5,170 research outputs found
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
Electron gas at the interface between two antiferromagnetic insulating manganites
We study theoretically the magnetic and electric properties of the interface
between two antiferromagnetic and insulating manganites: La0.5Ca0.5MnO3, a
strong correlated insulator, and CaMnO3, a band-insulator. We find that a
ferromagnetic and metallic electron gas is formed at the interface between the
two layers. We confirm the metallic character of the interface by calculating
the in-plane conductance. The possibility of increasing the electron gas
density by selective doping is also discussed.Comment: 6 pages, including 9 figure
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.
Fourier Decomposition of RR Lyrae light curves and the SX Phe population in the central region of NGC 3201
CCD time-series observations of the central region of the globular cluster
NGC~3201 were obtained with the aim of performing the Fourier decomposition of
the light curves of the RR~Lyrae stars present in that field. This procedure
gave the mean values, for the metallicity, of [Fe/H]
(statistical) (systematical), and for the distance, ~kpc (statistical) (systematical). The values found from two
RRc stars are consistent with those derived previously. The differential
reddening of the cluster was investigated and individual reddenings for the RR
Lyrae stars were estimated from their curves. We found an average value
of . An investigation of the light curves of stars in
the {\it blue stragglers} region led to the discovery of three new SX~Phe
stars. The period-luminosity relation of the SX~Phe stars was used for an
independent determination of the distance to the cluster and of the individual
reddenings. We found a distance of 5.0 kpcComment: To appear in Revista Mexicana de Astronom\'ia y Astrof\'isica,
Octuber 2014 issue, Vol 50. 17 pages, 10 figure
Re-entrant ferromagnetism in a generic class of diluted magnetic semiconductors
Considering a general situation where a semiconductor is doped by magnetic
impurities leading to a carrier-induced ferromagnetic exchange coupling between
the impurity moments, we show theoretically the possible generic existence of
three ferromagnetic transition temperatures, T_1 > T_2 > T_3, with two distinct
ferromagnetic regimes existing for T_1 > T > T_2 and T < T_3. Such an
intriguing re-entrant ferromagnetism, with a paramagnetic phase (T_2 > T > T_3)
between two ferromagnetic phases, arises from a subtle competition between
indirect exchange induced by thermally activated carriers in an otherwise empty
conduction band versus the exchange coupling existing in the impurity band due
to the bound carriers themselves. We comment on the possibility of observing
such a re-entrance phenomenon in diluted magnetic semiconductors and magnetic
oxides.Comment: 4 pages, 3 figure
On a Calderon-Zygmund commutator-type estimate
In this paper we extend a Calderon-Zygmund commutator-type estimate. This
estimate enables us to prove an embedding result concerning weighted function
spaces.Comment: 20 pages, no figure
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