Letter

Nature 453, 903-905 (12 June 2008) | doi:10.1038/nature07058; Received 2 April 2008; Accepted 5 May 2008; Published online 28 May 2008

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

F. Hunte1, J. Jaroszynski1, A. Gurevich1, D. C. Larbalestier1, R. Jin2, A. S. Sefat2, M. A. McGuire2, B. C. Sales2, D. K. Christen2 & D. Mandrus2

  1. National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32310, USA
  2. Materials Science & Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA

Correspondence to: F. Hunte1 Correspondence and requests for materials should be addressed to F.H. (Email: hunte@asc.magnet.fsu.edu).

The recent synthesis of the superconductor LaFeAsO0.89F0.11 with transition temperature T c approximately 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 B c2 compared to values expected from the slopes dB c2/dT approximately 2 T K-1 near T c, particularly at low temperatures where the deduced B c2(0) approximately 63–65 T exceeds the paramagnetic limit. We argue that oxypnictides represent a new class of high-field superconductors with B c2 values surpassing those of Nb3Sn, MgB2 and the Chevrel phases, and perhaps exceeding the 100 T magnetic field benchmark of the high-T c copper oxides.