Transcript Topological defects in graphene sheets

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.
Slater-Koster five-orbital tight-binding model
Tight-binding model to describe the Pn-Fe layers
Pn=As,P
- Hopping is restricted to second Fe nearest
neighbours. There are 18 hopping terms which can
be given in terms of just four fitting parameters.
-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 SlaterKoster framework in terms of the Pn-Fe (pd) and Fe-Fe (dd)
orbital overlap integrals .
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
Zhao, e al.
Nat. Mat. 7,
953 (2008)
.
(aLaFeAsO=33.2º)
LaFeAsO
has a
magnetic transition and high Tc.
LaFePO (aLaFePO=29.9º) has no
magnetic transition and low Tc.
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
Extended (Fe)
Brilloun zone
Reduced
Brilloun zone
on
AB initio calculation of LaFeAsO bands
in the reduced Brillouin zone,
C.H. Lee, e al.,
JPSJ 77,
083704 (2008)
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
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)).
V. Vildosola, e al. PRB 78, 64518 (2008)
Hole pockets in G can disappear
in elongated compounds
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