arXiv subjects

A. Vazquez

Publications and source records attributed to A. Vazquez.

17 recordsLinked to original sources

arXiv2025

Stability and optical quality of "windmill"-formed 8CB liquid crystal films for replenishable plasma mirrors

Liquid crystal (LC) film plasma mirrors (PMs) based on 4-octyl-4'-cyanobiphenyl (8CB) are an enabling technology for reflecting high-fluence laser pulses. These freestanding LC films can achieve high optical quality and are well-suited for rep-rated applications, as motorized devices continuously replenish films over an aperture following each destructive laser shot. However, a systematic characterization of film quality as a function of seminal operating conditions had not yet been performed for the LC "windmill" version of the device, which aims to match the repetition rate of an existing "spinning disk" (SDI) version and the angular stability of the "linear slider" (LSTI) version. We determined the 8CB film quality using low-power wavefront measurements, and studied the film-to-film wavefront stability and formation reliability. The film-formation reliability of 8CB LC films demonstrated >97% formation success at 2.7 mm/s film-forming speeds, but decreased to 45% at 10.8 mm/s. These reliability numbers will inform future designs to reach Hz-level repetition rates and beyond. Depending on area-of-interest within the 10 mm diameter film, the added wavefront root-mean-squared (RMS) variation was as small as 12 nm for a 2 mm diameter region, and <50 nm for a 3 mm diameter region. Within the optimal 21-22 degrees C operating regime, pointing fluctuations remained at or below 0.5 mrad. With a maximum effective film formation frequency of approximately 0.25 Hz, these results establish windmill-formed 8CB films as promising candidates to pursue next-iteration improvements towards rep-rated plasma-mirror operation.

physics.optics↗

arXiv2010

Phase transitions in geometrothermodynamics

Using the formalism of geometrothermodynamics, we investigate the geometric properties of the equilibrium manifold for diverse thermodynamic systems. Starting from Legendre invariant metrics of the phase manifold, we derive thermodynamic metrics for the equilibrium manifold whose curvature becomes singular at those points where phase transitions of first and second order occur. We conclude that the thermodynamic curvature of the equilibrium manifold, as defined in geometrothermodynamics, can be used as a measure of thermodynamic interaction in diverse systems with two and three thermodynamic degrees of freedom.

gr-qc↗

arXiv2009

Thermodynamic systems as bosonic strings

We apply variational principles in the context of geometrothermodynamics. The thermodynamic phase space ${\cal T}$ and the space of equilibrium states ${\cal E}$ turn out to be described by Riemannian metrics which are invariant with respect to Legendre transformations and satisfy the differential equations following from the variation of a Nambu-Goto-like action. This implies that the volume element of ${\cal E}$ is an extremal and that ${\cal E}$ and ${\cal T}$ are related by an embedding harmonic map. We explore the physical meaning of geodesic curves in ${\cal E}$ as describing quasi-static processes that connect different equilibrium states. We present a Legendre invariant metric which is flat (curved) in the case of an ideal (van der Waals) gas and satisfies Nambu-Goto equations. The method is used to derive some new solutions which could represent particular thermodynamic systems.

hep-th↗

arXiv2008

Impact of interactions on human dynamics

Queueing theory has been recently proposed as a framework to model the heavy tailed statistics of human activity patterns. The main predictions are the existence of a power-law distribution for the interevent time of human actions and two decay exponents $α=1$ and $α=3/2$. Current models lack, however, a key aspect of human dynamics, i.e. several tasks require, or are determined by, interactions between individuals. Here we introduce a minimal queueing model of human dynamics that already takes into account human-human interactions. To achieve large scale simulations we obtain a coarse-grained version of the model, allowing us to reach large interevent times and reliable scaling exponents estimations. Using this we show that the interevent distribution of interacting tasks exhibit the scaling exponents $α=2$, 3/2 and a series of numerable values between 3/2 and 1. This work demonstrates that, within the context of queueing models of human dynamics, interactions change the exponent of the power-law distributed interevent times. Beyond the study of human dynamics, these results are relevant to systems where the event of interest consists of the simultaneous occurrence of two (or more) events.

physics.soc-ph↗

arXiv2005

Reply to Comment on "The origin of bursts and heavy tails in human dynamics"

Understanding human dynamics is of major scientific and practical importance and can be increasingly addressed in a quantitative fashion thanks to electronic records capturing various human activity patterns. The authors of Ref. [1] revisit the datasets studied in Ref. [2], making four technical observations. Some of the observations of Ref. [1] are based on the authors' unfamiliarity with the details of the data collection process and have little relevance to the findings of Ref. [2] and others are resolved in quantitative fashion by other authors [3].

