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A. Alvarez

Publications and source records attributed to A. Alvarez.

17 recordsLinked to original sources
arXiv2026

Experiments towards a neutron target for measurements in inverse kinematics

Neutron-induced reactions play an important role in fundamental nuclear physics, nuclear astrophysics, and applications. In the case of reactions on rare isotopes, there are limited options for direct experimental measurements. The Neutron Target Demonstrator project at Los Alamos National Laboratory seeks to test the feasibility of moderating spallation neutrons within a 1~m$^3$ graphite cube to create a standing neutron target for neutron-induced reaction measurements in inverse kinematics. This paper presents the results of experimental neutron flux distribution tests using neutron sources (ranging from 1~keV to 50~MeV) created by accelerators at the University of Notre Dame and Texas A\&M University. Measurements were made with both the full graphite cube as well as a ''half cube'' setup in which half of the graphite cube was removed. The measured distributions agree with simulated distributions in the case of the full cube moderator, although there remain discrepancies in certain cases for the half cube moderator. The results shown here will provide useful information for an upcoming experimental campaign to test the neutron target proof-of-principle.

physics.ins-det↗
arXiv2023

Nuclear Recoil Identification in a Scientific Charge-Coupled Device

Charge-coupled devices (CCDs) are a leading technology in direct dark matter searches because of their eV-scale energy threshold and high spatial resolution. The sensitivity of future CCD experiments could be enhanced by distinguishing nuclear recoil signals from electronic recoil backgrounds in the CCD silicon target. We present a technique for event-by-event identification of nuclear recoils based on the spatial correlation between the primary ionization event and the lattice defect left behind by the recoiling atom, later identified as a localized excess of leakage current under thermal stimulation. By irradiating a CCD with an $^{241}$Am$^{9}$Be neutron source, we demonstrate $>93\%$ identification efficiency for nuclear recoils with energies $>150$ keV, where the ionization events were confirmed to be nuclear recoils from topology. The technique remains fully efficient down to 90 keV, decreasing to 50$\%$ at 8 keV, and reaching ($6\pm2$)$\%$ at 1.5--3.5 keV. Irradiation with a $^{24}$Na $\gamma$-ray source shows no evidence of defect generation by electronic recoils, with the fraction of electronic recoils with energies $<85$ keV that are spatially correlated with defects $<0.1$$\%$.

physics.ins-det↗
arXiv2023

Gravitationally decoupled Non-Schwarzschild black holes and wormhole space-times

In this article, using gravitational decoupling by means of minimal geometric deformation approach, we obtain a new spherically symmetric and static black hole solution. To progress, we close the system by assuming that the average pressure of the $\theta$-sector is vanishing. Also, we tackle the problem regarding how, for a given minimally deformed black hole solution, one can connect it to a wormhole space-time.

gr-qc↗
arXiv2023

Accommodating muon $\boldsymbol{(g-2)}$ and leptogenesis in a scotogenic model

We present a detailed study of a scotogenic model accommodating dark matter, neutrino masses and the anomalous magnetic moment of the muon while being consistent with the existing constraints on flavour violating decays of the leptons. Moreover, this model offers the possibility to explain the baryon asymmetry of the Universe via leptogenesis. We determine the viable regions of the model's parameter space in view of dark matter and flavour constraints using a Markov Chain Monte Carlo setup combined with a particular procedure to accommodate neutrino masses and the anomalous magnetic moment of the muon at the same time. We also discuss briefly the resulting collider phenomenology.

hep-ph↗
arXiv2022

Minimally deformed wormholes inspired by noncommutative geometry

In this article, new wormhole solutions in the framework of General Relativity are presented. Taking advantage of gravitational decoupling by means of minimal geometric deformation approach and, the so-called noncommutative geometry Gaussian and Lorentzian density profiles, the seminal Morris-Thorne space-time is minimally deformed providing new asymptotically wormhole solutions. Constraining the signature of some parameters, the dimensionless constant $\alpha$ is bounded using the flare-out and energy conditions. In both cases, this results in an energy-momentum tensor that violates energy conditions, thus the space-time is threading by exotic matter. However, it is possible to obtain a positive defined density at the wormhole throat and its neighborhood. To further support the study a thoroughly graphical analysis has been performed.

