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تسجيل مستخدم جديدPoint-contact study of ReFeAs(1-x)Fx (Re=La, Sm) superconducting films
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Point-contact (PC) Andreev-reflection (AR) measurements of the superconducting gap in iron-oxipnictide ReFeAsO_{1-x}F_x (Re=La, Sm) films have been carried out. The value of the gap is distributed in the range 2Delta simeq 5-10 meV (for Re=Sm) with a maximum in the distribution around 6 meV. Temperature dependence of the gap Delta(T) can be fitted well by BCS curve giving reduced gap ratio 2Delta /kT_c^*simeq 3.5 (here T_c^* is the critical temperature from the BCS fit). At the same time, an expected second larger gap feature was difficult to resolve distinctly on the AR spectra making determination reliability of the second gap detection questionable. Possible reasons for this and the origin of other features like clear-cut asymmetry in the AR spectra and current regime in PCs are discussed.
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A deep understanding of the character of superconductivity in the recently discovered Fe-based oxypnictides ReFeAsO1-xFx (Re = rare-earth) necessarily requires the determination of the number of the gaps and their symmetry in k space, which are funda
mental ingredients of any model for the pairing mechanism in these new superconductors. In the present paper, we show that point-contact Andreev-reflection experiments performed on LaFeAsO1-xFx (La-1111) polycrystals with Tc ~ 27 K and SmFeAsO0.8F0.2 (Sm-1111) ones with Tc ~ 53 K gave differential conductance curves exhibiting two peaks at low bias and two additional structures (peaks or shoulders) at higher bias, an experimental situation quite similar to that observed by the same technique in pure and doped MgB2. The single-band Blonder-Tinkham-Klapwijk model is totally unable to properly fit the conductance curves, while the two-gap one accounts remarkably well for the shape of the whole experimental dI/dV vs. V curves. These results give direct evidence of two nodeless gaps in the superconducting state of ReFeAsO1-xFx (Re = La, Sm): a small gap, Delta1, smaller than the BCS value (2Delta1/kBTc ~ 2.2 - 3.2) and a much larger gap Delta2 which gives a ratio 2Delta2/kBTc ~ 6.5 - 9. In Sm-1111 both gaps close at the same temperature, very similar to the bulk Tc, and follow a BCS-like behaviour, while in La-1111 the situation is more complex, the temperature dependence of the gaps showing remarkable deviations from the BCS behaviour at T close to Tc. The normal-state conductance reproducibly shows an unusual, but different, shape in La-1111 and Sm-1111 with a depression or a hump at zero bias, respectively. These structures survive up to T* ~ 140 K, close to the temperatures at which structural and magnetic transitions occur in the parent, undoped compound.
We report the electrical, magneto transport and specific heat of the layered polycrystalline RECoPO (RE = La, Nd and Sm) samples. These compounds are iso-structural to recently discovered superconductor LaFeAs(O/F). Bulk polycrystalline samples are s
ynthesized by solid state reaction route in an evacuated sealed quartz tube. All these compounds are crystallized in a tetragonal structure with space group P4/nmm. The Cobalt in these compounds is in itinerant state with its paramagnetic moment above 1.4muB and the same orders ferromagnetically (FM) with saturation moment of around 0.20muB below say 80K. Though, LaCoPO shows single paramagnetic (PM) to ferromagnetic (FM) transition near 35K, the NdCoPO and SmCoPO exhibit successive PM-FM-AFM transitions. Both FM and AFM transition temperatures vary with applied field. Although the itinerant ferromagnetism occurs with small saturation moment, typical anti-ferromagnetic (AFM) transitions (TN1, TN2) are observed at 69K and 14K for Nd and 57K and 45K for Sm. This FM-AFM transition of Co spins in NdCoPO and SmCoPO is both field and temperature dependent. The Magneto-transport of NdCoPO and SmCoPO distinctly follows their successive PM-FM-AFM transitions. It is clear that Sm/Nd (4f) interacts with the Co (3d) in first time synthesized Sm/NdCoPO.
We present point-contact (PC) Andreev-reflection measurements of a superconducting epitaxial c-axis oriented nickel borocarbide film LuNi2B2C (Tc=15.9 K). The averaged value of the superconducting gap is found to be 2.6 +/-0.2 meV in the one-gap appr
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oexistence 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.
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