305 research outputs found
Intrinsic surface depression of the order parameter under mixed (s+id)-wave pair symmetry and its effect on the critical current of high-Tc SIS Josephson junctions
An intrinsic gap depression at the Superconductor-Insulator interface due to
the very short value of the coherence length in High-Tc Superconductors [HTSs]
is considered, in the framework of a mixed (s+id)-wave pair symmetry for the
order parameter ranging from pure s to pure d-wave. This gap depression acts as
the main physical agent causing the relevant reduction of IcRn(T) values with
respect to BCS expectations in HTS SIS Josephson junctions. Good agreement with
various experimental data is obtained with both pure s-wave and pure d-wave
symmetries of the order parameter, but with amounts of gap depression depending
on the pair symmetry adopted. Regardless of the pair symmetry considered, these
results prove the importance of the surface order-parameter depression in the
correct interpretation of the Ic(T)Rn(T) data in HTS SIS junctions. In a case
of planar YBCO-based junction the use of the de Gennes condition allowed us to
tentatively obtain an upper limit for the amount of d-wave present in the order
parameter of YBCO.Comment: 11 pages REVTeX file, 6 PostScript figures, to be published in J.
Superconductivit
Possible d+id scenario in La_{2-x}Sr_{x}CuO_4 by point-contact measurements
We analyze the results of point-contact measurements in La_{2-x}Sr_{x}CuO_{4}
(LSCO) previously reported as a clear evidence of the separation between gap
and pseudogap in this copper oxide. Here we show that, in addition to this, the
conductance curves of our point-contact junctions -- showing clear Andreev
reflection features -- can be interpreted as supporting a nodeless
d_{x^2-y^2}+id_{xy}-wave symmetry of the gap in LSCO. The results of our
analysis, in particular the doping dependence of the subdominant d_{xy} gap
component, are discussed and compared to the predictions of different
theoretical models.Comment: 6 pages, 4 eps figures, presented at SATT11 Conference (Vietri sul
Mare, March 2002). To appear in Int. J. Mod. Phy
Independent determination of the two gaps by directional point-contact spectroscopy in MgB_2 single crystals
Directional point-contact spectroscopy measurements were performed for the
first time in state-of-the-art MgB_2 single crystals. The selective suppression
of the superconductivity in the "pi" band by means of a suitable magnetic field
allowed separating the partial contribution of each band to the total
point-contact conductance. By fitting the partial conductance curves
sigma_sigma(V) and sigma_pi(V), we got an independent determination of the two
gaps, Delta_sigma and Delta_pi, with a strong reduction of the experimental
uncertainty. Their temperature dependence was found to agree well with the
predictions of the two-band models for MgB_2.Comment: 6 pages, 4 eps figures. References added, abstract rewritten, text
slightly changed. Proceedings of the BOROMAG Conference, June 17-19, Genoa,
Ital
Evidence for two-gap nodeless superconductivity in SmFeAsOF from point-contact Andreev-reflection spectroscopy
Point-contact Andreev-reflection measurements were performed in
SmFeAsO_{0.8}F_{0.2} polycrystals with T_c \simeq 53 K. The experimental
conductance curves reproducibly exhibit peaks around \pm 6 mV and shoulders at
V \sim 16-20 mV, indicating the presence of two nodeless superconducting gaps.
While the single-band Blonder-Tinkham-Klapwijk fit can only reproduce a small
central portion of the conductance curve, the two-gap one accounts remarkably
well for the shape of the whole experimental dI/dV. The fits of the normalized
curves give Delta_1(0) = 6.15 \pm 0.45 meV and Delta_2(0) = 18 \pm 3 meV. Both
gaps close at the same temperature and follow a BCS-like behavior.Comment: 4 pages, 4 eps color figure
Evidence for pseudogap and phase-coherence gap separation by Andreev reflection experiments in Au/La_{2-x}Sr_{x}CuO_4 point-contact junctions
We present new Au/La_{2-x}Sr_{x}CuO_{4} (LSCO) point-contact conductance
measures as a function of voltage and temperature in samples with 0.08 <= x <=
0.2. Andreev reflection features disappear at about the bulk Tc, giving no
evidence of gap for T > Tc. The fit of the normalized conductance at any T < Tc
supports a (s + d)-wave symmetry of the gap, whose dominant low-T s component
follows the Tc(x) curve in contrast with recent angle-resolved photoemission
spectroscopy and quasiparticle tunneling data. These results prove the
separation between pseudogap and phase-coherence superconducting gap in LSCO at
x <= 0.2.Comment: 4 pages, 4 eps figures, 1 table (RevTeX). Labels added to Fig. 1;
Fig. 3 resized; references added; short discussion about ballistic contact
regime adde
Direct evidence for two-band superconductivity in MgB_2 single crystals from directional point-contact spectroscopy in magnetic fields
We present the results of the first directional point-contact spectroscopy
experiments in high-quality MgB_2 single crystals. Due to the directionality of
the current injection into the samples, the application of a magnetic field
allowed us to separate the contributions of the sigma and pi bands to the total
conductance of our point contacts. By using this technique, we were able to
obtain the temperature dependency of each gap independent of the other. The
consequent, strong reduction of the error on the value of the gap amplitude as
function of temperature allows a stricter test of the predictions of the
two-band model for MgB_2.Comment: 4 pages, 5 eps figures. References added, Fig.1 changed, some text
adde
Proximity Eliashberg theory of electrostatic field-effect-doping in superconducting films
We calculate the effect of a static electric field on the critical
temperature of a s-wave one band superconductor in the framework of proximity
effect Eliashberg theory. In the weak electrostatic field limit the theory has
no free parameters while, in general, the only free parameter is the thickness
of the surface layer where the electric field acts. We conclude that the best
situation for increasing the critical temperature is to have a very thin film
of a superconducting material with a strong increase of electron-phonon (boson)
constant upon charging.Comment: 9 pages, 5 figure
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