R. S. Meena

Superconductivity in SmFe1−xCoxAsO (x=0.0–0.30)

We report synthesis, structural details, and magnetization of SmFe1−xCoxAsO with x ranging from 0.0 to 0.30. It is found that Co substitutes fully at Fe site in SmFeAsO in an isostructural lattice with slightly compressed cell. The parent compound exhibited known as the spin density wave (SDW) character is below at around 140 K. Successive doping of Co at Fe site suppressed the SDW transition for x=0.05 and later induced superconductivity for x=0.10, 0.15, and 0.20, respectively, at 14, 15.5, and 9 K. The lower critical field as seen from magnetization measurements is below 200 Oe. The appearance of bulk superconductivity is established by wide open isothermal magnetization M(H) loops. Superconductivity is not observed for higher content of Co, i.e., x≥0.30. Clearly the Co substitution at Fe site in SmFe1−xCoxAsO diminishes the Fe SDW character, introduces bulk superconductivity for x between 0.10 and 0.20 and finally becomes nonsuperconducting for x above 0.20. The Fe2+ site Co3+ substitution injects mob...

Synthesis and physical properties of pure and f-doped REFeAsO (Re=Pr, Nd & Sm)

We report a comparative study of crystal structure, electrical, and magnetic properties of the pure and F-doped superconducting REFeAsO (Re = Pr, Nd, and Sm) samples. The presence of superconductivity in these iso-structural compounds provided an opportunity to understand the doping mechanism in oxy-pnictide superconductor. Bulk polycrystalline samples are synthesized by solid state reaction route in an evacuated sealed quartz tube. The Rietveld analysis of room temperature X-ray diffraction (XRD) data show that all the studied samples crystallize in single phase in a tetragonal structure with space group P4/nmm. The lattice parameters of the studied samples follow the well-known rare earth contraction. The decrease in c-parameter and the volume is indicative of successful substitution of F−1(RF = 1.33 Ǻ) at O2-(RO = 1.40 Ǻ) site. The ground state REFeAsO compounds shows a metallic step in resistivity measurements below say 150–130 K. This metallic step is attributed to structural and SDW transition. The superconductivity in the F-doped sample is confirmed by resistivity measurements as well as magnetic measurements. Superconducting transition temperature (Tc) is found to be at 51K, 48K and 38 K respectively for RE = Sm, Nd and Pr. The superconducting transition temperature of the F-doped samples increase with deceasing the ionic radii of the rare earth. The temperature dependent upper critical field Hc2(0) is calculated from detailed up to 14 Tesla R(T)H measurements using the extrapolation method employing Ginzburg- Landau (GL) theory. Thus calculated Hc2(0) is found to be above 200 Tesla, which is second best to the High Tc cuprates.

Superconductivity of Fe based pnictides and chalcogenides: Material aspects, doping routes, future prospects and challenges

I review here constant efforts of my superconducting materials group on synthesis and physical properties of title compounds from very beginning (May 2008) itself. Both direct (Fe/Co, Ni) and indirect O/F carrier doping routes are applied and superconductivity is achieved up to 14K and 50K respectively. It infers that in F doped samples the Tc is collective effect of electron doping and short range ordering of Fe spins. Whereas in Co/Ni doped samples occurrence of Tc is mainly due to electron doping. Seemingly magnetic excitations play an important role in achieving superconductivity. This along with drastic depression of Tc with Fe site Zn doping in superconducting LaFeAsO0.8 indicates towards s± coupling. Magneto transport measurements showed that these superconductors are quite robust against magnetic field having upper critical field [Hc2(0)] of up to 400 Tesla. It is found that though FeSe/Te superconductor possesses lower Tc, the same is even more robust against magnetic field than the REFeAsO. Seve...

Synthesis, Electric Transport, Mössbauer Spectroscopy, Specific Heat and Induction of Superconductivity in ReFeAsO (Re=La, Pr, Gd and Sm) †

We report an easy and versatile route of synthesis for ReFeAsO (Re = La, Gd, Pr and Sm) by both single step and precursor based double step solid-state reaction routes. The studied ground state compounds of the new FeAs based superconductor family crystallize in tetragonal P4/nmm structure, with lattice parameters variation following their ionic radii. The resistivity measurements exhibit clearly the appearance of a metallic step below say 150-130 K, which is reminiscent of the SDW (Spin Density Wave) magnetic character. Also, It is observed that oxygen stoichiometric ReFeAsO is not formed and rather the same is slightly oxygen deficient. The Mossbauer spectroscopy (MS) carried out on some of them at 290 K and 90 K i.e. both above and below the SDW temperature exhibits the Fe ordered state at 90 K. Further the MS is proved to be the most efficient technique in picking up the FeAs based impurities in these materials. The heat capacity measurements on SmFeAsO, clearly demonstrates the SDW temperature with a hump in Cp(T) at around 130 K. Further Sm spins are ordered anti-ferro-magnetically (AFM) at 4.5 K. Superconductivity is also induced in SmFeAsO with Co substitution at Fe site with Tc = 14 K. The general behavior of the ReFeAsO is discussed and it is concluded that (a) they can be synthesized by an easy and versatile methods, (b) their ground state is not oxygen stoichiometric, but deficient and (c) MS is the most efficient technique to pick up the Fe based impurities in these materials, that is yet rampant in this new class of superconductors.