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Rare earth Oxy-pnictides : A New Family of Superconductors

Categories: Chemistry and Chemical Biology (CCB)
Speaker: Ashok K Ganguli, Dept. of Chemistry, Indian Institute of Technology, Delhi
Date & Time: June 4, 2009 - 11:00am
Location: Room 202A

The discovery of superconductivity at 26 K [1] 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[9].  We have also observed superconductivity by Co-doping at Fe sites in CeOFeAs[9].  Sb-doped superconductors, LaO0.8F0.2FeAs1-xSbx, (x = 0.05 and 0.10) show an enhancement of the Tc to 30.1 K[10].  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 [11]. We discuss the transport properties, (resistivity, magneto-resistance, Seebeck and  Hall coefficient) , penetration depth and upper critical fields of these new superconductors.

 

References :

1. KAMIHARA Y., WATANABE T., HIRANO M. and HOSONO H., J. Am. Chem. Soc., 130 (2008) 3296.

2. Chen G F, Li Z, Wu D, Li G, Hu W Z, Dong J, Zheng P, Luo J L and Wang N L 2008 Phys. Rev. Lett. 100 247002.

3. Chen X H, Wu T, Wu G, Liu R H, Chen H and Fang D F  2008 Nature 453, 761.

4. Ren Z-A, Yang J, Lu W, Yi W, Shen X-L, Li Z-C, Che G-C, Dong X-L, Sun L-L, Zhou F and Zhao Z-X,  2008 Europhys. Lett. 82 57002.

5. J. Prakash,  S. Singh, , S. Patnaik and A. K. Ganguli, Solid State Commun., 149, 181(2009).

6. Yang J, Li Z-C, Lu W, Yi W, Shen X-L, Ren Z-A, Che G-C, Dong X-L, Sun L-L, Zhou F and Zhao Z-X 2008  Supercond. Sci. Technol. 21 082001.

7. J. Prakash,  S. Singh, S. L. Samal, S. Patnaik and A. K. Ganguli, Europhys. Letts., 84, 57003 (2008).

8. J. Prakash, S. J. Singh, S. Patnaik and A K Ganguli, Physica C 469, 300 (2009).

9. J. Prakash, S. J. Singh, S. Patnaik and A. K. Ganguli, J. Phys. Cond Matt. (in print),  2009.

10. J. Prakash,  S. Singh, , S. Patnaik and A. K. Ganguli, Supercon. Sci. & Tech. 22, 045017 (2009).

11.  J. Prakash,  S. Singh, , S. Patnaik and A. K. Ganguli, Phys Rev B (communicated) (2009).

Ashok K Ganguli
Department of Chemistry
Indian Institute of Technology, Delhi
New Delhi 110016, India
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