamauchi,K. Sugimoto,S. Takeshita,A. Tokuda,M. Takata,M. Udagawa,M. Takigawa,H. Harima,T. Arima,Z. Hiroi
Physics , 2012, DOI: 10.1103/PhysRevLett.108.247205
Abstract: Cd2Os2O7 shows a peculiar metal-insulator transition at 227 K with magnetic ordering in a frustrated pyrochlore lattice, but its magnetic structure in the ordered state and the transition origin are yet uncovered. We observed a commensurate magnetic peak by resonant x-ray scattering in a high-quality single crystal. X-ray diffraction and Raman scattering experiments confirmed that the transition is not accompanied with any spatial symmetry breaking. We propose a noncollinear all-in/all-out spin arrangement on the tetrahedral network made of Os atoms. Based on this we suggest that the transition is not caused by Slater mechanism as believed earlier but by an alternative mechanism related to the formation of the specific tetrahedral magnetic order on the pyrochlore lattice in the presence of strong spin-orbit interactions.
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