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).

Abstract

The recent synthesis of the superconductor LaFeAsO_0.89F_0.11 with transition temperature T_c  26 K (refs 1–4) has been quickly followed by reports of even higher transition temperatures in related compounds: 41 K in CeFeAsO_0.84F_0.16 (ref. 5), 43 K in SmFeAsO_0.9F_0.1 (ref. 6), and 52 K in NdFeAsO_0.89F_0.11 and PrFeAsO_0.89F_0.11 (refs 7, 8). These discoveries have generated much interest^{9, 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 LaFeAsO_0.89F_0.11 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  2 T K^-1 near T_c, particularly at low temperatures where the deduced B_c2(0)  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.

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