6,305 research outputs found

    Near degeneracy and pseudo Jahn-Teller effects in mixed-valence ladders: the phase transition in NaV2_2O5_5

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    We analyze the electronic structure of a mixed-valence ladder system. We find that structural anisotropy and complex electron correlations lead to on-rung charge localization and insulating character. Charge fluctuations within the rung of the ladder interact strongly to the lattice degrees of freedom, which gives rise to large pseudo Jahn--Teller effects. The phase transition in NaV2_2O5_5 should be driven by this kind of mechanism.Comment: 5 pages, 2 figures, 1 table, submitted to PR

    The Hessian biased singular value decomposition method for optimization and analysis of force fields

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    We present methodology (HBFF/SVD) for optimizing the form and parameters of force fields (FF) for molecular dynamics simulations through utilizing information about properties such as the geometry, Hessian, polarizability, stress (crystals), and elastic constants (crystals). This method is based on singular value decomposition (SVD) of the Jacobian describing the partial derivatives in various properties with respect to FF parameters. HBFF/SVD is effective for optimizing the parameters for accurate FFs of organic, inorganic, and transition metal compounds. In addition it provides information on the validity of the functional form of the FF for describing the properties of interest. This method is illustrated by application to organic molecules (CH2O, C2H4, C4H6, C6H8, C6H6, and naphthalene) and inorganic molecules (Cl2CrO2 and Cl2MoO2)

    The Emergence of El-Ni\~{n}o as an Autonomous Component in the Climate Network

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    We construct and analyze a climate network which represents the interdependent structure of the climate in different geographical zones and find that the network responds in a unique way to El-Ni\~{n}o events. Analyzing the dynamics of the climate network shows that when El-Ni\~{n}o events begin, the El-Ni\~{n}o basin partially loses its influence on its surroundings. After typically three months, this influence is restored while the basin loses almost all dependence on its surroundings and becomes \textit{autonomous}. The formation of an autonomous basin is the missing link to understand the seemingly contradicting phenomena of the afore--noticed weakening of the interdependencies in the climate network during El-Ni\~{n}o and the known impact of the anomalies inside the El-Ni\~{n}o basin on the global climate system.Comment: 5 pages,10 figure

    Diurnal experiment data report, 19-20 March 1974

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    Temperature and wind data are presented from 70 small meteorological sounding rockets launched from eight selected launch sites in the Western Hemisphere. Table 1 gives a complete listing of the launch sites involved and the altitude of temperature and wind observations successfully completed

    Half-Metallic Ferromagnetism and the spin polarization in CrO2_2

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    We present electronic structure calculations in combination with local and non-local many-body correlation effects for the half-metallic ferromagnet CrO2_2. Finite-temperature Dynamical Mean Field Theory results show the existence of non-quasiparticle states, which were recently observed as almost currentless minority spin states near the Fermi energy in resonant scattering experients. At zero temperatures, Variational Cluster Approach calculations support the half-metallic nature of CrO2_2 as seen in superconducting point contact spectroscopy. The combination of these two techniques allowed us to qualitatively describe the spin-polarization in CrO2_2.Comment: 5 pages, 3 figure

    Electron correlations and bond-length fluctuations in copper oxides: from Zhang--Rice singlets to correlation bags

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    We perform first principles, multiconfiguration calculations on clusters including several CuO6_6 octahedra and study the ground-state electron distribution and electron--lattice couplings when holes are added to the undoped d9p6d^9 p^6 configuration. We find that the so-called Zhang--Rice state on a single CuO4_4 plaquette is nearly degenerate with a state whose leading configuration is of the form Cu d9d^9-- O p5p^5-- Cu d9d^9. A strong coupling between the electronic and nuclear motion gives rise to large inter-site charge transfer effects for half-breathing displacements of the oxygen ions. Under the assumption of charge segregation into alternating hole-free and hole-rich stripes of Goodenough \cite{jbg_02,jbg_03}, our results seem to support the vibronic mechanism and the traveling charge-density wave model from Refs.\cite{jbg_02,jbg_03} for the superconductivity in copper oxides.Comment: submitted to Phys. Rev.

    Roles of Bond Alternation in Magnetic Phase Diagram of RMnO3

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    In order to investigate nature of the antiferromagnetic structures in perovskite RMnO3, we study a Heisenberg J1-J2 model with bond alternation using analytical and numerical approaches. The magnetic phase diagram which includes incommensurate spiral states and commensurate collinear states is reproduced. We discuss that the magnetic structure with up-up-down-down spin configuration (E-type structure) and the ferroelectricity emerge cooperatively to stabilize this phase. Magnetoelastic couplings are crucial to understand the magnetic and electric phase diagram of RMnO3.Comment: 5 pages, 6 figure

    Quantum computation with devices whose contents are never read

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    In classical computation, a "write-only memory" (WOM) is little more than an oxymoron, and the addition of WOM to a (deterministic or probabilistic) classical computer brings no advantage. We prove that quantum computers that are augmented with WOM can solve problems that neither a classical computer with WOM nor a quantum computer without WOM can solve, when all other resource bounds are equal. We focus on realtime quantum finite automata, and examine the increase in their power effected by the addition of WOMs with different access modes and capacities. Some problems that are unsolvable by two-way probabilistic Turing machines using sublogarithmic amounts of read/write memory are shown to be solvable by these enhanced automata.Comment: 32 pages, a preliminary version of this work was presented in the 9th International Conference on Unconventional Computation (UC2010

    Frustrated quantum-spin system on a triangle coupled with ege_g lattice vibrations - Correspondence to Longuet-Higgins et al.'s Jahn-Teller model -

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    We investigate the quantum three spin model (S1,S2,S3)({\bf S_1},{\bf S_2},{\bf S_3}) of spin=1/2=1/2 on a triangle, in which spins are coupled with lattice-vibrational modes through the exchange interaction depending on distances between spin sites. The present model corresponds to the dynamic Jahn-Teller system EgegE_g\otimes e_g proposed by Longuet-Higgins {\it et al.}, Proc.R.Soc.A.{\bf 244},1(1958). This correspondence is revealed by using the transformation to Nakamura-Bishop's bases proposed in Phys.Rev.Lett.{\bf 54},861(1985). Furthermore, we elucidate the relationship between the behavior of a chiral order parameter χ^=S1(S2×S3){\hat \chi}={\bf S_1\cdot(S_2\times S_3)} and that of the electronic orbital angular momentum ^z{\hat \ell_z} in EgegE_g\otimes e_g vibronic model: The regular oscillatory behavior of the expectation value forvibronicstructureswithincreasingenergycanalsobefoundinthatof for vibronic structures with increasing energy can also be found in that of . The increase of the additional anharmonicity(chaoticity) is found to yield a rapidly decaying irregular oscillation of
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