712 research outputs found
Conductivity anisotropy in the antiferromagnetic state of iron pnictides
Recent experiments on iron pnictides have uncovered a large in-plane
resistivity anisotropy with a surprising result: the system conducts better in
the antiferromagnetic x direction than in the ferromagnetic y direction. We
address this problem by calculating the ratio of the Drude weight along the x
and y directions, Dx/Dy, for the mean-field Q=(\pi,0) magnetic phase diagram of
a five-band model for the undoped pnictides. We find that Dx/Dy ranges between
0.3 < D_x/D_y < 1.4 for different interaction parameters. Large values of
orbital ordering favor an anisotropy opposite to the one found experimentally.
On the other hand D_x/D_y is strongly dependent on the topology and morfology
of the reconstructed Fermi surface. Our results points against orbital ordering
as the origin of the observed conductivity anisotropy, which may be ascribed to
the anisotropy of the Fermi velocity.Comment: 4 pages, 3 pdf figures. Fig 1(b) changed, one equation corrected,
minor changes in the text, references update
Tight binding model for iron pnictides
We propose a five-band tight-binding model for the Fe-As layers of iron
pnictides with the hopping amplitudes calculated within the Slater-Koster
framework. The band structure found in DFT, including the orbital content of
the bands, is well reproduced using only four fitting parameters to determine
all the hopping amplitudes. The model allows to study the changes in the
electronic structure caused by a modification of the angle formed by
the Fe-As bonds and the Fe-plane and recovers the phenomenology previously
discussed in the literature. We also find that changes in modify the
shape and orbital content of the Fermi surface sheets.Comment: 12 pages, 6 eps figures. Figs 1 and 2 modified, minor changes in the
text. A few references adde
The nature of correlations in the insulating states of twisted bilayer graphene
The recently observed superconductivity in twisted bilayer graphene emerges
from insulating states believed to arise from electronic correlations. While
there have been many proposals to explain the insulating behaviour, the
commensurability at which these states appear suggests that they are Mott
insulators. Here we focus on the insulating states with electrons or
holes with respect to the charge neutrality point. We show that the theoretical
expectations for the Mott insulating states are not compatible with the
experimentally observed dependence on temperature and magnetic field if, as
frequently assumed, only the correlations between electrons on the same site
are included. We argue that the inclusion of non-local (inter-site)
correlations in the treatment of the Hubbard model can bring the predictions
for the magnetic and temperature dependencies of the Mott transition to an
agreement with experiments and have consequences for the critical interactions,
the size of the gap, and possible pseudogap physics. The importance of the
inter-site correlations to explain the experimental observations indicates that
the observed insulating gap is not the one between the Hubbard bands and that
antiferromagnetic-like correlations play a key role in the Mott transition.Comment: 8 pages (including appendix), 5 figure
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