gr-qc↗
arXiv2021

f(R) wormholes embedded in a pseudo--Euclidean space $E^{5}$

This work is devoted to the study of analytic wormhole solutions within the framework of $f(R)$ gravity theory. To check the possibility of having wormhole structures satisfying energy conditions, by means of the class I approach the pair $\{\Phi(r), b(r)\}$ describing the wormhole geometry has been obtained. Then, in conjunction with a remarkably $f(R)$ gravity model, the satisfaction of the null and weak energy conditions at the wormhole throat and its neighborhood is investigated. To do so, some constant parameters have been bounded restricting the space parameter. In this concern, the $f(R)$ gravity model and its derivatives are playing a major role, specially in considering the violation of the non--existence theorem. Furthermore, the shape function should be bounded from above by the Gronwall--Bellman shape function, where the red--shift function plays a relevant role. By analyzing the main properties at the spatial stations and tidal accelerations at the wormhole throat, possibilities and conditions for human travel are explored.

gr-qc↗
arXiv2020

Zinc-Blende group III-V/group IV epitaxy: importance of the miscut

Here, we clarify the central role of the miscut during group III-V/ group IV crystal growth. We show that the miscut first impacts the initial antiphase domain distribution, with two distinct nucleation-driven and terraces-driven regimes. It is then inferred how the antiphase domain distribution mean phase and mean lateral length are affected by the miscut. An experimental confirmation is given through the comparison of antiphase domain distributions in GaP and GaSb/AlSb samples grown on nominal and vicinal Si substrates. The antiphase domain burying step of GaP/Si samples is then observed at the atomic scale by scanning tunneling microscopy. The steps arising from the miscut allow growth rate imbalance between the two phases of the crystal and the growth conditions can deeply modify the imbalance coefficient, as illustrated with GaAs/Si. We finally explain how a monodomain III-V semiconductor configuration can be achieved even on low miscut substrates.

cond-mat.mtrl-sci↗
arXiv2019

Overview of the DESI Legacy Imaging Surveys

The DESI Legacy Imaging Surveys are a combination of three public projects (the Dark Energy Camera Legacy Survey, the Beijing-Arizona Sky Survey, and the Mayall z-band Legacy Survey) that will jointly image approximately 14,000 deg^2 of the extragalactic sky visible from the northern hemisphere in three optical bands (g, r, and z) using telescopes at the Kitt Peak National Observatory and the Cerro Tololo Inter-American Observatory. The combined survey footprint is split into two contiguous areas by the Galactic plane. The optical imaging is conducted using a unique strategy of dynamically adjusting the exposure times and pointing selection during observing that results in a survey of nearly uniform depth. In addition to calibrated images, the project is delivering a catalog, constructed by using a probabilistic inference-based approach to estimate source shapes and brightnesses. The catalog includes photometry from the grz optical bands and from four mid-infrared bands (at 3.4, 4.6, 12 and 22 micorons) observed by the Wide-field Infrared Survey Explorer (WISE) satellite during its full operational lifetime. The project plans two public data releases each year. All the software used to generate the catalogs is also released with the data. This paper provides an overview of the Legacy Surveys project.

astro-ph.IM↗
arXiv2010

Dark-Bright Discrete Solitons: A Numerical Study of Existence, Stability and Dynamics

In the present work, we numerically explore the existence and stability properties of different types of configurations of dark-bright solitons, dark-bright soliton pairs and pairs of dark-bright and dark solitons in discrete settings, starting from the anti-continuum limit. We find that while single discrete dark-bright solitons have similar stability properties to discrete dark solitons, their pairs may only be stable if the bright components are in phase and are always unstable if the bright components are out of phase. Pairs of dark-bright solitons with dark ones have similar stability properties as individual dark or dark-bright ones. Lastly, we consider collisions between dark-bright solitons and between a dark-bright one and a dark one. Especially in the latter and in the regime where the underlying lattice structure matters, we find a wide range of potential dynamical outcomes depending on the initial soliton speed.

nlin.PS↗
arXiv2010

Estimation of the instantaneous volatility

This paper is concerned with the estimation of the volatility process in a stochastic volatility model of the following form: $dX_t=a_tdt+σ_tdW_t$, where $X$ denotes the log-price and $σ$ is a càdlàg semi-martingale. In the spirit of a series of recent works on the estimation of the cumulated volatility, we here focus on the instantaneous volatility for which we study estimators built as finite differences of the \textit{power variations} of the log-price. We provide central limit theorems with an optimal rate depending on the local behavior of $σ$. In particular, these theorems yield some confidence intervals for $σ_t$.

