Víctor Velázquez

Backbending in Dy isotopes within the Projected Shell Model

Abstract A systematic study of the yrast band in 154–164 Dy isotopes using the Projected Shell Model is presented. It is shown that, in the context of the present model, enlarging the mean field deformation by about 20% allows a very good description of the spectrum of the yrast band in these isotopes. The dependence of the B (E2) values on angular momentum is also better described when larger deformations are used. The observed oscillation of g -factors at low spin states remains an open question for this model.

Quantum chaos and nuclear mass systematics

Abstract The presence of quantum chaos in nuclear mass systematics is analyzed by considering the differences between measured and calculated nuclear masses as a time series described by the power law 1 f α . While for the liquid droplet model plus shell corrections a quantum chaotic behavior α ≈ 1 is found, errors in the microscopic mass formula have α ≈ 0.5 , closer to white noise. The chaotic behavior seems to arise from many body effects not included in the mass formula.

Scale invariance as a symmetry in physical and biological systems: listening to photons, bubbles and heartbeats

Many dynamical systems from different areas of knowledge can be studied within the theoretical framework of time series, where the system can be considered as a black box, that only needs to be “listened” to. In this framework, non‐correlated series (white noise) and strongly correlated series (brownian noise or periodic series) constitute two extremes. Certain dynamical systems auto‐organize in a critical state that is characterized by 1/f or flicker noise. The family of fβ noises (β≤0) is fractal because fragments of the series are statistically identical to the original time series. 1/f noise (β = −1) is critical because it maximizes important complexity‐related quantities as memory, information content, efficiency and fractality. 1/f noise has been observed in classical systems, but also in quantum systems, and could possibly offer a unifying bridge of understanding between the macroscopic and the quantum world. In the present article, we will discuss some examples from both worlds.

Utilization of a digital-versatile-disc pickup head for benchtop laser microfabrication

Laser microfabrication is now offering interesting solutions to rapidly produce high-resolution photomasks or microstructures. However, most works require expensive commercial lasers and computer numerical control platforms, limiting its use by a large public. In this paper, we report the construction of a simple, custom-made, easily reproducible, automated laser system, based on a DVD optical pickup head. A user-friendly computer interface specifically designed to operate a motorized three-axis platform with micrometric precision controls focus distance and in-plane displacements. Writing performance characterization for both direct ablation and sintering of commercial black toner demonstrated flexibility in tridimensional microfabrication resolution and speed thanks to precise management of laser power and exposure time, with a minimal resolution of 3.1 μm.

Bounds on the presence of quantum chaos in nuclear masses

Abstract.Differences between measured nuclear masses and those calculated using the Finite-Range Droplet Model are analyzed. It is shown that they have a well defined, clearly correlated oscillatory component as a function of the proton and neutron numbers. At the same time, they exhibit in their power spectrum the presence of chaos. Comparison with other mass calculations strongly suggest that this chaotic component arises from many body effects not included in the mass formula, and that they do not impose limits in the precision of mass calculations.

Self-consistency in the projected shell model

Abstract The projected shell model is a shell-model theory built up over a deformed BCS mean field. Ground state and excited bands in even-even nuclei are obtained through diagonalization of a pairing plus quadrupole Hamiltonian in an angular momentum projected 0-, 2-, and 4-quasiparticle basis. The residual quadrupole-quadrupole interaction strength is fixed self-consistently with the deformed mean field and the pairing constants are the same used in constructing the quasiparticle basis. Taking 160Dy as an example, we calculate low-lying states and compare them with experimental data. We exhibit the effect of changing the residual interaction strengths on the spectra. It is clearly seen that there are many Jπ = 0+, 1+, 4+ bandheads whose energies can only be reproduced using the self-consistent strengths. It is thus concluded that the projected shell model is a model with essentially no free parameters. The predicted energy of the 2+ bandhead lies however in nearly twice the experimental value.

Rapid fabrication of on-demand high-resolution optical masks with a CD-DVD pickup unit

A low-cost, direct fabrication technique with a micrometer range resolution has been implemented for rapid prototyping of optical masks for photolithography and structured light and diffraction optics applications. Using a setup based on the optical unit of a compact disc-digital versatile disc burner, a low-energy infrared laser beam was focused on a thin polymeric layer with embedded absorbing carbon nanopowder coated on a transparent glass substrate. This allowed for the generation of a custom-made transparent pattern in a computer numerical control fashion. In addition to its great simplicity and repeatability, the method also enables grayscale contrasts for each pixel individually, and fabricated masks proved to resist high intensities.

Microscopic study of neutron-rich dysprosium isotopes

Microscopic studies in heavy nuclei are very scarce due to large valence spaces involved. This computational problem can be avoided by means of the use of symmetry-based models. Ground-state, γ and β bands, and their B(E2) transition strengths in 160–168Dy isotopes, are studied in the framework of the pseudo-SU(3) model which includes the preserving symmetry Q · Q term and the symmetry-breaking Nilsson and pairing terms, systematically parametrized. Additionally, three rotor-like terms are considered, whose free parameters, fixed for all members of the chain, are used to fine tune the moment of inertia of rotational bands and the band head of γ and β bands. The model succesfully describes in a systematic way rotational features in these nuclei and allows to extrapolate toward the midshell nucleus 170Dy. The results presented show that it is possible to study a full chain of isotopes or isotones in the region with the present model.

The Hong–Ou–Mandel interferometer in the undergraduate laboratory

The Hong?Ou?Mandel interferometer is an optical device that allows us to prove the quantum nature of light experimentally via the quantum amplitude superposition of two indistinguishable photons. We have implemented this experiment as an advanced undergraduate laboratory experience. We were able to overcome well-known difficulties using techniques reported recently by Thomas et al (2009 Rev. Sci. Instrum. 80 036101).

Nuclear mass forecasting: can observed pattern determine mass values?

Theoretical prediction of nuclear masses is analyzed as a pattern recognition problem on the N‐Z plane. A global pattern is observed by plotting the differences between measured masses and Liquid Drop Model (LDM) predictions. After unfolding the data by removing the smooth LDM mass contributions, the remaining microscopic effects have proved difficult to model, although they display a striking pattern. These deviations carry information related to shell closures, nuclear deformation and the residual nuclear interactions. In the present work the more than 2000 known nuclear masses are studied as an array in the N‐Z plane viewed through a mask, behind which the approximately 7000 unknown unstable nuclei that can exist between the proton and neutron drip lines are hidden. Employing a Fourier transform deconvolution method these masses can be predicted. Measured masses are reconstructed with and r.m.s. error of less than 200 keV. The existence of an island of stability around (Z≈ 116, N≈ 194) is strongly sugg...