arXiv subjects

J. J. Rodriguez

Publications and source records attributed to J. J. Rodriguez.

3 recordsLinked to original sources

arXiv2022

First measurements and upgrade plans of the MAGIC intensity interferometer

The two MAGIC 17-m diameter Imaging Atmospheric Cherenkov Telescopes have been equipped to work also as an intensity interferometer with a deadtime-free, 4-channel, GPU-based, real-time correlator. Operating with baselines between approx. 40 and 90 m the MAGIC interferometer is able to measure stellar diameters of 0.5-1 mas in the 400-440 nm wavelength range with a sensitivity roughly 10 times better than that achieved in the 1970s by the Narrabri Stellar Intensity Interferometer. Besides, active mirror control allows to split the primary mirrors into sub-mirrors. This allows to make simultaneous calibration measurements of the zero-baseline correlation or to simultaneously collect six baselines below 17 m with almost arbitrary orientation, corresponding to angular scales of approx. 1-50 mas. We plan to perform test observations adding the nearby Cherenkov Telescope Array (CTA) LST-1 23 m diameter telescope by next year. All three telescope pairs will be correlated simultaneously. Adding LST-1 is expected to increase the sensitivity by at least 1 mag and significantly improve the u-v plane coverage. If successful, the proposed correlator setup is scalable enough to be implemented to the full CTA arrays.

astro-ph.IM↗

arXiv1997

Double fluctuations on the attractive Hubbard model: ladder approximation

We explore, for the first time, the effect of double fluctuations on both the diagonal and off-diagonal self-energy. We use the T-Matrix equations below $T_c$, developed recently by the Zürich group (M.H. Pedersen et al) for the local pair attraction Hamiltonian. Here, we include as well the effect of fluctuations on the order parameter (beyond the BCS solution) up to second order in $U/t$. This is equivalent to approximating the effective interaction by $U$ in the off-diagonal self-energy. For $U/t = -6.0$, $T/t = 0.05$, $μ/t = - 5.5$ and $Δ/t = 1.5$, we find four peaks both for the diagonal, $A(n(π/16,π/16),ω)$, and off-diagonal, $B(n(π/16,π/16),ω)$, spectral functions. These peaks are not symmetric in pairs as previously found. In addition: (a) in $A(n(π/16,π/16),ω)$, the far left peak has a vanishing small weight; (b) in $B(n(π/16,π/16),ω)$ the far left and far right peaks have very small weights. The physical picture is, then, that the pair physics in the normal phase ($T > T_c$) is still valid below $T_c$. However, the condensation of the e-h pairs produces an additional gap around the chemical potential as in BCS, in other words, superconductivity opens a gap in the lower branch of a Hubbard-type-I solution.

cond-mat.str-el↗

arXiv1996

Some Global Properties of the Attractive Hubbard Model in the Superconducting Phase: T-Matrix Approximation in 2D

We have applied the Fast Fourier transform (FFT), which allows to compute efficiently convolution sums, to solve the set of self-consistent T-matrix equations to get the Green function of the two dimensional attractive-U Hubbard modelbelow $T_c$, extending previous calculations of the same authors. Using a constant order parameter $Δ(T)$, we calculated $T_c$ as a function of electron density and interaction strength $U$. These global results deviate from the BCS behavior remarkably.

cond-mat.supr-con↗