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Superconductivity at 41.0 K in the F-doped LaFeAsO1−xFx
Solid State Communications 148 (2008) 168–170
Contents lists available at ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Superconductivity at 41.0 K in the F-doped LaFeAsO1−x Fx Wei Lu, Xiao-Li Shen, Jie Yang, Zheng-Cai Li, Wei Yi, Zhi-An Ren ∗ , Xiao-Li Dong, Guang-Can Che, Li-Ling Sun, Fang Zhou, Zhong-Xian Zhao ∗∗ National Laboratory for Superconductivity, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100190, PR China Beijing National Laboratory for Condensed Matter Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100190, PR China
article
info
Article history: Received 15 May 2008 Received in revised form 15 July 2008 Accepted 17 July 2008 by E.G. Wang Available online 23 July 2008 PACS: 74.70.-b 74.70.Dd 74.25.Fy 74.25.Ha
a b s t r a c t Here we report the enhanced superconductivity in the LaFeAsO1−x Fx system prepared by high pressure synthesis. The highest onset superconducting transition temperature (Tc ) in this La-based system is 41.0 K with the nominal composition of LaFeAsO1−x Fx (x = 0.6), which is higher than that reported previously by ambient pressure synthesis. The increase of Tc can be attributed to further shrinkage of the crystal lattice that causes stronger chemical pressure on the Fe–As plane, which is induced by the increased F-doping level under the high pressure synthesis. © 2008 Elsevier Ltd. All rights reserved.
Keywords: A. Superconductors A. High-Tc superconductors D. Electronic transport
1. Introduction Since the discovery of high temperature copper oxide superconductors about two decades ago [1], much effort has been devoted to searching for new high-Tc superconductors. Recently research interests have been triggered by the report [2] that a layered F-doped arsenic-oxide LaFeAsO1−x Fx shows a superconducting transition temperature (Tc ) of 26 K. Following the above result, a series of new superconductors of ReTAsO1−x Fx and ReTAsO1−x (Re = La, Ce, Pr, Nd, Sm, Gd, and T = Fe, Ni) were synthesized [3–10] with a highest Tc of 55 K in the Sm-based system [8, 9]. The crystal structure of these quaternary equiatomic ReTAsO compounds is simple compared with cuprates, and can be categorized as ZrCuSiAs-type, showing a tetragonal layered structure with a space group P4/nmm, where the Fe–As layer and Re–O layer stack alternately along the c-axis. A recent theoretical calculation
∗ Corresponding address: National Laboratory for Superconductivity, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing, 100190, PR China. Tel.: +86 10 82649188; fax: +86 10 82649486. ∗∗ Corresponding address: National Laboratory for Superconductivity, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing, 100190, PR China. Tel.: +86 10 82649190; fax: +86 10 82649486. E-mail addresses: [email protected] (Z.-A. Ren), [email protected] (Z.-X. Zhao). 0038-1098/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2008.07.032
shows the electron–phonon coupling is not strong enough to generate such a high superconducting transition temperature in the LaFeAsO1−x Fx [11]. It’s also surprising that the strong magnetic element Fe may play an important role in superconductivity, which may supply some helpful information for studying the puzzling origin of unconventional high temperature superconductors. Here we report the superconducting transition at 41 K in the LaFeAsO1−x Fx system. 2. Experimental A series of polycrystalline LaFeAsO1−x Fx samples were synthesized by a high pressure (HP) method. At first, LaAs powder was prepared by La chips and As pieces, which were sealed in vacuumed quartz tube and sintered in the muffle furnace at 650 ◦ C for 12 h and then at 950 ◦ C for 12 h. Subsequently LaAs powder and Fe, Fe2 O3 , FeF2 powders (the purities of all starting chemicals are better than 99.99%) were mixed together according to the nominal chemical formula of LaFeAsO1−x Fx , then ground thoroughly and pressed into small pellets. These pellets were sealed in boron nitride crucibles and sintered in a high pressure synthesis apparatus at 1250 ◦ C under pressure of 6 GPa for 2 h. A series of polycrystalline LaFeAsO1−x samples were also prepared for comparison. Compared with the ambient pressure (AP) method using an evacuated quartz tube, the HP method is more convenient and
W. Lu et al. / Solid State Communications 148 (2008) 168–170
169
Fig. 1. The X-ray diffraction patterns of the HP-synthesized nominal LaFeAsO0.4 F0.6 and an undoped AP sample LaFeAsO. The vertical bars at the bottom are the theoretical simulation for the diffraction peaks of the undoped LaFeAsO sample.
