25,248 research outputs found

    Electronic structures of antiperovskite superconductors: MgXNi3_3 (X=B,C,N)

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    We have investigated electronic structures of a newly discovered antiperovskite superconductor MgCNi3_3 and related compounds MgBNi3_3 and MgNNi3_3. In MgCNi3_3, a peak of very narrow and high density of states is located just below EF\rm E_F, which corresponds to the π\pi^* antibonding state of Ni-3d and C-2p2p but with the predominant Ni-3d character. The prominent nesting feature is observed in the Γ\Gamma-centered electron Fermi surface of an octahedron-cage-like shape that originates from the 19th band. The estimated superconducting parameters based on the simple rigid-ion approximation are in reasonable agreement with experiment, suggesting that the superconductivity in MgCNi3_3 is described well by the conventional phonon mechanism.Comment: 5 pages, 5 figure

    Electronic Structures of Antiperovskite Superconductor MgCNi3_3 and Related Compounds

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    Electronic structure of a newly discovered antiperovskite superconductor MgCNi3_3 is investigated by using the LMTO band method. The main contribution to the density of states (DOS) at the Fermi energy EFE_{\rm F} comes from Ni 3dd states which are hybridized with C 2pp states. The DOS at EFE_{\rm F} is varied substantially by the hole or electron doping due to the very high and narrow DOS peak located just below EFE_{\rm F}. We have also explored electronic structures of C-site and Mg-site doped MgCNi3_3 systems, and described the superconductivity in terms of the conventional phonon mechanism.Comment: 3 pages, presented at ORBITAL2001 September 11-14, 2001 (Sendai, JAPAN

    Electronic structure of metallic antiperovskite compound GaCMn3_3

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    We have investigated electronic structures of antiperovskite GaCMn3_3 and related Mn compounds SnCMn3_3, ZnCMn3_3, and ZnNMn3_3. In the paramagnetic state of GaCMn3_3, the Fermi surface nesting feature along the ΓR\Gamma{\rm R} direction is observed, which induces the antiferromagnetic (AFM) spin ordering with the nesting vector {\bf Q} ΓR\sim \Gamma{\rm R}. Calculated susceptibilities confirm the nesting scenario for GaCMn3_3 and also explain various magnetic structures of other antiperovskite compounds. Through the band folding effect, the AFM phase of GaCMn3_3 is stabilized. Nearly equal densities of states at the Fermi level in the ferromagnetic and AFM phases of GaCMn3_3 indicate that two phases are competing in the ground state.Comment: 4 pages, 5 figure

    The Solar pp and hep Processes in Effective Field Theory

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    The strategy of modern effective field theory is exploited to pin down accurately the flux SS factors for the pppp and hephep processes in the Sun. The technique used is to combine the high accuracy established in few-nucleon systems of the "standard nuclear physics approach" (SNPA) and the systematic power counting of chiral perturbation theory (ChPT) into a consistent effective field theory framework. Using highly accurate wave functions obtained in the SNPA and working to \nlo3 in the chiral counting for the current, we make totally parameter-free and error-controlled predictions for the pppp and hephep processes in the Sun.Comment: 5 pages, aipproc macros are included. Talk given at International Nuclear Physics Conference 2001, Berkeley, California, July 30 - August 3, 200

    Superfluid-insulator transitions of two-species Bosons in an optical lattice

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    We consider a realization of the two-species bosonic Hubbard model with variable interspecies interaction and hopping strength. We analyze the superfluid-insulator (SI) transition for the relevant parameter regimes and compute the ground state phase diagram for odd filling at commensurate densities. We find that in contrast to the even commensurate filling case, the superfluid-insulator transition occurs with (a) simultaneous onset of superfluidity of both species or (b) coexistence of Mott insulating state of one species and superfluidity of the other or, in the case of unit filling, (c) complete depopulation of one species. The superfluid-insulator transition can be first order in a large region of the phase diagram. We develop a variational mean-field method which takes into account the effect of second order quantum fluctuations on the superfluid-insulator transition and corroborate the mean-field phase diagram using a quantum Monte Carlo study.Comment: 12 pages, 11 figure

    Ultra-high-Q microcavity operation in H2O and D2O

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    Optical microcavities provide a possible method for boosting the detection sensitivity of biomolecules. Silica-based microcavities are important because they are readily functionalized, which enables unlabeled detection. While silica resonators have been characterized in air, nearly all molecular detections are performed in solution. Therefore, it is important to determine their performance limits in an aqueous environment. In this letter, planar microtoroid resonators are used to measure the relationship between quality factor and toroid diameter at wavelengths ranging from visible to near-IR in both H2O and D2O, and results are then compared to predictions of a numerical model. Quality factors (Q) in excess of 10^8, a factor of 100 higher than previous measurements in an aqueous environment, are observed in both H2O and D2O

    Magnetic field switching in parallel quantum dots

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    We show that the Coulomb blockade in parallel dots pierced by magnetic flux Φ\Phi completely blocks the resonant current for any value of Φ\Phi except for integer multiples of the flux quantum Φ0\Phi_0. This non-analytic (switching) dependence of the current on Φ\Phi arises only when the dot states that carry the current are of the same energy. The time needed to reach the steady state, however, diverges when ΦnΦ0\Phi\to n\Phi_0.Comment: additional explanations added, Europhysics Letters, in pres
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