Figure 1

Schematic diagram of the reciprocal space probed, transport and neutron scattering data on Ba_0.67K_0.33Fe₂As₂.

(a) Real and reciprocal space of the FeAs plane. The light and dark As atoms indicate As positions below and above the Fe-planes, respectively. (b) Fermi surfaces at k_z = 0 calculated using the tight-binding model of Graser et al.⁴² for electron and hole-doped BaFe₂As₂. The different colors indicate orbital weights around the Fermi surface with red = d_xz , green = d_yz and blue = d_xy . (c) Temperature dependence of the in-plane resistivity ρ(T) shows the onset of superconductivity at T_c = 38 K. We find no resistivity anomaly that might be associated with structural or AF phase transitions above T_c . The inset shows the temperature dependence of the bulk susceptibility for a 1 mT in-plane magnetic field giving T_c = 38 K. (d) Elastic neutron scattering along the (H, 0, L) direction with fixed values of L = 0, 2 and 3 at 2 K, demonstrating that there is no static AF order in our samples. (e) Energy scans at the AF signal [Q = (1,0,0)] and background [Q = (1.3,0,0)] positions from 0.5 to 8.5 meV at 45 K and 2 K. (f) χ”(Q,ω), obtained by subtracting the background and removing the Bose population factor, clearly shows that a spin gap opens below E∼5.5 meV at 2 K. (g) Energy scans at Q = (1,0,1) and background Q = (1.3,0,1) positions from 0.5 to 8.5 meV at 45 K and 2 K. (h) χ”(Q,ω), obtained using the identical method as in (f), shows quite different behavior from the results in (f). Solid lines are guides to the eye. Data in (d) are from HB-3 and those in (e–h) are from cold triple-axis SPINS. The error bars indicate one sigma throughout the paper.