Fig. 2. The temperature dependence of electrical resistivity for the HP-synthesized nominal LaFeAsO0.4 F0.6 . The inset shows the enlargement of the superconducting transition.
efficient for the synthesis of a gas-releasing compound with superhigh pressure-seal. The resulting samples were characterized at room temperature by powder X-ray diffraction (XRD) analysis on an MXP18A-HF type diffractometer with Cu–Kα (λ = 1.54056 Å) radiation from 20◦ to 80◦ with a step of 0.01◦ . The electrical resistivity measurements for the LaFeAsO1−x Fx samples were performed by a standard fourprobe method down to 4.5 K at ambient pressure with no applied field. We measured the temperature dependence of magnetization at a fixed magnetic field using a MPMS XL-1 system (Quantum Design). All the magnetization measurements were done during a warming cycle under an applied field of 1 Oe after zero field cooling (ZFC) and a field cooling (FC) process, respectively.
the strong inner chemical pressure in this sample, caused by a much higher level of F-doping by high pressure synthesis. A recent work reported in Ref. [12] raised the onset Tc of LaFeAsO0.89 F0.11 from 26 K to 43 K by measuring resistivity under a pressure of 4 GPa, while the zero resistivity starts at about 28 K in their data. The consistency of these results indicates that either a physical pressure or a chemical pressure can have a positive effect on the superconductivity of the La-based superconductors. The DC-susceptibility for the HP sample with the nominal composition of LaFeAsO0.4 F0.6 is shown in Fig. 3. The sharp magnetic transition of the susceptibility curve and the large shielding signal indicate the good quality of the main superconducting component compared with all previous reported results. The onset diamagnetic transition can be clearly observed from the enlarged ZFC & FC curves at 40 K, as shown in the inset of the left plot in Fig. 3, and the differential ZFC curve also shows the superconducting transition at 40 K. This corresponds to the onset of resistivity transition, while the bulk Meissner transition starts at 31 K, near the Tc (zero) on the resistivity curve. The wide transition in the resistivity data is from the inhomogeneity of F-doping in the LaFeAsO0.4 F0.6 bulk sample, which indicates that, by improving the sample synthesis method, it is possible to achieve good quality samples or even higher Tc in this La-system. Compared with the oxygen-deficient superconductors LaFeAsO1−x that we previously reported [9], the F-doping samples have better superconducting properties. This is because F-doping can help in forming a more stable structural phase than oxygen vacancies for this La-based system, due to the larger crystal lattice, which vacancies will make more distorted and unstable. For the phase diagram of the La-based system, our observed results indicate that the Tc keeps increasing with more fluorine doping, it seems to be different from that of Nd-based systems and will be further studied later. The high doping level of fluorine by high pressure synthesis caused the further shrinkage of the crystal lattice and also created more carriers on the Fe–As plane, which is considered to be responsible for the observed enhanced superconductivity in the nominal compound of LaFeAsO0.4 F0.6 .
