46 research outputs found
Superconducting gap structure and pinning in disordered MgB2 films
We have performed a comparative study of two thin films of magnesium diboride
(MgB2) grown by different techniques. The critical current density at different
temperatures and magnetic fields was evaluated from magnetisation curves, the
structure of superconducting order parameter was obtained from point-contact
spectroscopy, and the scattering rates were evaluated by fitting the
temperature dependent normal-state resistivity to the two-band model. The films
have similar critical temperatures close to 39 K, but the upper critical fields
were different by a factor of 2 (5.2T and 2.5 T at 20 K). We have found that
the film with higher Hc2 also had stronger scattering in the sigma band and
smaller value of the superconducting gap in this band. As the scattering in
sigma band is primarily due to the defects in boron plane, our results are
consistent with the assumption that disordering the boron planes leads to
enhanced Hc2 and better pinning properties in magnetic field.Comment: Paper presented at EUCAS'0
Electron diffusivities in MgB2 from point contact spectroscopy
We demonstrate that the variation of the Andreev reflection with applied
magnetic field provides a direct means of comparing the properties of MgB2 with
the theory for a dirty two-band superconductor, and we find good agreement
between the two. The ratio of electron diffusivities in the s and p bands can
be inferred from this experiment. We find that the field dependence of the
density of states at the Fermi level in the p band is independent of the field
direction, and that the anisotropic upper critical field is determined by the
anisotropic diffusivity in the s band
The effect of magnetic field on the two superconducting gaps in MgB2
Double-gap superconductivity in an epitaxial MgB2 film has been studied by
means of point-contact spectroscopy in magnetic field up to 8 Tesla. The
relatively fast disappearance of the feature associated with the pi-band gap at
a field around 1 T is caused by the line broadening due to strong pair breaking
rather than to a collapse of the double-gap state. This pair breaking was found
to increase linearly with field. Field dependences of the order parameters
Delta_pi and Delta_sigma in the pi and sigma bands were measured in field
applied parallel and perpendicular to the film, at T = 4.2 K. In perpendicular
field, both order parameters survive to a common Hc2, which is about 6.5 T for
this direction. In parallel field, the decrease of Delta_sigma is much more
gradual, consistent with the Hc2 being about 4 times greater in this
ori-entation. The difference in Delta_pi measured in the two field orientations
is however smaller than the difference of Delta_sigma. We compare these results
with the data on tunnelling spectroscopy and specific heat measurements of MgB2
single crystals and find consistency between the different experimental
approaches.Comment: 4 pages, 5 figures Revised: discussion of the distribution of gap
value
Peak effect in single crystal MgB superconductor for -axis
We have studied the phase diagram of MgB superconductor using a single
crystal for -axis. For the first time we report the
existence of peak effect in the screening current in MgB single crystal
for -axis. In the magnetic field regime kOe the
local fundamental diamagnetic moment displays a very narrow diamagnetic step,
with a temperature width of the same size as the zero dc-magnetic field
transition. For higher field this step is transformed to a peak which is
related with the peak effect in the screening current. Finally, for kOe
the diamagnetic step is transformed to a gradual transition. Our findings for
the vortex matter phase diagram for the MgB are closely related with
theoretical predictions concerning the vortex matter phase diagram of a type II
superconductor in the presence of weak point disorder.Comment: Has been submitted for publication in PRL since 21 February 200
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
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
Magnetoresistivity and Complete in
Detailed magneto-transport data on dense wires of are reported for
applied magnetic fields up to 18 T. The temperature and field dependencies of
the electrical resistivity are consistent with behaving like a simple
metal and following a generalized form of Kohler's rule. In addition, given the
generally high values and narrow resistive transition widths associated
with synthesized in this manner, combined with applied magnetic fields
of up to 18 T, an accurate and complete curve could be determined.
This curve agrees well with curves determined from lower field measurements on
sintered pellets and wires of . is linear in over a wide
range of temperature (7 K 32 K) and has an upward curvature for
close to . These features are similar to other high , clean limit,
boron-bearing intermetallics: and .Comment: minor changes in styl
Effects of C, Cu and Be substitutions in superconducting MgB2
Density functional calculations are used to investigate the effects of
partial substitutional alloying of the B site in MgB2 with C and Be alone and
combined with alloying of the Mg site with Cu. The effect of such substitutions
on the electronic structure, electron phonon coupling and superconductivity are
discussed. We find that Be substitution for B is unfavorable for
superconductivity as it leads to a softer lattice and weaker electron-phonon
couplings. Replacement of Mg by Cu leads to an increase in the stiffness and
doping level at the same time, while the carrier concentration can be
controlled by partial replacement of B by C. We estimate that with full
replacement of Mg by Cu and fractional substitution of B by C, Tc values of 50K
may be attainable.Comment: 5 pages, 4 figure
Point-Contact Spectroscopy in MgB_2: from Fundamental Physics to Thin-Film Characterization
In this paper we highlight the advantages of using point-contact spectroscopy
(PCS) in multigap superconductors like MgB_2, both as a fundamental research
tool and as a non-destructive diagnostic technique for the optimization of
thin-film characteristics. We first present some results of crucial fundamental
interest obtained by directional PCS in MgB_2 single crystals, for example the
temperature dependence of the gaps and of the critical fields and the effect of
a magnetic field on the gap amplitudes. Then, we show how PCS can provide
useful information about the surface properties of MgB_2 thin films (e.g. Tc,
gap amplitude(s), clean or dirty-limit conditions) in view of their
optimization for the fabrication of tunnel and Josephson junctions for
applications in superconducting electronics.Comment: 11 pages, 7 figures; Proceedings of 6th EUCAS Conference (14-18
September 2003, Sorrento - Italy
Electron and hole transmission through superconductor - normal metal interfaces
We have investigated the transmission of electrons and holes through
interfaces between superconducting aluminum (Tc = 1.2 K) and various normal
non-magnetic metals (copper, gold, palladium, platinum, and silver) using
Andreev-reflection spectroscopy at T = 0.1 K. We analyzed the point contacts
with the modified BTK theory that includes Dynes' lifetime as a fitting
parameter G in addition to superconducting energy gap 2D and normal reflection
described by Z. For contact areas from 1 nm^2 to 10000 nm^2 the BTK Z parameter
was 0.5, corresponding to transmission coefficients of about 80 %, independent
of the normal metal. The very small variation of Z indicates that the
interfaces have a negligible dielectric tunneling barrier. Fermi surface
mismatch does not account for the observed transmission coefficient.Comment: 9 pages, 4 figures, submitted to Proceedings of the 19th
International Conference on Magnetism ICM2012 (Busan 2012