q-fin.ST↗
arXiv2007

Discrete moving breather collisions in a Klein-Gordon chain of oscillators

We study collision processes of moving breathers with the same frequency, traveling with opposite directions within a Klein-Gordon chain of oscillators. Two types of collisions have been analyzed: symmetric and non-symmetric, head-on collisions. For low enough frequency the outcome is strongly dependent of the dynamical states of the two colliding breathers just before the collision. For symmetric collisions, several results can be observed: breather generation, with the formation of a trapped breather and two new moving breathers; breather reflection; generation of two new moving breathers; and breather fusion bringing about a trapped breather. For non-symmetric collisions the possible results are: breather generation, with the formation of three new moving breathers; breather fusion, originating a new moving breather; breather trapping with also breather reflection; generation of two new moving breathers; and two new moving breathers traveling as a ligand state. Breather annihilation has never been observed.

nlin.PS↗
arXiv2006

Breather trapping and breather transmission in a DNA model with an interface

We study the dynamics of moving discrete breathers in an interfaced piecewise DNA molecule. This is a DNA chain in which all the base pairs are identical and there exists an interface such that the base pairs dipole moments at each side are oriented in opposite directions. The Hamiltonian of the Peyrard--Bishop model is augmented with a term that includes the dipole--dipole coupling between base pairs. Numerical simulations show the existence of two dynamical regimes. If the translational kinetic energy of a moving breather launched towards the interface is below a critical value, it is trapped in a region around the interface collecting vibrational energy. For an energy larger than the critical value, the breather is transmitted and continues travelling along the double strand with lower velocity. Reflection phenomena never occur. The same study has been carried out when a single dipole is oriented in opposite direction to the other ones. When moving breathers collide with the single inverted dipole, the same effects appear. These results emphasize the importance of this simple type of local inhomogeneity as it creates a mechanism for the trapping of energy. Finally, the simulations show that, under favorable conditions, several launched moving breathers can be trapped successively at the interface region producing an accumulation of vibrational energy. Moreover, an additional colliding moving breather can produce a saturation of energy and a moving breather with all the accumulated energy is transmitted to the chain.

nlin.PS↗
arXiv2005

Classical and quantum nonlinear localized excitations in discrete systems

Discrete breathers, or intrinsic localized modes, are spatially localized, time--periodic, nonlinear excitations that can exist and propagate in systems of coupled dynamical units. Recently, some experiments show the sighting of a form of discrete breather that exist at the atomic scale in a magnetic solid. Other observations of breathers refer to systems such as Josephson--junction arrays, photonic crystals and optical-switching waveguide arrays. All these observations underscore their importance in physical phenomena at all scales. The authors review some of their latest theoretical contributions in the field of classical and quantum breathers, with possible applications to these widely different physical systems and to many other such as DNA, proteins, quantum dots, quantum computing, etc.

nlin.PS↗
arXiv2003

Demonstration of the stability or instability of multibreathers at low coupling

Whereas there exists a mathematical proof for one--site breathers stability, and an unpublished one for two--sites breathers, the methods for determining the stability properties of multibreathers rely in numerical computation of the Floquet multipliers or in the weak nonlinearity approximation leading to discrete non--linear Schrödinger equations. Here we present a set of multibreather stability theorems (MST) that provides with a simple method to determine multibreathers stability in Klein--Gordon systems. These theorems are based in the application of degenerate perturbation theory to Aubry's band theory. We illustrate them with several examples.

nlin.PS↗
arXiv2002

Dark breathers in Klein-Gordon lattices. Band analysis of their stability properties

Discrete bright breathers are well known phenomena. They are localized excitations that consist of a few excited oscillators in a lattice and the rest of them having very small amplitude or none. In this paper we are interested in the opposite kind of localization, or discrete dark breathers, where most of the oscillators are excited and one or a few units of them have very small amplitude. We investigate using band analysis, Klein--Gordon lattices at frequencies not close to the linear ones. Dark breathers at low coupling are shown to be stable for Klein--Gordon chains with soft on--site potentials and repulsive dispersive interaction, and with hard on--site potentials and attractive dispersive interactions. At higher coupling dark breathers lose their stability via subharmonic, harmonic or oscillatory bifurcations, depending on the model. However, most of these bifurcations are harmless in the sense that they preserve dark localization. None of these bifurcations disappear when the system is infinite. Dark breathers in dissipative systems are found to be stable for both kinds of dispersive interaction.

nlin.PS↗
arXiv1999

Noise-induced flow in quasigeostrophic turbulence with bottom friction

Randomly-forced fluid flow in the presence of scale-unselective dissipation develops mean currents following topographic contours. Known mechanisms based on the scale-selective action of damping processes are not at work in this situation. Coarse-graining reveals that the phenomenon is a kind of noise-rectification mechanism, in which lack of detailed balance and the symmetry-breaking provided by topography play an important role.

chao-dyn↗