3. Results and discussions Fig. 1 shows the XRD patterns of the HP nominal sample LaFeAsO0.4 F0.6 and an undoped AP sample LaFeAsO, respectively, and the vertical bars at the bottom indicate the theoretical simulation of diffraction peaks. It’s obvious that each of the comparatively strong diffraction peaks in the theoretical simulation has its counterpart in the patterns of the HP nominal sample LaFeAsO0.4 F0.6 and the undoped AP sample LaFeAsO in spite of few extra weak peaks coming from impurities, which indicates that the principal phase can be well indexed on the basis of a tetragonal ZrCuSiAs type structure with the space group P4/nmm. Fig. 1 also shows that the diffraction peaks of the HP nominal sample LaFeAsO0.4 F0.6 shift to a high diffraction angle in comparison with those of the undoped AP sample LaFeAsO, indicating the lattice has shrunk due to sufficient F-doping by the high pressure synthesis method. The refined lattice parameter for this HP sample is a = 3.991(5) Å and c = 8.700(8) Å, compared with the AP sample (a = 4.033(5) Å and c = 8.739(1) Å), the a-axis shrinks by 1% while the c-axis shrinks by 0.45%. The observed shrinkage on the Fe–As plane of this sample is the largest among all the reported LaFeAsO1−x Fx and LaFeAsO1−x samples, which indicates a strong inner chemical pressure on the Fe–As plane. Fig. 2 shows the temperature dependence of resistivity for the HP sample with the nominal composition of LaFeAsO0.4 F0.6 . The electrical resistivity gradually decreases when the temperature decreases from ambient temperature, then drops dramatically below 41.0 K, which we refer to as the onset Tc , and finally becomes unmeasurable at 30 K. The transition width is about 11 K, as shown in the inset of Fig. 2. The onset Tc of 41.0 K is the highest one among all the reported data of La-based compounds in the Fe–As family measured under ambient pressure, which we attribute to
4. Conclusion In conclusion, the nominal samples LaFeAsO1−x Fx were synthesized by a high pressure method. Among these samples the highest onset superconducting transition temperature Tc was achieved at 41.0 K for the nominal composition x = 0.6, which is considerably higher than that reported previously. The XRD analysis indicates a further shrinkage of the crystal lattice that generates the stronger chemical pressure on the Fe–As plane, which is believed to be responsible for the large enhancement of the superconducting transition temperature.
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W. Lu et al. / Solid State Communications 148 (2008) 168–170
Foundation of China (NSFC, No. 50571111 & 10734120) and 973 program of China (No. 2006CB601001 & 2007CB925002). We also acknowledge the support from EC under the project COMEPHS TTC. References
Fig. 3. The temperature dependence of DC susceptibility (left panel) and differential ZFC (right panel) for the HP nominal sample LaFeAsO0.4 F0.6 . The inset shows the enlargement of the magnetic transitions.
Acknowledgements We thank Mrs. Shun-Lian Jia for her kind helps in resistivity measurements. This work is supported by Natural Science
[1] J.G. Bednorz, K.A. Muller, Z. Phys. B 64 (1986) 189. [2] Y. Kamihara, T. Watanabe, M. Hirano, H. Hosono, J. Am. Chem. Soc. 160 (2008) 3296–3297. [3] X.H. Chen, T. Wu, G. Wu, R.H. Liu, H. Chen, D.F. Fang, Nature 453 (2008) 761. [4] G.F. Chen, Z. Li, D. Wu, G. Li, W.Z. Hu, J. Dong, P. Zheng, J.L. Luo, N.L. Wang, Phys. Rev. Lett. 100 (2008) 247002. [5] Z. Li, G.F. Chen, J. Dong, G. Li, W.Z. Hu, J. Zhou, D. Wu, S.K. Su, P. Zheng, N.L. Wang, J.L. Luo. cond-mat/arXiv0803.2572v1. [6] Z.A. Ren, J. Yang, W. Lu, W. Yi, G.C. Che, X.L. Dong, L.L. Sun, Z.X. Zhao, Mater. Res. Innov. 12 (2008) 1. [7] Z.A. Ren, J. Yang, W. Lu, W. Yi, X.L. Shen, Z.C. Li, G.C. Che, X.L. Dong, L.L. Sun, F. Zhou, Z.X. Zhao, Europhys. Lett. 82 (2008) 57002. [8] Z.A. Ren, W. Lu, J. Yang, W. Yi, X.L. Shen, Z.C. Li, G.C. Che, X.L. Dong, L.L. Sun, F. Zhou, Z.X. Zhao, Chin. Phys. Lett. 25 (2008) 2215. [9] Z.A. Ren, G.C. Che, X.L. Dong, J. Yang, W. Lu, W. Yi, X.L. Shen, Z.C. Li, L.L. Sun, F. Zhou, Z.X. Zhao, Europhys. Lett. 83 (2008) 17002. [10] R.H. Liu, G. Wu, T. Wu, D.F. Fang, H. Chen, S.Y. Li, K. Liu, Y.L. Xie, X.F. Wang, R.L. Yang, C. He, D.L. Feng, X.H. Chen. cond-mat/arXiv0804.2105v1. [11] L. Boeri, O.V. Dolgov, A.A. Golubov. cond-mat/arXiv0803.2703v1. [12] H. Takahashi, K. Igawa, K. Arii, Y. Kamihara, M. Hirano, H. Hosono, Nature 453 (2008) 376.
