Two-band superconductivity in LaFeAsO_(0.89)F_(0.11) at very high magnetic fields

摘要

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: 41K in CeFeAsO_(0.84)F_(0.16)(ref. 5), 43K 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 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 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 Nb_3Sn, MgB_2 and the Chevrel phases, and perhaps exceeding the 100 T magnetic field benchmark of the high-T_c copper oxides.