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The role of yttrium doping on the structural and superconducting properties of SmFeAsO $$_{1-x}$$ F $$_x$$ (Sm1111) is studied in detail and is compared with other yttrium-doped RE1111 (RE-rare earth) iron oxypnictide superconductors on the basis of the existing reports. It is found that the partial substitution of lanthanides such as La, Ce, and Nd with yttrium systematically enhances $$T_\mathrm{C}$$ of the system to different levels depending on their crystal ionic radii difference with yttrium. The isovalent substitution of these lanthanides at relatively smaller yttrium ion site causes inner chemical pressure and enhances the $$T_\mathrm{C}$$ . However, yttrium doping does not seem to be beneficial for Sm1111 because of its larger ionic size. [ABSTRACT FROM AUTHOR]

The optical properties of the superconducting single crystal Rb0.8Fe1.68Se2 with Tc ... 31 K have been measured over a wide frequency range in the ab plane. We found that the optical conductivity is dominated by a series of infrared-active phonon modes at low-frequency region as well as several other high-frequency bound excitations. The low-frequency optical conductivity has rather low value and shows quite small Drude-like response, indicating low carriers density in this material. Furthermore, the phonon modes increase continuously in frequency with decreasing temperature; specifically, the phonon mode around 200 cm-1 shows an enhanced asymmetry effect at low temperatures, suggesting an increasing electron-phonon coupling in this system. [ABSTRACT FROM AUTHOR]

Abstract: The transport and magnetic properties of the undoped and yttrium doped NdFeAs(O,F) superconductor are investigated in detail for the first time. The transition temperatures of the yttrium doped samples are found to increase monotonically and attain a maximum of 53.3K for 30% yttrium doping, ∼4K higher as compared to the yttrium free sample. The TC values observed from the temperature dependent ac susceptibility measurements are also in agreement with the resistivity measurements. Yttrium doping is found to decrease the lattice parameters of the system; causing an inner chemical pressure and thereby accomplishing a TC enhancement. Moreover, the substitution of relatively smaller yttrium ion at Nd site creates lattice defects which act as effective pinning centers thereby enhancing the JC (H) performance of the system. [Copyright &y& Elsevier]

In this paper we analyze transport data of different families of Fe-based superconductors, within a pseudogap framework. The Fe(Te, Se) samples exhibit s-shaped resistivity ρ( T) curves, while SmFeAs(O, F) ones present a departure from their high-temperature linear behavior. In either case, a characteristic temperature can be identified. These temperatures correspond to those at which abrupt changes in the temperature behavior of Hall resistance and of Seebeck coefficient occur, suggesting that they are signatures of pseudogaps opening in the density of states. The direct correlation between these characteristic temperatures and the superconducting transition temperatures suggests that the pseudogap and the superconducting state originate from the same mechanism. Scaling procedures of resistivity curves confirm such proportionality. On the other hand, excess Fe content in Fe(Te, Se) samples affects the pseudogap temperature much more strongly than the superconducting T. Finally, we find out that the pseudogap in the 1111 family is almost insensitive to disorder, as in high- T cuprates. [ABSTRACT FROM AUTHOR]

Abstract: Isoelectronic Tellurium (Te) substitution for Selenium (Se) in the tetragonal phase of FeSe (β-FeSe) increases the superconducting transition temperature (Tc ) by applying a negative pressure on the lattice. However, the normal state resistivity increases and shows semi-metallic behavior for samples with higher Te concentration. With increasing Te concentration, the Tc increases and reaches a maximum for FeSe0.5Te0.5 and then decreases with further increase of Te. We have investigated the effect of Cobalt (Co) and Nickel (Ni) doping in FeSe0.5Te0.5 in the nominal composition range Fe1− x TM x Se0.5Te0.5 (TM=Co (x =0.05, 0.1, 0.15, 0.2) and Ni (x =0.05, 0.1)). Both Co and Ni doping suppress Tc and drives the system to metal–insulator transition. The in-plane (‘a’) and out-of-plane (‘c’) lattice constants decrease with increasing dopant concentration. [Copyright &y& Elsevier]