Very High Field Two-Band Superconductivity in LaFeAsO_0.89F_0.11

TL;DR

This study reveals that LaFeAsO_0.89F_0.11 exhibits very high upper critical magnetic fields exceeding 60 Tesla, indicating strong coupling and multiband effects that position it as a promising high-field superconductor surpassing many known materials.

Contribution

The paper provides experimental evidence of extremely high upper critical fields in LaFeAsO_0.89F_0.11, highlighting the role of multiband effects and strong coupling in its superconductivity.

Findings

B_c2(0) ~ 63-65 T exceeds paramagnetic limit

High-field resistance measurements up to 45T

Indications of multiband superconductivity

Abstract

The ongoing search for new superconductors has recently yielded a new family of oxypnictides composed of alternating La_2O_{2-x}F_x and Fe_2As_2 layers [1-4] with transition temperatures T_c 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 B_c2 at low temperatures, as compared to those expected from the already high slopes of dB_c2/dT ~ 2T/K near T_c . The deduced B_c2(0) ~ 63-65 T exceeds the paramagnetic limit, consistent with strong coupling and important two-band effects in LaFeAsO_0.89F_0.11. We argue that oxypnictides are emerging as a new class of high-field superconductors surpassing the B_c2 of Nb_3Sn, MgB_2, and the Chevrel phases and perhaps approaching the 100T field benchmark of the high-T_c cuprates.