Contents lists available at ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Superconductivity at 41.0 K in the F-doped LaFeAsO1−x Fx Wei Lu, Xiao-Li Shen, Jie Yang, Zheng-Cai Li, Wei Yi, Zhi-An Ren ∗ , Xiao-Li Dong, Guang-Can Che, Li-Ling Sun, Fang Zhou, Zhong-Xian Zhao ∗∗ National Laboratory for Superconductivity, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100190, PR China Beijing National Laboratory for Condensed Matter Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100190, PR China
article
info
Article history: Received 15 May 2008 Received in revised form 15 July 2008 Accepted 17 July 2008 by E.G. Wang Available online 23 July 2008 PACS: 74.70.-b 74.70.Dd 74.25.Fy 74.25.Ha
a b s t r a c t Here we report the enhanced superconductivity in the LaFeAsO1−x Fx system prepared by high pressure synthesis. The highest onset superconducting transition temperature (Tc ) in this La-based system is 41.0 K with the nominal composition of LaFeAsO1−x Fx (x = 0.6), which is higher than that reported previously by ambient pressure synthesis. The increase of Tc can be attributed to further shrinkage of the crystal lattice that causes stronger chemical pressure on the Fe–As plane, which is induced by the increased F-doping level under the high pressure synthesis. © 2008 Elsevier Ltd. All rights reserved.
Keywords: A. Superconductors A. High-Tc superconductors D. Electronic transport
1. Introduction Since the discovery of high temperature copper oxide superconductors about two decades ago [1], much effort has been devoted to searching for new high-Tc superconductors. Recently research interests have been triggered by the report [2] that a layered F-doped arsenic-oxide LaFeAsO1−x Fx shows a superconducting transition temperature (Tc ) of 26 K. Following the above result, a series of new superconductors of ReTAsO1−x Fx and ReTAsO1−x (Re = La, Ce, Pr, Nd, Sm, Gd, and T = Fe, Ni) were synthesized [3–10] with a highest Tc of 55 K in the Sm-based system [8, 9]. The crystal structure of these quaternary equiatomic ReTAsO compounds is simple compared with cuprates, and can be categorized as ZrCuSiAs-type, showing a tetragonal layered structure with a space group P4/nmm, where the Fe–As layer and Re–O layer stack alternately along the c-axis. A recent theoretical calculation
∗ Corresponding address: National Laboratory for Superconductivity, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing, 100190, PR China. Tel.: +86 10 82649188; fax: +86 10 82649486. ∗∗ Corresponding address: National Laboratory for Superconductivity, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing, 100190, PR China. Tel.: +86 10 82649190; fax: +86 10 82649486. E-mail addresses: [email protected] (Z.-A. Ren), [email protected] (Z.-X. Zhao). 0038-1098/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2008.07.032
shows the electron–phonon coupling is not strong enough to generate such a high superconducting transition temperature in the LaFeAsO1−x Fx [11]. It’s also surprising that the strong magnetic element Fe may play an important role in superconductivity, which may supply some helpful information for studying the puzzling origin of unconventional high temperature superconductors. Here we report the superconducting transition at 41 K in the LaFeAsO1−x Fx system. 2. Experimental A series of polycrystalline LaFeAsO1−x Fx samples were synthesized by a high pressure (HP) method. At first, LaAs powder was prepared by La chips and As pieces, which were sealed in vacuumed quartz tube and sintered in the muffle furnace at 650 ◦ C for 12 h and then at 950 ◦ C for 12 h. Subsequently LaAs powder and Fe, Fe2 O3 , FeF2 powders (the purities of all starting chemicals are better than 99.99%) were mixed together according to the nominal chemical formula of LaFeAsO1−x Fx , then ground thoroughly and pressed into small pellets. These pellets were sealed in boron nitride crucibles and sintered in a high pressure synthesis apparatus at 1250 ◦ C under pressure of 6 GPa for 2 h. A series of polycrystalline LaFeAsO1−x samples were also prepared for comparison. Compared with the ambient pressure (AP) method using an evacuated quartz tube, the HP method is more convenient and
W. Lu et al. / Solid State Communications 148 (2008) 168–170
169
Fig. 1. The X-ray diffraction patterns of the HP-synthesized nominal LaFeAsO0.4 F0.6 and an undoped AP sample LaFeAsO. The vertical bars at the bottom are the theoretical simulation for the diffraction peaks of the undoped LaFeAsO sample.
