Five-orbital tight-binding model and effect of the tetrahedral distortion in iron pnictides

1. CONCLUSIONS • Abstract Results for the LaFeAsO bands • Angle dependence in experiments, LDA and in our tight binding model Results: Fermi surfaces and orbital components of the Fermi surfaces Slater-Koster five-orbital tight-binding model Five-orbital tight-binding model and effect of the tetrahedral distortion in iron pnictides M.J. Calderón(1), B. Valenzuela(1,2) and E. Bascones(1) (1) Instituto de Ciencias de Materiales de Madrid, ICMM-CSIC (Spain), (2) Universidad Autónoma de Madrid (Spain). Tight-binding model to describe the Pn-Fe layers • Hopping is restricted to second Fe nearest neighbours. There are 18 hopping terms which can • be given in terms of justfour fitting parameters. Iron pnictides are layered materials characterized by Pnictogen (Pn)-Fe layers, Pn=As,P. Fe-Pn bonds form an angle a with the Fe plane. Experiments and first principle calculations seem to indicate a dependence of the superconducting and magnetic properties on a. Inclusion of the five 3d-Fe orbitals is believed to be relevant to describe these materials. Interband scattering is sensitive to the orbital makeup of the Fermi surface sheets. We propose a 5-orbital tight-binding model to describe the pnictogen-iron layers. We use it to study the influence of the Fe-Pn angle a on the band structure. We find that small changes in a have a strong impact on the bands and on the shape and orbital content of the Fermi surface. Pn=As,P -All five d-Fe orbitals are included. - Pnictogen (As, P) atoms only enter via Fe-Fe hopping amplitudes Fe - Indirect hopping between Fe atoms via Pn induces a dependence of the hopping amplitudes on a. - Hopping parameters are calculated within the Slater-Koster framework in terms of the Pn-Fe (pd) and Fe-Fe (dd) orbital overlap integrals . pdp pds dds ddp ddd Slater and Koster, Phys. Rev 94, 1498 (1954). W.A. Harrison, “ElementaryElectronicStructure”, WorldScientific (2004) • . on As in LDA it is found a switch in M between a d3z2-r2 pocket for aLaFePO=29.9º and a dxy pocket for aLaFeAsO=33.2º LaFeAsO (aLaFeAsO=33.2º) has a magnetic transition and high Tc. LaFePO(aLaFePO=29.9º) has no magnetic transition and low Tc. CeFeAsO1-xF Good agreement for the hole and electron pockets and for the orbital character of the bands Zhao, e al. Nat. Mat. 7, 953 (2008) Extended (Fe) Brilloun zone Reduced Brilloun zone C.H. Lee, e al., JPSJ 77, 083704 (2008) From LDA: Fermi pocket with dxy character in LaFeAsO but with d3z2-r2 character in LaFePO a differs among compounds and depends on doping or applied pressure. Differences in the value of a have been proposed as the origin of the different superconducting and magnetic properties among compounds. Hole pockets in G can disappear in elongated compounds AB initio calculation of LaFeAsO bands in the reduced Brillouin zone, Boeri, e al. PRL 101, 26403 (2008) Tight-binding LaFeAsO bands in the extended Brilloun zone V. Vildosola, e al. PRB 78, 64518 (2008) • We propose a five orbital Slater-Koster tight binding model for the iron-pnictogen layers which allows to analyze the depencence of the electronic properties of iron pnictides on the Fe-As angle. It reproduces the bands and orbital component using only four parameters to parametrize all hopping terms. For the angle dependence: • The hopping amplitudes depend strongly on the Fe-As angle. This is expected to be important for weak coupling models (via nesting) and strong coupling models (via superexchange). • This angle-dependence is also present in the shape and topology of the Fermi surface (crucial for properties based on nesting). • In agreement with LDA calculations at M (in the extended Brillouin zone) close to the Fermi surface the bands with 3z2-r2 and xy character switch in energy as a function of the angle. • The orbital component of the Fermi surfaces also depends on the Fe-As angle, what could influence the symmetry of the superconducting order parameter The Fermi surface orbital makeup has been claimed to determine the symmetry of the superconducting order parameter (Maier et al. PRB 79, 224510 (2009) ; Kuroki et al. , PRB 79, 224511 (2009)). Change on the shape and topology of the Fermi surface when the Fe-Pn angle varies Change on the orbital content of the Fermi surface when the Fe-Pn angle varies Calderon, Valenzuela and Bascones, arXiv:0907.1259