Printed October 26, 2018 | I.I. Mazin, D.J. Singh, M.D. Johannes, and M.H. Du
The authors argue that the recently discovered superconductivity in the Fe-based layered compound LaFeAsO$_{1-x}$F$_x$ is unconventional and mediated by antiferromagnetic (AF) spin fluctuations. This state is characterized by extended s-wave pairing with a sign reversal of the order parameter (OP) between different Fermi surface sheets. The doping in this compound primarily serves to reduce the density of states and suppress pair-breaking ferromagnetic fluctuations. The pure compound, LaFeAsO, is on the verge of a magnetic instability, with a high magnetic susceptibility and significant renormalization compared to density functional theory (DFT) calculations. Doping with F increases this susceptibility, suggesting a near critical point and non-trivial competition between different spin fluctuations. The calculated Fermi surfaces for doped LaFeAsO show a highly 2D electronic structure with two heavy hole cylinders and two lighter electron cylinders, which simplifies the fermiology and moves the system away from ferromagnetic instability. The authors propose that the main role of AF spin fluctuations near the M point in the Brillouin zone is to induce a superconducting state with OPs of opposite signs on the electron and hole pockets, forming an "s±" state. This state is analogous to previously proposed states in semimetals and bilayer cuprates, and it has implications for experimental probes such as neutron scattering and Josephson junction experiments.The authors argue that the recently discovered superconductivity in the Fe-based layered compound LaFeAsO$_{1-x}$F$_x$ is unconventional and mediated by antiferromagnetic (AF) spin fluctuations. This state is characterized by extended s-wave pairing with a sign reversal of the order parameter (OP) between different Fermi surface sheets. The doping in this compound primarily serves to reduce the density of states and suppress pair-breaking ferromagnetic fluctuations. The pure compound, LaFeAsO, is on the verge of a magnetic instability, with a high magnetic susceptibility and significant renormalization compared to density functional theory (DFT) calculations. Doping with F increases this susceptibility, suggesting a near critical point and non-trivial competition between different spin fluctuations. The calculated Fermi surfaces for doped LaFeAsO show a highly 2D electronic structure with two heavy hole cylinders and two lighter electron cylinders, which simplifies the fermiology and moves the system away from ferromagnetic instability. The authors propose that the main role of AF spin fluctuations near the M point in the Brillouin zone is to induce a superconducting state with OPs of opposite signs on the electron and hole pockets, forming an "s±" state. This state is analogous to previously proposed states in semimetals and bilayer cuprates, and it has implications for experimental probes such as neutron scattering and Josephson junction experiments.