The discovery of superconductivity at 26 K  in oxy-pnictides of the type La(O/F)FeAs has generated global interest. Structurally, these oxy-pnictides, LnOFeAs (Ln= rare earth metals), are similar to the layered ZrCuSiAs network consisting of two dimensional layers of (Ln-O)- and (Fe-As)+ in which iron is tetrahedrally coordinated. The parent compound LaOFeAs is an antiferromagnetic semimetal and shows anomalies ~ 150 K in both resistivity and magnetic susceptibility. Electron injection by of the alio-valent substitution of fluorine at the oxygen site [or oxygen deficiency] yields superconductivity which enhances upon the substitution of smaller rare earth metal ions[2-6]. A maximum Tc of 55 K is reported in the system SmO1-xFxFeAs.
We discuss our recent results [7,8] on a new route for electron doping, using KF or NaF in LaOFeAs system which leads to an increase in the upper critical field as would be expected for a multi-band superconductor. We also report the highest Tc (onset) of 28.50 K and highest upper critical field (122 T) at ambient pressure in the family of La based oxypnictides. Substitution of Th4+ at the La site in LaOFeAs also leads to electron superconductors. We have also observed superconductivity by Co-doping at Fe sites in CeOFeAs. Sb-doped superconductors, LaO0.8F0.2FeAs1-xSbx, (x = 0.05 and 0.10) show an enhancement of the Tc to 30.1 K. Simultaneous substitution at both the Ln-O and M-As layers has been investigated for the first time to examine the role of charge carriers in these two layers. This study has lead to superconductivity with enhanced transition temperatures (48K) in Ce-based superconductors . We discuss the transport properties, (resistivity, magneto-resistance, Seebeck and Hall coefficient) , penetration depth and upper critical fields of these new superconductors.
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Ashok K Ganguli
Department of Chemistry
Indian Institute of Technology, Delhi
New Delhi 110016, India