Three-dimensional electronic structure of the nematic and antiferromagnetic phases of NaFeAs from detwinned angle-resolved photoemission spectroscopy

We report a comprehensive angle-resolved photoemission spectroscopy study of NaFeAs, a prototypical parent compound of the Fe-based superconductors. By mechanically detwinning the samples, we show that in the nematic phase (below the structural transition at T_s=54 K but above the antiferromagnetic transition at T_N=43 K) spectral weight is detected on only the elliptical electron pocket along the longer a_orth axis. This dramatic anisotropy is likely to arise as a result of coupling to a fluctuating antiferromagnetic order in the nematic phase. In the long-range ordered antiferromagnetic state below TN, this single electron pocket is backfolded and hybridizes with the hole bands, leading to the reconstructed Fermi surface. By careful analysis of the k_z variation, we show that the backfolding of spectral weight in the magnetic phase has a wave vector of (π,0,π), with the c-axis component being in agreement with the magnetic ordering in NaFeAs observed by neutron scattering. Our results clarify the origin of the tiny Fermi surfaces of NaFeAs at low temperatures and highlight the importance of the three-dimensional aspects of the electronic and magnetic properties of Fe-based superconductors.