2,014 research outputs found
Phonon dispersion and electron-phonon coupling in MgB_2 and AlB_2
We present a first principles investigation of the lattice dynamics and
electron-phonon coupling of the superconductor MgB_2 and the isostructural
AlB_2 within the framework of density functional perturbation theory using a
mixed-basis pseudopotential method. Complete phonon dispersion curves and
Eliashberg functions \alpha^2F are calculated for both systems. We also report
on Raman measurements, which support the theoretical findings. The calculated
generalized density-of-states for MgB_2 is in excellent agreement with recent
neutron-scattering experiments. The main differences in the calculated phonon
spectra and \alpha^2F are related to high frequency in-plane boron vibrations.
As compared to AlB_2, they are strongly softened in MgB_2 and exhibit an
exceptionally strong coupling to electronic states at the Fermi energy. The
total coupling constants are \lambda_{MgB_2}=0.73 and \lambda_{AlB_2}=0.43.
Implications for the superconducting transition temperature are briefly
discussed.Comment: 10 pages, 4 figures, to appear in Phys. Rev. Let
Lattice dynamics and electron-phonon interaction in (3,3) carbon nanotubes
We present a detailed study of the lattice dynamics and electron-phonon
coupling for a (3,3) carbon nanotube which belongs to the class of small
diameter based nanotubes which have recently been claimed to be
superconducting. We treat the electronic and phononic degrees of freedom
completely by modern ab-initio methods without involving approximations beyond
the local density approximation. Using density functional perturbation theory
we find a mean-field Peierls transition temperature of approx 40K which is an
order of magnitude larger than the calculated superconducting transition
temperature. Thus in (3,3) tubes the Peierls transition might compete with
superconductivity. The Peierls instability is related to the special 2k_F
nesting feature of the Fermi surface. Due to the special topology of the (n,n)
tubes also a q=0 coupling between the two bands crossing the Fermi energy at
k_F is possible which leads to a phonon softening at the Gamma point.Comment: 4 pages, 3 figures; to be published in Phys. Rev. Let
Lattice dynamics and electron-phonon coupling in transition metal diborides
The phonon density-of-states of transition metal diborides TMB2 with TM = Ti,
V, Ta, Nb and Y has been measured using the technique of inelastic neutron
scattering. The experimental data are compared with ab initio density
functional calculations whereby an excellent agreement is registered. The
calculations thus can be used to obtain electron-phonon spectral functions
within the isotropic limit. A comparison to similar data for MgB2 and AlB2
which were subject of prior publications as well as parameters important for
the superconducting properties are part of the discussion.Comment: 4 pages, 3 figure
Staggered Pairing Phenomenology for UPd_2Al_3 and UNi_2Al_3
We apply the staggered-pairing Ginzburg-Landau phenomenology to describe
superconductivity in UPd_2Al_3 and UNi_2Al_3. The phenomenology was applied
successfully to UPt_3 so it explains why these materials have qualitatively
different superconducting phase diagrams although they have the same
point-group symmetry. UPd_2Al_3 and UNi_2Al_3 have a two-component
superconducting order parameter transforming as an H-point irreducible
representation of the space group. Staggered superconductivity can induce
charge-density waves characterized by new Bragg peaks suggesting experimental
tests of the phenomenology.Comment: 4 pages, REVTeX, 2 Postscript figure
Witnessing effective entanglement in a continuous variable prepare&measure setup and application to a QKD scheme using postselection
We report an experimental demonstration of effective entanglement in a
prepare&measure type of quantum key distribution protocol. Coherent
polarization states and heterodyne measurement to characterize the transmitted
quantum states are used, thus enabling us to reconstruct directly their
Q-function. By evaluating the excess noise of the states, we experimentally
demonstrate that they fulfill a non-separability criterion previously presented
by Rigas et al. [J. Rigas, O. G\"uhne, N. L\"utkenhaus, Phys. Rev. A 73, 012341
(2006)]. For a restricted eavesdropping scenario we predict key rates using
postselection of the heterodyne measurement results.Comment: 12 pages, 12 figures, 2 table
Properties of the phonon-induced pairing interaction in YBaCuO within the local density approximation
The properties of the phonon-induced interaction between electrons are
studied using the local density approximation (LDA). Restricting the electron
momenta to the Fermi surface we find generally that this interaction has a
pronounced peak for large momentum transfers and that the interband
contributions between bonding and antibonding band are of the same magnitude as
the intraband ones. Results are given for various symmetry averages of this
interaction over the Fermi surface. In particular, we find that the
dimensionless coupling constant in the d-wave channel , relevant for
superconductivity, is only 0.022, i.e., even about ten times smaller than the
small value of the s-wave channel. Similarly, the LDA contribution to the
resistivity is about a factor 10 times smaller than the observed resistivity
suggesting that phonons are not the important low-energy excitations in
high-T oxides.Comment: 6 pages, 7 figure
Direct user guidance in e-dictionaries for text production and text reception — The verbal relative in Sepedi as a case study
This article introduces a prototype of a writing (and learning) assistant for verbal relative clauses of the African language Sepedi, accessible from within a dictionary or from a word processor. It is an example of how a user support tool for complicated grammatical structures in a scarcely resourced language can be compiled. We describe a dynamic light-weight tool aimed at combining user-knowledge with text production support, i.e., user-involved interactive text pro-duction of the complicated verbal relative in Sepedi. In this article, the focus is on access in a dic-tionary use situation. Although the tool is intended as a writing assistant to support users in text production; it also satisfies text reception and cognitive needs, but its focus is on solving text pro-duction issues related with the interaction between lexical items and complex grammatical structures in the African (Bantu) languages and for learning by users and/or training users in this interaction.Keywords:Writing Tools, User-Guidance, User Support, Text Production, E-Dictionaries, African Languages, Sepedi, Complex Grammatical Struc Tures, Relative Constructio
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