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

Oxypnictide superconductors: Behaviour in very high magnetic fields The synthesis of the novel superconductor LaFeAsO 0.89 F 0.11 with transition temperature T c ∼ 26 K has been quickly followed by reports of even higher T c s. These discoveries have generated much interest in the mechanisms and manifestations of unconventional superconductivity in the family of doped quaternary layered oxypnictides, because many features of these materials set them apart from other superconductors. Hunte et al . report very high-field resistance measurements of LaFeAsO 0.89 F 0.11 up to 45 T, which show a remarkable enhancement of the upper critical field B c2 as compared to values expected from the slopes d B c2 /d T ≈ 2 T/K near T c , particularly at low temperatures where the deduced B c2 (0) ≈ 63–65 T exceeds the paramagnetic limit. The recent synthesis of the superconductor LaFeAsO 0.89 F 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 0.84 F 0.16 (ref. 5 ), 43 K in SmFeAsO 0.9 F 0.1 (ref. 6 ), and 52 K in NdFeAsO 0.89 F 0.11 and PrFeAsO 0.89 F 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 0.89 F 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 d B c2 /d T ≈ 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 Nb 3 Sn, MgB 2 and the Chevrel phases, and perhaps exceeding the 100 T magnetic field benchmark of the high- T c copper oxides.