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 $T_c$ 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 $T_c \times n$. 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 $T_c$ 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, TcT_c 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 $T_c$ 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 $CuO_2$ 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 $T_{d}$. 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 $T_{d}$ 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 $3d$ 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 ($T_c$). 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 $T_c$ in different compounds of Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$, 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 $E_J$ 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