Fig. 2. The temperature dependence of electrical resistivity for the HP-synthesized nominal LaFeAsO0.4 F0.6 . The inset shows the enlargement of the superconducting transition.
efficient for the synthesis of a gas-releasing compound with superhigh pressure-seal. The resulting samples were characterized at room temperature by powder X-ray diffraction (XRD) analysis on an MXP18A-HF type diffractometer with Cu–Kα (λ = 1.54056 Å) radiation from 20◦ to 80◦ with a step of 0.01◦ . The electrical resistivity measurements for the LaFeAsO1−x Fx samples were performed by a standard fourprobe method down to 4.5 K at ambient pressure with no applied field. We measured the temperature dependence of magnetization at a fixed magnetic field using a MPMS XL-1 system (Quantum Design). All the magnetization measurements were done during a warming cycle under an applied field of 1 Oe after zero field cooling (ZFC) and a field cooling (FC) process, respectively.
the strong inner chemical pressure in this sample, caused by a much higher level of F-doping by high pressure synthesis. A recent work reported in Ref. [12] raised the onset Tc of LaFeAsO0.89 F0.11 from 26 K to 43 K by measuring resistivity under a pressure of 4 GPa, while the zero resistivity starts at about 28 K in their data. The consistency of these results indicates that either a physical pressure or a chemical pressure can have a positive effect on the superconductivity of the La-based superconductors. The DC-susceptibility for the HP sample with the nominal composition of LaFeAsO0.4 F0.6 is shown in Fig. 3. The sharp magnetic transition of the susceptibility curve and the large shielding signal indicate the good quality of the main superconducting component compared with all previous reported results. The onset diamagnetic transition can be clearly observed from the enlarged ZFC & FC curves at 40 K, as shown in the inset of the left plot in Fig. 3, and the differential ZFC curve also shows the superconducting transition at 40 K. This corresponds to the onset of resistivity transition, while the bulk Meissner transition starts at 31 K, near the Tc (zero) on the resistivity curve. The wide transition in the resistivity data is from the inhomogeneity of F-doping in the LaFeAsO0.4 F0.6 bulk sample, which indicates that, by improving the sample synthesis method, it is possible to achieve good quality samples or even higher Tc in this La-system. Compared with the oxygen-deficient superconductors LaFeAsO1−x that we previously reported [9], the F-doping samples have better superconducting properties. This is because F-doping can help in forming a more stable structural phase than oxygen vacancies for this La-based system, due to the larger crystal lattice, which vacancies will make more distorted and unstable. For the phase diagram of the La-based system, our observed results indicate that the Tc keeps increasing with more fluorine doping, it seems to be different from that of Nd-based systems and will be further studied later. The high doping level of fluorine by high pressure synthesis caused the further shrinkage of the crystal lattice and also created more carriers on the Fe–As plane, which is considered to be responsible for the observed enhanced superconductivity in the nominal compound of LaFeAsO0.4 F0.6 .
