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Register a new userStudy of the Order Parameter symmetry in overdoped Y1-xCaxBa2Cu3O7-delta by measurement of Andreev reflection, evidence for enhancement of the sub dominant order parameter amplitude in high transparency contacts
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We have measured the differential Conductance of Au/Y1-xCaxBa2Cu3O7-delta point contacts in the regime where it is dominated by Andreev reflection, which enhances its value at low bias. We find that the characteristics can not be fitted by a pure d-wave Order Parameter (OP). Using the formalism developed by Kashiwaya and Tanaka[1], the best fits are obtained by adding a sub-dominant imaginary OP, whose amplitude appears to depend on the transparency of the contact. At high transparencies it can reach up to 60% of the total amplitude of the OP. for lower transparencies it is substantially smaller. We attribute this enhancement, at high transparencies to a proximity effect between the Au tip and the superconducting electrode
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We have measured Andreev reflections between an Au tip and Y_{1-x}Ca_{x}Ba_{2}Cu_{3}O_{7 - delta} thin films in the in-plane orientation. The conductance spectra are best fitted with a pair potential having the d_{x^{2}-y^{2}+is symmetry. We find that the amplitude of the is component is enhanced as the contact transparency is increased. This is an indication for an unusual proximity effect that modifies the pair potential in the superconductor near the surface with the normal metal.
We report on the results of directional point-contact Andreev-reflection (PCAR) measurements in Ba(Fe_{1-x}Co_x)2As2 single crystals and epitaxial c-axis oriented films with x = 0.08 as well as in Ca(Fe_{1-x}Co_x)2As2 single crystals with x = 0.06. The PCAR spectra are analyzed within the two-band 3D version of the Blonder-Tinkham-Klapwijk model for Andreev reflection we recently developed, and that makes use of an analytical expression for the Fermi surface that mimics the one calculated within the density-functional theory (DFT). The spectra in Ca(Fe_{0.94}Co_{0.06})2As2 unambiguously demonstrate the presence of nodes or zeros in the small gap. In Ba(Fe_{0.92}Co_{0.08})2As2, the ab-plane spectra in single crystals can be fitted by assuming two nodeless gaps, but this model fails to fit the c-axis ones in epitaxial films. All these results are discussed in comparison with recent theoretical predictions about the occurrence of accidental 3D nodes and the presence of hot spots in the gaps of 122 compounds.
We have performed a detailed study of the conductance characteristics obtained by point contact junctions realized between a normal Pt/Ir tip and syntered RuSr2GdCu2O8 (Ru-1212) samples. Indeed, this compound is subject of great interest due to the coexistence of both magnetic order and bulk superconductivity. In our experiments, the low temperature tunneling spectra reproducibly show a zero bias conductance peak that can be well reproduced by a generalized BTK model in the case of d-wave symmetry of the superconducting order parameter.
We report a scanning tunneling spectroscopy investigation of polycrystalline SmFeAsO0.85 having a superconducting transition at 52 K. On large regions of the sample surface the tunneling spectra exhibited V-shaped gap structures with no coherence peaks, indicating degraded surface properties. In some regions, however, the coherence peaks were clearly observed, and the V-shaped gaps could be fit to the theory of tunneling into a d-wave superconductor, yielding gap values between 8 to 8.5 meV, corresponding to the ratio 2D/KTc ~ 3.55 - 3.8, which is slightly above the BCS weak-coupling prediction. In other regions the spectra exhibited zero-bias conductance peaks, consistent with a d-wave order parameter symmetry.
Using intrinsic multiple Andreev reflections effect (IMARE) spectroscopy, we studied ballistic superconductor - normal metal - superconductor (SnS) contacts in layered oxypnictide superconductors NdFeAsO$_{0.6}$H$_{0.36}$ with critical temperatures $T_c = 45-48$ K. We directly determined the magnitude of two bulk superconducting order parameters, the large gap $Delta_L approx 10.4$ meV, and a possible small gap $Delta_S approx 1.8$ meV, and their temperature dependence. Additionally, a resonant coupling with a characteristic bosonic mode was observed. The boson energy at 4.2 K, $varepsilon_0 = 10.5-11.0$ meV being less than the indirect gap ($Delta_L < varepsilon_0 < Delta_L +Delta_S$).
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