Anisotropy in the magnetic state of undoped iron pnictides

The recently discovered high temperature superconductors iron pnictides
present singular magnetism. The undoped compound is metallic with Q=(pi,0) columnar ordering instead of being a Mott insulator with Néel order as in cuprates. It presents a very low magnetic moment[1], even lower than predicted in ab-initio calculations. Another interesting aspect is a strong anisotropy found in transport[2], optical[3] and inelastic neutron experiments[4]. In particular, the resistivity anisotropy is the opposite to the expected one since the system conducts better in the antiferromagnetic x direction than in the ferromagnetic y direction. This situation has put forward orbital ordering as a possible theoretical scenario to understand the anisotropies found in these experiments. Orbital ordering is also consistent with the dominance of zx orbital at the Fermi surface in ARPES[5]. In our work we calculate the mean field Q=(pi,0) magnetic phase diagram using a five orbital tight-binding model[6]. For intermediate values of the interaction, two different meta!
llic regimes with low and high magnetic moments arise. Orbital ordering is concomitant with magnetism in both regimes. The low moment state is characterized by on-site antiparallel orbital magnetic moments[7]. This metallic low moment state is consistent with the strong exchange anisotropy found in neutron experiments. The orbital ordering found in the metallic region of the phase diagram reproduces the large zx weight seen around Gamma in ARPES experiments. We also calculate the ratio of the Drude weight along the x and y directions, Dx/Dy[8]. We find that Dx/Dy ranges between 0.2 < Dx/Dy < 1.7 for different interaction parameters. Large values of orbital ordering favor an anisotropy opposite to the one found experimentally. On the other hand Dx/Dy is strongly dependent on the topology and morphology of the reconstructed Fermi surface. This anisotropy extends to higher frequencies and changes direction as seen in optical experiments. Our results points against orbital ordering as the origin of the observed conductivity anisotropy, which may be ascribed to the anisotropy built by the magnetic state[8,9].
[1] J. Zhao, et al. Nat.Mat 7, 953 (2008)
[2] Chu et al. Science 329, 824 (2010)
[3] Dusza et al, EPL 93 37002 (2011)
[4] Zhao et al., Nat. Phys. 5, 555 (2009)
[5] T. Shimojima et al., PRL 104 057002 (2010)
[6] MJ Calderon, B.V, E Bascones PRB 80, 94531 (2009)
[7] E. Bascones, M.J. Calderón, B. V., PRL 104 , 227201 (2010)
[8] B. Valenzuela, E. Bascones, M.J. Calderón, PRL 105, 207202 (2010)
[9] G. León, E. Bascones, M.J. Calderón, B. Valenzuela, in preparation

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