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

The recent synthesis of the superconductor LaFeAsO0.89F0.11 with transition temperature T-c approximate to 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 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 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 approximate to 2 T K-1 near Tc, particularly at low temperatures where the deduced B-c2(0) approximate to 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.