274,335 research outputs found
The extended Hubbard model applied to phase diagram and the pressure effects in \Bi superconductors
We use the two dimensional extended Hubbard Hamiltonian with the position of
the attractive potential as a variable parameter with a BCS type approach to
study the interplay between the superconductor transition temperature and
hole content for high temperature superconductors. This novel method gives some
insight on the range and intensity of the Cooper pair interaction and why
different compounds have different values for their measured coherence lengths
and it describes well the experimental results of the superconducting phase
diagram . The calculations may also be used to study the effect
of the applied pressure with the assumption that it increases the attractive
potential which is accompanied by an increase in the superconductor gap. In
this way we obtain a microscopic interpretation for the intrinsic term and a
general expansion for in terms of the pressure which reproduces well the
experimental measurements on the \Bi superconductors.Comment: 11 pags in RevTex, 5 fi2s. in eps, accepted in Braz. J. of Physic
Theory of the Fermi Arcs, the Pseudogap, and the Anisotropy in k-space of Cuprate Superconductors
The appearance of the Fermi arcs or gapless regions at the nodes of the Fermi
surface just above the critical temperature is described through
self-consistent calculations in an electronic disordered medium. We develop a
model for cuprate superconductors based on an array of Josephson junctions
formed by grains of inhomogeneous electronic density derived from a phase
separation transition. This approach provides physical insights to the most
important properties of these materials like the pseudogap phase as forming by
the onset of local (intragrain) superconducting amplitudes and the zero
resistivity critical temperature due to phase coherence activated by
Josephson coupling. The formation of the Fermi arcs and the dichotomy in
k-space follows from the direction dependence of the junctions tunneling
current on the d-wave symmetry on the planes. We show that this
semi-phenomenological approach reproduces also the main future of the cuprates
phase diagram.Comment: 5 pages 7 fig
Nonsequential Double Ionization with Polarization-gated Pulses
We investigate laser-induced nonsequential double ionization by a
polarization-gated laser pulse, constructed employing two counter-rotating
circularly polarized few cycle pulses with a time delay . We address the
problem within a classical framework, and mimic the behavior of the
quantum-mechanical electronic wave packet by means of an ensemble of classical
electron trajectories. These trajectories are initially weighted with the
quasi-static tunneling rate, and with suitably chosen distributions for the
momentum components parallel and perpendicular to the laser-field polarization,
in the temporal region for which it is nearly linearly polarized. We show that,
if the time delay is of the order of the pulse length, the
electron-momentum distributions, as functions of the parallel momentum
components, are highly asymmetric and dependent on the carrier-envelope (CE)
phase. As this delay is decreased, this asymmetry gradually vanishes. We
explain this behavior in terms of the available phase space, the quasi-static
tunneling rate and the recollision rate for the first electron, for different
sets of trajectories. Our results show that polarization-gating technique may
provide an efficient way to study the NSDI dynamics in the single-cycle limit,
without employing few-cycle pulses.Comment: 17 pages, 6 figure
Effective three-band structure in Fe-based superconductors
We present self-consistent calculations of the multi-gap structure measured
in some Fe-based superconductors. These materials are known to have structural
disorder in real space and a multi-gap structure due to the Fe-orbitals
contributing to a complex Fermi surface topology with hole and electron
pockets. Different experiments identify three s-wave like superconducting gaps
with a single critical temperature (). We investigate the temperature
dependence of these gaps by a multi-band Bogoliubov-deGennes theory at
different pockets in the presence of effective hybridizations between some
bands and an attractive temperature dependent intra-band interaction. We show
that this approach reproduces the three observed gaps and single in
different compounds of BaKFeAs, providing some insights
on the inter-band interactions
Room temperature GW bar detector with opto-mechanical readout
We present the full implementation of a room-temperature gravitational wave
bar detector equipped with an opto-mechanical readout. The mechanical
vibrations are read by a Fabry--Perot interferometer whose length changes are
compared with a stable reference optical cavity by means of a resonant laser.
The detector performance is completely characterized in terms of spectral
sensitivity and statistical properties of the fluctuations in the system output
signal. The new kind of readout technique allows for wide-band detection
sensitivity and we can accurately test the model of the coupled oscillators for
thermal noise. Our results are very promising in view of cryogenic operation
and represent an important step towards significant improvements in the
performance of massive gravitational wave detectors.Comment: 7 figures, submitted to Phys. Rev.
Electronic Phase Separation Transition as the Origin of the Superconductivity and the Pseudogap Phase of Cuprates
We propose a new phase of matter, an electronic phase separation transition
that starts near the upper pseudogap and segregates the holes into high and low
density domains. The Cahn-Hilliard approach is used to follow quantitatively
this second order transition. The resulting grain boundary potential confines
the charge in domains and favors the development of intragrain superconducting
amplitudes. The zero resistivity transition arises only when the intergrain
Josephson coupling is of the order of the thermal energy and phase
locking among the superconducting grains takes place. We show that this
approach explains the pseudogap and superconducting phases in a natural way and
reproduces some recent scanning tunneling microscopy dataComment: 4 pages and 5 eps fig
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