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).
Abstract
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.
Pn=As,P
Slater-Koster five-orbital tight-binding model Tight-binding model to describe the Pn-Fe layers -All five d-Fe orbitals are included. - Pnictogen (As, P) atoms only enter via Fe-Fe hopping amplitudes - Hopping parameters are calculated within the SlaterKoster framework in terms of the Pn-Fe (pd) and Fe-Fe (dd) orbital overlap integrals . - Hopping is restricted to second Fe nearest neighbours. There are 18 hopping terms which can be given in terms of just four fitting parameters. - Indirect hopping between Fe atoms via Pn induces a dependence of the hopping amplitudes on a.
Fe
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.
pds
pdp
dds
ddp
ddd
Slater and Koster, Phys. Rev 94, 1498 (1954).
W.A. Harrison, “Elementary Electronic Structure”, World Scientific (2004)
Results for the LaFeAsO bands
Angle dependence in experiments, LDA and in our tight binding model
CeFeAsO1-xF
Good agreement for the hole and electron pockets and for the orbital character of the bands
Extended (Fe) Brilloun zone Reduced Brilloun zone
Zhao, e al. Nat. Mat. 7, 953 (2008)
LaFeAsO has a magnetic transition and high Tc. LaFePO (aLaFePO=29.9º) has no magnetic transition and low Tc.
(aLaFeAsO=33.2º)
.
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
on
C.H. Lee, e al., JPSJ 77, 083704 (2008)
AB initio calculation of LaFeAsO bands in the reduced Brillouin zone,
Tight-binding LaFeAsO bands in the extended Brilloun zone
Boeri, e al. PRL 101, 26403 (2008)
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.
From LDA: Fermi pocket with dxy character in LaFeAsO but with d3z2-r2 character in LaFePO
V. Vildosola, e al. PRB 78, 64518 (2008)
Hole pockets in G can disappear in elongated compounds
Results: Fermi surfaces and orbital components of the Fermi surfaces 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)).
CONCLUSIONS
• 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).
Change on the shape and topology of the Fermi surface when the Fe-Pn angle varies
• 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.
Change on the orbital content of the Fermi surface when the Fe-Pn angle varies
•The orbital component of the Fermi surfaces also depends on the Fe-As angle, what could influence the symmetry of the superconducting order parameter Calderon, Valenzuela and Bascones, arXiv:0907.1259