5,728 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
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.
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
Slow light in molecular aggregates nanofilms
We study slow light performance of molecular aggregates arranged in nanofilms
by means of coherent population oscillations (CPO). The molecular cooperative
behavior inside the aggregate enhances the delay of input signals in the GHz
range in comparison with other CPO-based devices. Moreover, the problem of
residual absorption present in CPO processes, is removed. We also propose an
optical switch between different delays by exploiting the optical bistability
of these aggregates.Comment: 4 pages, 4 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
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
Nonleptonic two-body B-decays including axial-vector mesons in the final state
We present a systematic study of exclusive charmless nonleptonic two-body B
decays including axial-vector mesons in the final state. We calculate branching
ratios of B\to PA, VA and AA decays, where A, V and P denote an axial-vector, a
vector and a pseudoscalar meson, respectively. We assume naive factorization
hypothesis and use the improved version of the nonrelativistic ISGW quark model
for form factors in B\to A transitions. We include contributions that arise
from the effective \Delta B=1 weak Hamiltonian H_{eff}. The respective
factorized amplitude of these decays are explicitly showed and their penguin
contributions are classified. We find that decays B^-to a_1^0\pi^-,\barB^0\to
a_1^{\pm}\pi^{\mp}, B^-\to a_1^-\bar K^0, \bar B^0\to a_1^+K^-, \bar B^0\to
f_1\bar K^0, B^-\to f_1K^-, B^-\to K_1^-(1400)\etap, B^-\to b_1^-\bar K^{0},
and \bar B^0\to b_1^+\pi^-(K^-) have branching ratios of the order of 10^{-5}.
We also study the dependence of branching ratios for B \to K_1P(V,A) decays
(K_1=K_1(1270),K_1(1400)) with respect to the mixing angle between K_A and K_B.Comment: 28 pages, 2 tables and one reference added, notation changed in
appendices, some numerical results and abstract correcte
Effect of strain on the orbital and magnetic ordering of manganite thin films and their interface with an insulator
We study the effect of uniform uniaxial strain on the ground state electronic
configuration of a thin film manganite. Our model Hamiltonian includes the
double-exchange, the Jahn-Teller electron-lattice coupling, and the
antiferromagnetic superexchange. The strain arises due to the lattice mismatch
between an insulating substrate and a manganite which produces a tetragonal
distortion. This is included in the model via a modification of the hopping
amplitude and the introduction of an energy splitting between the Mn e_g
levels. We analyze the bulk properties of half-doped manganites and the
electronic reconstruction at the interface between a ferromagnetic and metallic
manganite and the insulating substrate. The strain drives an orbital selection
modifying the electronic properties and the magnetic ordering of manganites and
their interfaces.Comment: 8 pages, 8 figure
Magnetic field-assisted manipulation and entanglement of Si spin qubits
Architectures of donor-electron based qubits in silicon near an oxide
interface are considered theoretically. We find that the precondition for
reliable logic and read-out operations, namely the individual identification of
each donor-bound electron near the interface, may be accomplished by
fine-tuning electric and magnetic fields, both applied perpendicularly to the
interface. We argue that such magnetic fields may also be valuable in
controlling two-qubit entanglement via donor electron pairs near the interface.Comment: 4 pages, 4 figures. 1 ref and 1 footnote adde
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