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

1
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 just four fitting
  parameters.

Iron pnictides are layered materials
characterized by Pnictogen (Pn)-Fe layers,
PnAs,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.
PnAs,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, Elementary Electronic Structure,
World Scientific (2004)
.
on
As in LDA it is found a switch in M between a
d3z2-r2 pocket for aLaFePO29.9º and a dxy pocket
for aLaFeAsO33.2º
LaFeAsO (aLaFeAsO33.2º) has a magnetic
transition and high Tc. LaFePO (aLaFePO29.9º)
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, arXiv0907.1259