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We report a new methodology for synthesizing oxypnictide superconductors with the commonly available potassium fluoride (KF) as a source of fluorine instead of expensive LaF3. This results in simultaneous doping of potassium at lanthanum sites and leads to a three-fold increase in the upper critical field. We report the Tc (onset) of 28.50 K by F-doping and highest upper critical field (\sim122 T) at ambient pressure in the family of La–based oxypnictides. To study the contribution from inter and intra-granular current density, we compare remanent magnetization measurements on La1.03{\O}0.9{\F}0.2{\FeAs} and La0.8{\K}0.2{\O}0.8{\F}0.2{\FeAs} superconductors that show only one major peak indicating substantial electromagnetic granularity. The Seebeck effect (S) with respect to temperature has negative sign and exhibits unconventional temperature dependence. Analysis of resistivity transition broadening under magnetic field reveals the signatures of Arrhenius behavior and the calculated activation energy follows power-law dependence with respect to magnetic field. [ABSTRACT FROM AUTHOR]

Structural and superconducting properties of yttrium substituted Ce1-xYx(O/F)FeAs superconductors have been investigated for the first time. All the compounds crystallize in the tetragonal ZrCuSiAs structure type. There is a decrease in both the a and c lattice parameters on increasing yttrium substitution (with fixed F content) along with a substantial enhancement of the superconducting transition temperature (Tc) and upper critical field (Hc2) indicating the influence of chemical pressure. Interestingly the maximum Tc (~48 K) was observed for an intermediate composition (Ce0.5Y0.5O0.9F0.1FeAs) which is higher than either of the parent Y or Ce-compounds, (YO0.9F0.1FeAs (~10 K) and Ce(O/F)FeAs (~42 K)). The transition temperature was also found to be nearly independent of the electron -doping introduced by fluoride substitution (0.1 to 0.2 moles per formula unit) indicating the significance of the charge reservoir layer (Ce-O). The yttrium substituted (fluoride free) compositions of the type, Ce1-xYxOFeAs were found to be semimetallic like the parent compound CeOFeAs with the shift in the anomaly temperature towards low temperature on substitution of yttrium ions. Hall coefficient and thermopower measurements show an increase in charge carriers (electrons) through Y-doping in fluorine doped CeOFeAs. [ABSTRACT FROM AUTHOR]

The effect of ytterbium substitution at the lanthanum site on the superconducting properties of La1-xYbxO0·8F0·2FeAs (x = 0·10, 0·20 and 0·30) oxypnictides has been investigated. Powder X-ray diffraction studies show the presence of Yb2O3 and LaOF as secondary phases. The superconducting transition temperature (Tc) of 31·3 (± 0·05) K has been observed in x = 0·1 composition which is the maximum Tc so far in the La(O/F)FeAs superconductor family at ambient pressure. Further increase in x leads to suppression and broadening of superconducting transition. The resistive transition curves under different magnetic fields were investigated, leading to determination of upper critical field Hc2 (T) of this new superconductor. The value of Hc2 at zero temperature is estimated to be about 46 T corresponding to coherence length ∼27 Å. [ABSTRACT FROM AUTHOR]

We report significant enhancement in superconducting properties of yttrium substituted Ce1-xYxOFFeAs superconductors. The polycrystalline samples were prepared by two step solid state reaction technique. X-ray diffraction confirmed tetragonal ZrCuSiAs structure with decrease in both a and c lattice parameters on increasing yttrium substitution (with fixed F content). With smaller ion Y in place of Ce, the transition temperature increased by 6 K. Yttrium doping also lead to higher critical fields as well as broader magnetization loops, particularly at elevated temperature. [ABSTRACT FROM AUTHOR]