105 research outputs found
Field emission properties of as-grown multiwalled carbon nanotube films
Multiwalled carbon nanotubes have been produced by ethylene catalytic
chemical vapor deposition and used to fabricate thick and dense freestanding
films ("buckypapers") by membrane filtering. Field emission properties of
buckypapers have been locally studied by means of high vacuum atomic force
microscopy with a standard metallic cantilever used as anode to collect
electrons emitted from the sample. Buckypapers showed an interesting linear
dependence in the Fowler-Nordheim plots demonstrating their suitability as
emitters. By precisely tuning the tip-sample distance in the submicron region
we found out that the field enhancement factor is not affected by distance
variations up to 2um. Finally, the study of current stability showed that the
field emission current with intensity of about 3,3*10-5A remains remarkably
stable (within 5% fluctuations) for several hours.Comment: 18 pages, 5 figure
Effect of density of state on isotope effect exponent of two-band superconductors
The exact formula of Tc's equation and the isotope effect exponent of
two-band s-wave superconductors in weak-coupling limit are derived by
considering the influence of two kinds of density of state : constant and van
Hove singularity. The pairing interaction in each band consisted of 2 parts :
the electron-phonon interaction and non-electron-phonon interaction are
included in our model. We find that the interband interaction of
electron-phonon show more effect on isotope exponent than the intraband
interaction and the isotope effect exponent with constant density of state can
fit to an experimental data,MgB2, and high-Tc superconductors, better than van
Hove singularity density of state.Comment: 11 pages. accepted in Physica
Specific heat of MgB in a one- and a two-band model from first-principles calculations
The heat capacity anomaly at the transition to superconductivity of the
layered superconductor MgB is compared to first-principles calculations
with the Coulomb repulsion, , as the only parameter which is fixed to
give the measured . We solve the Eliashberg equations for both an
isotropic one-band and a two-band model with different superconducting gaps on
the and Fermi surfaces. The agreement with experiments is
considerably better for the two-band model than for the one-band model.Comment: final published versio
Theory for phonon-induced superconductivity in MgB
We analyze superonductivity in MgB observed below K resulting
from electron-phonon coupling involving a mode at meV and
most importantly the in-plane B-B vibration at
meV. The quasiparticles originating from - and -states couple
strongly to the low-frequency mode and the -vibrations respectively.
Using two-band Eliashberg theory, and , we calculate the gap functions (,
).
Our results provide an explanation of recent tunneling experiments.
We get .Comment: revised version, accepted for publication in PR
Retention of Two-Band Superconductivity in Highly Carbon-Doped MgB2
Tunneling data on MgB_{1.8}C_{0.2} show a reduction in the energy gap of the
pi-bands by a factor of two from undoped MgB2 that is consistent with the Tc
reduction, but inconsistent with the expectations of the dirty limit.
Dirty-limit theory for undoped MgB2 predicts a single gap about three times
larger than measured and a reduced Tc comparable to that measured. Our
heavily-doped samples exhibit a uniform dispersion of C suggestive of
significantly enhanced scattering, and we conclude that the retention of
two-band superconductivity in these samples is caused by a selective
suppression of interband scattering.Comment: 4 pages, 4 figures; added one figure, added one reference, minor
changes to the text, manuscript accepted for publication as a Phys. Rev. B
Rapid Communicatio
Mixed state properties of superconducting MgB2 single crystals
We report on measurements of the magnetic moment in superconducting MgB2
single crystals. We find \mu_0H_{c2}^c(0) = 3.2 T, \mu_0H_{c2}^{ab}(0) = 14.5
T, \gamma = 4.6, \mu_0H_c(0) = 0.28 T, and \kappa(T_c) = 4.7. The standard
Ginzburg-Landau and London model relations lead to a consistent data set and
indicate that MgB2 is a clean limit superconductor of intermediate coupling
strength with very pronounced anisotropy effects
Synthesis and Properties of c-axis Oriented Epitaxial MgB2 Thin Films
We report the growth and properties of epitaxial MgB2 thin films on (0001)
Al2O3 substrates. The MgB2 thin films were prepared by depositing boron films
via RF magnetron sputtering, followed by a post-deposition anneal at 850C in
magnesium vapor. X-ray diffraction and cross-sectional TEM reveal that the
epitaxial MgB2 films are oriented with their c-axis normal to the (0001) Al2O3
substrate and a 30 degree rotation in the ab-plane with respect to the
substrate. The critical temperature was found to be 35 K and the anisotropy
ratio, Hc2(parallel to the film) / Hc2(pendicular to the film), about 3 at 25K.
The critical current densities at 4.2 K and 20 K (at 1 T perpendicular magnetic
field) are 5x10E6 A/cm2 and 1x10E6 A/cm2, respectively. The controlled growth
of epitaxial MgB2 thin films opens a new avenue in both understanding
superconductivity in MgB2 and technological applications.Comment: 10 pages, 6 figure
Multiband model for tunneling in MgB2 junctions
A theoretical model for quasiparticle and Josephson tunneling in multiband
superconductors is developed and applied to MgB2-based junctions. The gap
functions in different bands in MgB2 are obtained from an extended Eliashberg
formalism, using the results of band structure calculations. The temperature
and angle dependencies of MgB2 tunneling spectra and the Josephson critical
current are calculated. The conditions for observing one or two gaps are given.
We argue that the model may help to settle the current debate concerning
two-band superconductivity in MgB2.Comment: minor corrections, published in Phys. Rev. B 65, 180517(R) (2002
Evidence for High-frequency Phonon Mediated S-wave Superconductivity : 11B-NMR Study of Al-doped MgB2
We report B-NMR study on Al-doped MgB that addresses a possible
mechanism for a high superconducting (SC) transition temperature () of
K in recently discovered MgB. The result of nuclear spin lattice
relaxation rate in the SC state revealed that the size in the SC gap is
not changed by substituting Al for Mg. The reduction on by Al-doping is
shown to be due to the decrease of . According to the McMillan
equation, the experimental relation between and the relative change in
allowed us to estimate a characteristic phonon frequency K and an electron-phonon coupling constant . These
results suggest that the high- superconductivity in MgB is mediated by
the strong electron-phonon coupling with high-frequency phonons.Comment: 6pages, 3figure
Multiband model for penetration depth in MgB2
The results of first principles calculations of the electronic structure and
the electron-phonon interaction in MgB2 are used to study theoretically the
temperature dependence and anisotropy of the magnetic field penetration depth.
The effects of impurity scattering are essential for a proper description of
the experimental results. We compare our results with experimental data and we
argue that the two-band model describes the data rather well.Comment: submitted to Phys. Rev.
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