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Two-band superconductivity in LaFeAsO0.89F0.11 at very high magnetic fields


The recent synthesis of the superconductor LaFeAsO0.89F0.11 with transition temperature Tc ≈ 26 K (refs 1–4) has been quickly followed by reports of even higher transition temperatures in related compounds: 41 K in CeFeAsO0.84F0.16 (ref. 5), 43 K in SmFeAsO0.9F0.1 (ref. 6), and 52 K in NdFeAsO0.89F0.11 and PrFeAsO0.89F0.11 (refs 7, 8). These discoveries have generated much interest9,10 in the mechanisms and manifestations of unconventional superconductivity in the family of doped quaternary layered oxypnictides LnOTMPn (Ln: La, Pr, Ce, Sm; TM: Mn, Fe, Co, Ni; Pn: P, As), because many features of these materials set them apart from other known superconductors. Here we report resistance measurements of LaFeAsO0.89F0.11 at high magnetic fields, up to 45 T, that show a remarkable enhancement of the upper critical field Bc2 compared to values expected from the slopes dBc2/dT ≈ 2 T K-1 near Tc, particularly at low temperatures where the deduced Bc2(0) ≈ 63–65 T exceeds the paramagnetic limit. We argue that oxypnictides represent a new class of high-field superconductors with Bc2 values surpassing those of Nb3Sn, MgB2 and the Chevrel phases, and perhaps exceeding the 100 T magnetic field benchmark of the high-Tc copper oxides.

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Figure 1: Variation of LaFeAsO 0.89 F 0.11 sample resistance with applied magnetic field at fixed temperatures in the range 4.2 K to 25 K.
Figure 2: Upper critical field–temperature phase diagram of LaFeAsO0.89F0.11.


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Work at the NHMFL was supported by IHRP under NSF Cooperative Agreement, by the State of Florida, by the DOE, by the NSF Focused Research Group on Magnesium Diboride (FRG), and by AFOSR. Work at ORNL was supported by the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences. We are grateful for discussions with G. Boebinger, E. Hellstrom, P. Lee, J. Jiang, and C. Tarantini at the NHMFL.

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Hunte, F., Jaroszynski, J., Gurevich, A. et al. Two-band superconductivity in LaFeAsO0.89F0.11 at very high magnetic fields. Nature 453, 903–905 (2008).

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