4,216 research outputs found
On the checkerboard pattern and the autocorrelation of photoemission data in high temperature superconductors
In the pseudogap state the spectrum of the autocorrelation of angle resolved
photoemission (AC-ARPES) data of Bi2212 presents non-dispersive peaks in
momentum space which compare well with those responsible of the checkerboard
pattern found in the density of states by Scanning Tunneling Microscopy. This
similarity suggests that the checkerboard pattern originates from peaks in the
joint density of states, as the dispersive peaks found in the superconducting
state do. Here we show that the experimental AC-ARPES spectrum can be
reproduced within a model for the pseudogap with no charge-ordering or symmetry
breaking. We predict that, because of the competition of superconductivity and
pseudogap, in the superconducting state, the AC-ARPES data of underdoped
cuprates will present both dispersive and non-dispersive peaks and they will be
better observed in cuprates with low critical temperature. We finally argue
that the AC-ARPES data is a complementary and convenient way to measure the arc
length.Comment: 5 pages, 3 eps figure
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
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
Orbital differentiation and the role of orbital ordering in the magnetic state of Fe superconductors
We analyze the metallic (pi,0) antiferromagnetic state of a five-orbital
model for iron superconductors. We find that with increasing interactions the
system does not evolve trivially from the pure itinerant to the pure localized
regime. Instead we find a region with a strong orbital differentiation between
xy and yz, which are half-filled gapped states at the Fermi level, and
itinerant zx, 3z^2-r^2 and x^2-y^2. We argue that orbital ordering between yz
and zx orbitals arises as a consequence of the interplay of the exchange energy
in the antiferromagnetic x direction and the kinetic energy gained by the
itinerant orbitals along the ferromagnetic y direction with an overall
dominance of the kinetic energy gain. We indicate that iron superconductors are
close to the boundary between the itinerant and the orbital differentiated
regimes and that it could be possible to cross this boundary with doping.Comment: 6 pages, including 7 figures. As accepted in Phys. Rev.
Optical conductivity and Raman scattering of iron superconductors
We discuss how to analyze the optical conductivity and Raman spectra of
multi-orbital systems using the velocity and the Raman vertices in a similar
way Raman vertices were used to disentangle nodal and antinodal regions in
cuprates. We apply this method to iron superconductors in the magnetic and
non-magnetic states, studied at the mean field level. We find that the
anisotropy in the optical conductivity at low frequencies reflects the
difference between the magnetic gaps at the X and Y electron pockets. Both gaps
are sampled by Raman spectroscopy. We also show that the Drude weight
anisotropy in the magnetic state is sensitive to small changes in the lattice
structure.Comment: 14 pages, 10 figures, as accepted in Phys. Rev. B,
explanations/discussion added in Secs. II, III and V
Non-resonant Raman response of inhomogeneous structures in the electron doped Hubbard model
We calculate the non-resonant Raman response, the single particle spectra and
the charge-spin configuration for the electron doped Hubbard model using
unrestricted Hartree-Fock calculations. We discuss the similarities and
differences in the response of homogeneous versus inhomogeneous structures.
Metallic antiferromagnetism dominates in a large region of the phase
diagram but at high values of the on-site interaction and for intermediate
doping values, inhomogeneous configurations are found with lower energy. This
result is in contrast with the case of hole doped cuprates where
inhomogeneities are found already at very low doping. The inhomogeneities found
are in-phase stripes compatible with inelastic neutron scattering experiments.
They give an incoherent background in the Raman response. The signal
can show a quasiparticle-like component even when no Fermi surface is found in
the nodal direction.Comment: 8 pages, 10 figures, accepted for publication in Phys. Rev.
Understanding the spiral structure of the Milky Way using the local kinematic groups
We study the spiral arm influence on the solar neighbourhood stellar
kinematics. As the nature of the Milky Way (MW) spiral arms is not completely
determined, we study two models: the Tight-Winding Approximation (TWA) model,
which represents a local approximation, and a model with self-consistent
material arms named PERLAS. This is a mass distribution with more abrupt
gravitational forces. We perform test particle simulations after tuning the two
models to the observational range for the MW spiral arm properties. We explore
the effects of the arm properties and find that a significant region of the
allowed parameter space favours the appearance of kinematic groups. The
velocity distribution is mostly sensitive to the relative spiral arm phase and
pattern speed. In all cases the arms induce strong kinematic imprints for
pattern speeds around 17 km/s/kpc (close to the 4:1 inner resonance) but no
substructure is induced close to corotation. The groups change significantly if
one moves only ~0.6 kpc in galactocentric radius, but ~2 kpc in azimuth. The
appearance time of each group is different, ranging from 0 to more than 1 Gyr.
Recent spiral arms can produce strong kinematic structures. The stellar
response to the two potential models is significantly different near the Sun,
both in density and kinematics. The PERLAS model triggers more substructure for
a larger range of pattern speed values. The kinematic groups can be used to
reduce the current uncertainty about the MW spiral structure and to test
whether this follows the TWA. However, groups such as the observed ones in the
solar vicinity can be reproduced by different parameter combinations. Data from
velocity distributions at larger distances are needed for a definitive
constraint.Comment: 18 pages, 21 figures, 4 tables; acccepted for publication in MNRA
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