1,222 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
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
Linear Response Calculations of Lattice Dynamics in Strongly Correlated Systems
We introduce a new linear response method to study the lattice dynamics of
materials with strong correlations. It is based on a combination of dynamical
mean field theory of strongly correlated electrons and the local density
functional theory of electronic structure of solids. We apply the method to
study the phonon dispersions of a prototype Mott insulator NiO. Our results
show overall much better agreement with experiment than the corresponding local
density predictions.Comment: 4 pages, 2 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
Phonon spectrum and soft-mode behavior of MgCNi_3
Temperature dependent inelastic neutron-scattering measurements of the
generalized phonon density-of-states for superconducting MgCNi_3, T_c=8 K, give
evidence for a soft-mode behavior of low-frequency Ni phonon modes. Results are
compared with ab initio density functional calculations which suggest an
incipient lattice instability of the stoichiometric compound with respect to Ni
vibrations orthogonal to the Ni-C bond direction.Comment: 4 pages, 5 figure
Superconductivity and hybrid soft modes in TiSe
The competition between superconductivity and other ground states of solids
is one of the challenging topics in condensed matter physics. Apart from
high-temperature superconductors [1,2] this interplay also plays a central role
in the layered transition-metal dichalcogenides, where superconductivity is
stabilized by suppressing charge-density-wave order to zero temperature by
intercalation [3] or applied pressure [4-7]. 1T-TiSe forms a prime example,
featuring superconducting domes on intercalation as well as under applied
pressure. Here, we present high energy-resolution inelastic x-ray scattering
measurements of the CDW soft phonon mode in intercalated CuTiSe and
pressurized 1T-TiSe along with detailed ab-initio calculations for the
lattice dynamical properties and phonon-mediated superconductivity. We find
that the intercalation-induced superconductivity can be explained by a solely
phonon-mediated pairing mechanism, while this is not possible for the
superconducting phase under pressure. We argue that a hybridization of phonon
and exciton modes in the pairing mechanism is necessary to explain the full
observed temperature-pressure-intercalation phase diagram. These results
indicate that 1T-TiSe under pressure is close to the elusive state of the
excitonic insulator
Electron-phonon coupling in the conventional superconductor YNiBC at high phonon energies studied by time-of-flight neutron spectroscopy
We report an inelastic neutron scattering investigation of phonons with
energies up to 159 meV in the conventional superconductor YNiBC. Using
the SWEEP mode, a newly developed time-of-flight technique involving the
continuous rotation of a single crystal specimen, allowed us to measure a four
dimensional volume in (Q,E) space and, thus, determine the dispersion surface
and linewidths of the (~ 102 meV) and (~ 159 meV) type phonon
modes for the whole Brillouin zone. Despite of having linewidths of , modes do not strongly contribute to the total electron-phonon
coupling constant . However, experimental linewidths show a remarkable
agreement with ab-initio calculations over the complete phonon energy range
demonstrating the accuracy of such calculations in a rare comparison to a
comprehensive experimental data set.Comment: accepted for publication in PR
Theory of "ferrisuperconductivity" in
We construct a two component Ginzburg-Landau theory with coherent pair motion
and incoherent quasiparticles for the phase diagram of .
The two staggered superconducting states live at the Brillouin zone center and
the zone boundary, and coexist for temperatures at concentrations
. We predict below
appearance of a charge density wave (CDW) and Be-sublattice distortion. The
distortion explains the SR relaxation anomaly, and Th-impurity mediated
scattering of ultrasound to CDW fluctuations explains the attenuation peak.Comment: 4 pages, 4 eps figures, REVTe
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