Very High Field Two-Band Superconductivity in LaFeAsO0.89F0.11
Abstract
The ongoing search for new superconductors has recently yielded a new family of oxypnictides composed of alternating La2O2-xFx and Fe2As2 layers [1-4] with transition temperatures Tc of 25-28 K, which can be raised to 40-43 K by replacing La with Ce [5] or Sm [6] or to 52 K by replacing La with Nd and Pr [7, 8]. Recent experiments and band structure calculations have suggested an unconventional multiband superconductivity in the layers of paramagnetic Fe ions, which would normally destroy superconductivity in the traditional mechanism of the s-wave Cooper pairing. Here we report very high-field resistance measurements up to 45T, which show a remarkable enhancement of the upper critical fields Bc2 at low temperatures, as compared to those expected from the already high slopes of dBc2/dT ~ 2T/K near Tc . The deduced Bc2(0) ~ 63-65 T exceeds the paramagnetic limit, consistent with strong coupling and important two-band effects in LaFeAsO0.89F0.11. We argue that oxypnictides are emerging as a new class of high-field superconductors surpassing the Bc2 of Nb3Sn, MgB2, and the Chevrel phases and perhaps approaching the 100T field benchmark of the high-Tc cuprates.