2,915 research outputs found
Specific heat of single crystal MgB_2: a two-band superconductor with two different anisotropies
Heat-capacity measurements of a 39 microgramm MgB_2 single crystal in fields
up to 14 T and below 3 K allow the determination of the low-temperature linear
term of the specific heat, its field dependence and its anisotropy. Our results
are compatible with two-band superconductivity, the band carrying the small gap
being isotropic, that carrying the large gap having an anisotropy of ~ 5. Three
different upper critical fields are thus needed to describe the superconducting
state of MgB2.Comment: 4 pages, 4 figures - V2: Bibliography updated and some typo
corrected. One reference added - V3: version accepted for publication in PRL,
changes made in the tex
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
Thermal properties of MgB2: the effect of disorder on gap amplitudes and relaxation times of p and s bands
We present thermal conductivity and specific heat measurements on MgB2 and
Mg-AlB2 samples. Thermal properties have been analysed by using a two-gap model
in order to estimate the gap amplitudes, D(0)p and D(0)s and the intra-band
scattering rates, Gss and Gpp. As a function of Al doping and disorder D(0)s
rapidly decreases, while D(0)p is rather constant. Gss and Gpp are increased by
the disorder, being Gpp more affected than Gss.Comment: 2 pages, 3 figures, presented to the conference M2S-HTSC, 25-30 May
2003, Rio de Janeir
AGAPE, an experiment to detect MACHO's in the direction of the Andromeda galaxy
The status of the Agape experiment to detect Machos in the direction of the
andromeda galaxy is presented.Comment: 4 pages, 1 figure in a separate compressed, tarred, uuencoded uufile.
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The Zero Age Main Sequence of WIMP burners
We modify a stellar structure code to estimate the effect upon the main
sequence of the accretion of weakly interacting dark matter onto stars and its
subsequent annihilation. The effect upon the stars depends upon whether the
energy generation rate from dark matter annihilation is large enough to shut
off the nuclear burning in the star. Main sequence WIMP burners look much like
protostars moving on the Hayashi track, although they are in principle
completely stable. We make some brief comments about where such stars could be
found, how they might be observed and more detailed simulations which are
currently in progress. Finally we comment on whether or not it is possible to
link the paradoxically young OB stars found at the galactic centre with WIMP
burners.Comment: 4 pages, 3 figs. Matches published versio
Thermal conductivity of MgB in the superconducting state
We present thermal conductivity measurements on very pure and dense bulk
samples, as indicated by residual resistivity values as low as 0.5 mW cm and
thermal conductivity values higher than 200 W/mK. In the normal state we found
that the Wiedemann Franz law, in its generalized form, works well suggesting
that phonons do not contribute to the heat transport. The thermal conductivity
in the superconducting state has been analysed by using a two-gap model. Thank
to the large gap anisotropy we were able to evaluate quantitatively intraband
scattering relaxation times of and bands, which depend on the
disorder in different way; namely, as the disorder increases, it reduces more
effectively the relaxation times of than of bands, as
suggested by a recent calculation [1].Comment: 12 pages, 5 figure
Vortex Imaging in the pi-Band of Magnesium Diboride
We report scanning tunneling spectroscopy imaging of the vortex lattice in
single crystalline MgB2. By tunneling parallel to the c-axis, a single
superconducting gap (Delta = 2.2 meV) associated with the pi-band is observed.
The vortices in the pi-band have a large core size compared to estimates based
on Hc2, and show an absence of localized states in the core. Furthermore,
superconductivity between the vortices is rapidly suppressed by an applied
field. These results suggest that superconductivity in the pi-band is, at least
partially, induced by the intrinsically superconducting sigma-band.Comment: 4 pages, 3 figure
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