3. Results and discussions Fig. 1 shows the XRD patterns of the HP nominal sample LaFeAsO0.4 F0.6 and an undoped AP sample LaFeAsO, respectively, and the vertical bars at the bottom indicate the theoretical simulation of diffraction peaks. It’s obvious that each of the comparatively strong diffraction peaks in the theoretical simulation has its counterpart in the patterns of the HP nominal sample LaFeAsO0.4 F0.6 and the undoped AP sample LaFeAsO in spite of few extra weak peaks coming from impurities, which indicates that the principal phase can be well indexed on the basis of a tetragonal ZrCuSiAs type structure with the space group P4/nmm. Fig. 1 also shows that the diffraction peaks of the HP nominal sample LaFeAsO0.4 F0.6 shift to a high diffraction angle in comparison with those of the undoped AP sample LaFeAsO, indicating the lattice has shrunk due to sufficient F-doping by the high pressure synthesis method. The refined lattice parameter for this HP sample is a = 3.991(5) Å and c = 8.700(8) Å, compared with the AP sample (a = 4.033(5) Å and c = 8.739(1) Å), the a-axis shrinks by 1% while the c-axis shrinks by 0.45%. The observed shrinkage on the Fe–As plane of this sample is the largest among all the reported LaFeAsO1−x Fx and LaFeAsO1−x samples, which indicates a strong inner chemical pressure on the Fe–As plane. Fig. 2 shows the temperature dependence of resistivity for the HP sample with the nominal composition of LaFeAsO0.4 F0.6 . The electrical resistivity gradually decreases when the temperature decreases from ambient temperature, then drops dramatically below 41.0 K, which we refer to as the onset Tc , and finally becomes unmeasurable at 30 K. The transition width is about 11 K, as shown in the inset of Fig. 2. The onset Tc of 41.0 K is the highest one among all the reported data of La-based compounds in the Fe–As family measured under ambient pressure, which we attribute to
4. Conclusion In conclusion, the nominal samples LaFeAsO1−x Fx were synthesized by a high pressure method. Among these samples the highest onset superconducting transition temperature Tc was achieved at 41.0 K for the nominal composition x = 0.6, which is considerably higher than that reported previously. The XRD analysis indicates a further shrinkage of the crystal lattice that generates the stronger chemical pressure on the Fe–As plane, which is believed to be responsible for the large enhancement of the superconducting transition temperature.
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W. Lu et al. / Solid State Communications 148 (2008) 168–170
Foundation of China (NSFC, No. 50571111 & 10734120) and 973 program of China (No. 2006CB601001 & 2007CB925002). We also acknowledge the support from EC under the project COMEPHS TTC. References
Fig. 3. The temperature dependence of DC susceptibility (left panel) and differential ZFC (right panel) for the HP nominal sample LaFeAsO0.4 F0.6 . The inset shows the enlargement of the magnetic transitions.
Acknowledgements We thank Mrs. Shun-Lian Jia for her kind helps in resistivity measurements. This work is supported by Natural Science
[1] J.G. Bednorz, K.A. Muller, Z. Phys. B 64 (1986) 189. [2] Y. Kamihara, T. Watanabe, M. Hirano, H. Hosono, J. Am. Chem. Soc. 160 (2008) 3296–3297. [3] X.H. Chen, T. Wu, G. Wu, R.H. Liu, H. Chen, D.F. Fang, Nature 453 (2008) 761. [4] G.F. Chen, Z. Li, D. Wu, G. Li, W.Z. Hu, J. Dong, P. Zheng, J.L. Luo, N.L. Wang, Phys. Rev. Lett. 100 (2008) 247002. [5] Z. Li, G.F. Chen, J. Dong, G. Li, W.Z. Hu, J. Zhou, D. Wu, S.K. Su, P. Zheng, N.L. Wang, J.L. Luo. cond-mat/arXiv0803.2572v1. [6] Z.A. Ren, J. Yang, W. Lu, W. Yi, G.C. Che, X.L. Dong, L.L. Sun, Z.X. Zhao, Mater. Res. Innov. 12 (2008) 1. [7] Z.A. Ren, J. Yang, W. Lu, W. Yi, X.L. Shen, Z.C. Li, G.C. Che, X.L. Dong, L.L. Sun, F. Zhou, Z.X. Zhao, Europhys. Lett. 82 (2008) 57002. [8] Z.A. Ren, W. Lu, J. Yang, W. Yi, X.L. Shen, Z.C. Li, G.C. Che, X.L. Dong, L.L. Sun, F. Zhou, Z.X. Zhao, Chin. Phys. Lett. 25 (2008) 2215. [9] Z.A. Ren, G.C. Che, X.L. Dong, J. Yang, W. Lu, W. Yi, X.L. Shen, Z.C. Li, L.L. Sun, F. Zhou, Z.X. Zhao, Europhys. Lett. 83 (2008) 17002. [10] R.H. Liu, G. Wu, T. Wu, D.F. Fang, H. Chen, S.Y. Li, K. Liu, Y.L. Xie, X.F. Wang, R.L. Yang, C. He, D.L. Feng, X.H. Chen. cond-mat/arXiv0804.2105v1. [11] L. Boeri, O.V. Dolgov, A.A. Golubov. cond-mat/arXiv0803.2703v1. [12] H. Takahashi, K. Igawa, K. Arii, Y. Kamihara, M. Hirano, H. Hosono, Nature 453 (2008) 376.