1,007 research outputs found

    Soft x-ray spectroscopy experiments on the near K-edge of B in MB2 (M=Mg, Al, Ta, and Nb)

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    Soft X-ray absorption and emission measurements are performed for the K- edge of B in MB2_2 (M=Mg, Al, Ta and Nb). Unique feature of MgB2_2 with a high density of B 2pxy(σ)p_{xy}(\sigma)-state below and above the Fermi edge, which extends to 1 eV above the edge, is confirmed. In contrast, the B 2pp density of states in AlB2_2 and TaB2_2, both of occupied and unoccupied states, decreased linearly towards the Fermi energy and showed a dip at the Fermi energy. Furthermore, there is a broadening of the peaks with pσp\sigma-character in XES and XAS of AlB2_2, which is due to the increase of three dimensionality in the pσp\sigma-band in AlB2_2. The DOS of NbB2_2 has a dip just below the Fermi energy. The present results indicate that the large DOS of B-2pσp\sigma states near the Fermi energy are crucial for the superconductivity of MgB2_2.Comment: 3 pages text and 4 pages figures. accepted for publication to Phys. Rev.

    Effects of Al doping on the structural and electronic properties of Mg(1-x)Al(x)B2

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    We have studied the structural and electronic properties of Mg(1-x)Al(x)B2 within the Virtual Crystal Approximation (VCA) by means of first-principles total-energy calculations. Results for the lattice parameters, the electronic band structure, and the Fermi surface as a function of Al doping for 0<x<0.6 are presented. The ab initio VCA calculations are in excellent agreement with the experimentally observed change in the lattice parameters of Al doped MgB2. The calculations show that the Fermi surface associated with holes a the boron planes collapses gradually with aluminum doping and vanishes for x=0.56. In addition, an abrupt topological change in the sigma-band Fermi surface was found for x=0.3. The calculated hole density correlates closely with existing experimental data for Tc(x), indicating that the observed loss of superconductivity in Mg(1-x)Al(x)B2 is a result of hole bands filling.Comment: 4 pages (revtex) and 4 figures (postscript

    Structural and Superconducting Transitions in Mg_{1-x}Al_{x}B_2

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    From systematic ab initio calculations of the alloy system Mg_{1-x}Al_{x}B_2, we find a strong tendency for the formation of a superstructure characterized by Al-rich layers. We also present a simple model, based on calculated energies and an estimate of the configurational entropy, which suggests that the alloy has two separate concentration regimes of phase separation, with critical points near x = 0.25 and x = 0.75. These results, together with calculations of electronic densities of states in several ionic arrangements, give a qualitative explanation for the observed structural instabilities, as well as the x-dependence of the superconducting T_c for x<0.6.Comment: 4 pp./4 figs.; revisions in responce to Referee comment

    Dielectric functions and collective excitations in MgB_2

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    The frequency- and momentum-dependent dielectric function ϵ(q,ω)\epsilon{(\bf q,\omega)} as well as the energy loss function Im[-ϵ1(q,ω)\epsilon^{-1}{(\bf q,\omega)}\protect{]} are calculated for intermetallic superconductor MgB2MgB_2 by using two {\it ab initio} methods: the plane-wave pseudopotential method and the tight-binding version of the LMTO method. We find two plasmon modes dispersing at energies 2\sim 2-8 eV and 18\sim 18-22 eV. The high energy plasmon results from a free electron like plasmon mode while the low energy collective excitation has its origin in a peculiar character of the band structure. Both plasmon modes demonstrate clearly anisotropic behaviour of both the peak position and the peak width. In particular, the low energy collective excitation has practically zero width in the direction perpendicular to boron layers and broadens in other directions.Comment: 3 pages with 10 postscript figures. Submitted to PRB on May 14 200

    Neuropsychological constraints to human data production on a global scale

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    Which are the factors underlying human information production on a global level? In order to gain an insight into this question we study a corpus of 252-633 Million publicly available data files on the Internet corresponding to an overall storage volume of 284-675 Terabytes. Analyzing the file size distribution for several distinct data types we find indications that the neuropsychological capacity of the human brain to process and record information may constitute the dominant limiting factor for the overall growth of globally stored information, with real-world economic constraints having only a negligible influence. This supposition draws support from the observation that the files size distributions follow a power law for data without a time component, like images, and a log-normal distribution for multimedia files, for which time is a defining qualia.Comment: to be published in: European Physical Journal

    Dicyclic Horizontal Symmetry and Supersymmetric Grand Unification

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    It is shown how to use as horizontal symmetry the dicyclic group Q6SU(2)Q_6 \subset SU(2) in a supersymmetric unification SU(5)SU(5)SU(2)SU(5)\otimes SU(5)\otimes SU(2) where one SU(5)SU(5) acts on the first and second families, in a horizontal doublet, and the other acts on the third. This can lead to acceptable quark masses and mixings, with an economic choice of matter supermultiplets, and charged lepton masses can be accommodated.Comment: 10 pages, LaTe

    Effects of C, Cu and Be substitutions in superconducting MgB2

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    Density functional calculations are used to investigate the effects of partial substitutional alloying of the B site in MgB2 with C and Be alone and combined with alloying of the Mg site with Cu. The effect of such substitutions on the electronic structure, electron phonon coupling and superconductivity are discussed. We find that Be substitution for B is unfavorable for superconductivity as it leads to a softer lattice and weaker electron-phonon couplings. Replacement of Mg by Cu leads to an increase in the stiffness and doping level at the same time, while the carrier concentration can be controlled by partial replacement of B by C. We estimate that with full replacement of Mg by Cu and fractional substitution of B by C, Tc values of 50K may be attainable.Comment: 5 pages, 4 figure

    QED Effective Action at Finite Temperature: Two-Loop Dominance

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    We calculate the two-loop effective action of QED for arbitrary constant electromagnetic fields at finite temperature T in the limit of T much smaller than the electron mass. It is shown that in this regime the two-loop contribution always exceeds the influence of the one-loop part due to the thermal excitation of the internal photon. As an application, we study light propagation and photon splitting in the presence of a magnetic background field at low temperature. We furthermore discover a thermally induced contribution to pair production in electric fields.Comment: 34 pages, 4 figures, LaTe

    Horizontal Symmetry for Quark and Squark Masses in Supersymmetric SU(5)

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    Recent interest in horizontal symmetry model building has been driven mainly by the large top mass and hence strong hierarchy in quark masses, and the possibility of appropriately constrained soft squark mass matrices, in place of an assumed universality condition, for satisfying the relevant FCNC constraints. Here we present the first successful SUSY-SU(5)SU(5) model that has such a feature. The horizontal symmetry is a gauged (Q12×U(1))H(Q_{12} \times U(1))_H ((SU(2)×U(1))H\subset (SU(2) \times U(1))_H). All nonrenormalizable terms compatible with the symmetry are allowed in the mass matrix constructions. Charged lepton masses can also be accommodated.Comment: 15 pages, latex, 1 latex figure included version to be published in Phys. Rev. Lett. ; some small changes in notations and presentation, a small paragragh and 3 references adde

    MSSM Higgs sector CP violation at photon colliders: Revisited

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    We present a comprehensive analysis on the MSSM Higgs sector CP violation at photon colliders including the chargino contributions as well as the contributions of other charged particles. The chargino loop contributions can be important for the would-be CP odd Higgs production at photon colliders. Polarization asymmetries are indispensable in determining the CP properties of neutral Higgs bosons.Comment: 24 pages, 40 figure
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