6,132 research outputs found
Phonon dispersion and lifetimes in MgB2
We measure phonon dispersion and linewidth in a single crystal of MgB_2 along
the Gamma-A, Gamma-M and A-L directions using inelastic X-Ray scattering. We
use Density Functional Theory to compute the effect of both electron-phonon
coupling and anharmonicity on the linewidth, obtaining excellent agreement with
experiment. Anomalous broadening of the E_2g phonon mode is found all along
Gamma-A. The dominant contribution to the linewidth is always the
electron-phonon coupling.Comment: 4 pages, 3 figure
Multiband effects on beta-FeSe single crystals
We present the upper critical fields Hc2(T) and Hall effect in beta-FeSe
single crystals. The Hc2(T) increases as the temperature is lowered for field
applied parallel and perpendicular to (101), the natural growth facet of the
crystal. The Hc2(T) for both field directions and the anisotropy at low
temperature increase under pressure. Hole carriers are dominant at high
magnetic fields. However, the contribution of electron-type carriers is
significant at low fields and low temperature. Our results show that multiband
effects dominate Hc2(T) and electronic transport in the normal state
Two-gap superconductivity in MgB: clean or dirty?
A large number of experimental facts and theoretical arguments favor a
two-gap model for superconductivity in MgB. However, this model predicts
strong suppression of the critical temperature by interband impurity scattering
and, presumably, a strong correlation between the critical temperature and the
residual resistivity. No such correlation has been observed. We argue that this
fact can be understood if the band disparity of the electronic structure is
taken into account, not only in the superconducting state, but also in normal
transport
A Comprehensive Survey of Brane Tilings
An infinite class of gauge theories can be engineered on
the worldvolume of D3-branes probing toric Calabi-Yau 3-folds. This kind of
setup has multiple applications, ranging from the gauge/gravity correspondence
to local model building in string phenomenology. Brane tilings fully encode the
gauge theories on the D3-branes and have substantially simplified their
connection to the probed geometries. The purpose of this paper is to push the
boundaries of computation and to produce as comprehensive a database of brane
tilings as possible. We develop efficient implementations of brane tiling tools
particularly suited for this search. We present the first complete
classification of toric Calabi-Yau 3-folds with toric diagrams up to area 8 and
the corresponding brane tilings. This classification is of interest to both
physicists and mathematicians alike.Comment: 39 pages. Link to Mathematica modules provide
The form factor of the pion in "point-form" of relativistic dynamics revisited
The electromagnetic form factor of the pion is calculated in the "point-form"
of relativistic quantum mechanics using simple, phenomenological wave
functions. It is found that the squared charge radius of the pion is predicted
one order of magnitude larger than the experimental value and the asymptotic
behavior expected from QCD cannot be reproduced. The origin of these
discrepancies is analyzed. The present results confirm previous ones obtained
from a theoretical model and call for major improvements in the implementation
of the "point-form" approach.Comment: 8 pages, 3 eps figure
Electromagnetic form factors of pion and rho in the three forms of relativistic kinematics
The electromagnetic form factors of the and the are obtained
using the three forms of relativistic kinematics, instant form, point form and
(light) front form. Simple representations of the mass operator together with
single quark currents are employed with all the forms. The Poincar\'e covariant
current operators are generated by the dynamics from single-quark currents that
are covariant under the kinematic subgroup. Front and instant forms allow to
reproduce the available data for the pion form factor. On the other hand point
form is not able to reproduce qualitatively the experimental data with
reasonable values for the wave function parameters. For the
electromagnetic form factors, instant and front forms provide a consistent
picture. The obtained results do not depend appreciably on the wave function
used.Comment: 10 pages, color figures. Revised references and discussion. Accepted
in Phys. Letts.
Extreme Electron-Phonon Coupling in Boron-based Layered Superconductors
The phonon-mode decomposition of the electron-phonon coupling in the
MgB2-like system Li_{1-x}BC is explored using first principles calculations. It
is found that the high temperature superconductivity of such systems results
from extremely strong coupling to only ~2% of the phonon modes. Novel
characteristics of E_2g branches include (1) ``mode lambda'' values of 25 and
greater compared to a mean of for other modes, (2) a precipitous
Kohn anomaly, and (3) E_2g phonon linewidths within a factor of ~2 of the
frequency itself, indicating impending breakdown of linear electron-phonon
theory. This behavior in borne out by recent inelastic x-ray scattering studies
of MgB2 by Shukla et al.Comment: 4 two-column pages, 4 figures. Equations simplified. Figure 4
changed. Comparison with new data include
Finite-Band-width Effects on the Transition Temperature and NMR Relaxation Rate of Impure Superconductors
We study the thermodynamic properties of impure superconductors by explicitly
taking into consideration the finiteness of electronic bandwidths within the
phonon-mediated Eliashberg formalism. For a finite electronic bandwidth, the
superconducting transition temperature, , is suppressed by nonmagnetic
impurity scatterings. This is a consequence of a reduction in the effective
electron-phonon coupling, . The reduced is
reflected in the observation that the coherence peak in , where
is the nuclear spin-lattice relaxation time and is the temperature,
is enhanced by impurity scatterings for a finite bandwidth. Calculations are
presented for and as bandwidths and impurity scattering rates
are varied. Implications for doped C superconductors are discussed in
connection with and measurements.Comment: 10 pages. REVTeX. 5 postscript figures. Scheduled to be published in
Physical Review B, March 1. The previous submission is revised and two
figures are adde
Coulomb suppression of NMR coherence peak in fullerene superconductors
The suppressed NMR coherence peak in the fullerene superconductors is
explained in terms of the dampings in the superconducting state induced by the
Coulomb interaction between conduction electrons. The Coulomb interaction,
modelled in terms of the onsite Hubbard repulsion, is incorporated into the
Eliashberg theory of superconductivity with its frequency dependence considered
self-consistently at all temperatures. The vertex correction is also included
via the method of Nambu. The frequency dependent Coulomb interaction induces
the substantial dampings in the superconducting state and, consequently,
suppresses the anticipated NMR coherence peak of fullerene superconductors as
found experimentally.Comment: 4 pages, Revtex, and 2 figures. Revised and final version to appear
in Phys. Rev. Lett. (1998
The possible explanation of electric-field-doped C60 phenomenology in the framework of Eliashberg theory
In a recent paper (J.H. Schon, Ch. Kloc, R.C. Haddon and B. Batlogg, Nature
408 (2000) 549) a large increase in the superconducting critical temperature
was observed in C60 doped with holes by application of a high electric field.
We demonstrate that the measured Tc versus doping curves can be explained by
solving the (four) s-wave Eliashberg equations in the case of a finite,
non-half-filled energy band. In order to reproduce the experimental data, we
assume a Coulomb pseudopotential depending on the filling in a very simple and
plausible way. Reasonable values of the physical parameters involved are
obtained. The application of the same approach to new experimental data (J.H.
Schon, Ch. Kloc and B. Batlogg, Science 293 (2001) 2432) on electric
field-doped, lattice-expanded C60 single crystals (Tc=117 K in the hole-doped
case) gives equally good results and sets a theoretical limit to the linear
increase of Tc at the increase of the lattice spacing.Comment: latex2e, 6 pages, 7 figures, 1 table, revised versio
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