physics.data-an↗

arXiv2005

Modeling bursts and heavy tails in human dynamics

Current models of human dynamics, used from risk assessment to communications, assume that human actions are randomly distributed in time and thus well approximated by Poisson processes. We provide direct evidence that for five human activity patterns the timing of individual human actions follow non-Poisson statistics, characterized by bursts of rapidly occurring events separated by long periods of inactivity. We show that the bursty nature of human behavior is a consequence of a decision based queuing process: when individuals execute tasks based on some perceived priority, the timing of the tasks will be heavy tailed, most tasks being rapidly executed, while a few experiencing very long waiting times. We discuss two queueing models that capture human activity. The first model assumes that there are no limitations on the number of tasks an individual can hadle at any time, predicting that the waiting time of the individual tasks follow a heavy tailed distribution with exponent alpha=3/2. The second model imposes limitations on the queue length, resulting in alpha=1. We provide empirical evidence supporting the relevance of these two models to human activity patterns. Finally, we discuss possible extension of the proposed queueing models and outline some future challenges in exploring the statistical mechanisms of human dynamics.

physics.soc-ph↗

arXiv2005

The topological relationship between the large-scale attributes and local interaction patterns of complex networks

Recent evidence indicates that the abundance of recurring elementary interaction patterns in complex networks, often called subgraphs or motifs, carry significant information about their function and overall organization. Yet, the underlying reasons for the variable quantity of different subgraph types, their propensity to form clusters, and their relationship with the networks' global organization remain poorly understood. Here we show that a network's large-scale topological organization and its local subgraph structure mutually define and predict each other, as confirmed by direct measurements in five well studied cellular networks. We also demonstrate the inherent existence of two distinct classes of subgraphs, and show that, in contrast to the low-density type II subgraphs, the highly abundant type I subgraphs cannot exist in isolation but must naturally aggregate into subgraph clusters. The identified topological framework may have important implications for our understanding of the origin and function of subgraphs in all complex networks.

cond-mat.dis-nn↗

arXiv2003

Global protein function prediction in protein-protein interaction networks

The determination of protein functions is one of the most challenging problems of the post-genomic era. The sequencing of entire genomes and the possibility to access gene's co-expression patterns has moved the attention from the study of single proteins or small complexes to that of the entire proteome. In this context, the search for reliable methods for proteins' function assignment is of uttermost importance. Previous approaches to deduce the unknown function of a class of proteins have exploited sequence similarities or clustering of co-regulated genes, phylogenetic profiles, protein-protein interactions, and protein complexes. We propose to assign functional classes to proteins from their network of physical interactions, by minimizing the number of interacting proteins with different categories. The function assignment is made on a global scale and depends on the entire connectivity pattern of the protein network. Multiple functional assignments are made possible as a consequence of the existence of multiple equivalent solutions. The method is applied to the yeast Saccharomices Cerevisiae protein-protein interaction network. Robustness is tested in presence of a high percentage of unclassified proteins and under deletion/insertion of interactions.

cond-mat.stat-mech↗

arXiv2002

Critical load and congestion instabilities in scale-free networks

We study the tolerance to congestion failures in communication networks with scale-free topology. The traffic load carried by each damaged element in the network must be partly or totally redistributed among the remaining elements. Overloaded elements might fail on their turn, triggering the occurrence of failure cascades able to isolate large parts of the network. We find a critical traffic load above which the probability of massive traffic congestions destroying the network communication capabilities is finite.

cond-mat.stat-mech↗

arXiv2002

Internet topology at the router and autonomous system level

We present a statistical analysis of different metrics characterizing the topological properties of Internet maps, collected at two different resolution scales: the router and the autonomous system level. The metrics we consider allow us to confirm the presence of scale-free signatures in several statistical distributions, as well as to show in a quantitative way the hierarchical nature of the Internet. Our findings are relevant for the development of more accurate Internet topology generators, which should include, along with the scale-free properties of the connectivity distribution, the hierarchical signatures unveiled in the present work.

cond-mat.dis-nn↗

arXiv2002

Levy Flights and Earthquakes

Levy flights representation is proposed to describe earthquake characteristics like the distribution of waiting times and position of hypocenters in a seismic region. Over 7500 microearthquakes and earthquakes from 1985 to 1994 were analyzed to test that its spatial and temporal distributions are such that can be described by a Levy flight with anomalous diffusion (in this case in a subdiffusive regime). Earthquake behavior is well described through Levy flights and Levy distribution functions such as results show.

cond-mat.soft↗

arXiv2002

Ferromagnetic ordering in graphs with arbitrary degree distribution

We present a detailed study of the phase diagram of the Ising model in random graphs with arbitrary degree distribution. By using the replica method we compute exactly the value of the critical temperature and the associated critical exponents as a function of the minimum and maximum degree, and the degree distribution characterizing the graph. As expected, there is a ferromagnetic transition provided <= < \infty. However, if the fourth moment of the degree distribution is not finite then non-trivial scaling exponents are obtained. These results are analyzed for the particular case of power-law distributed random graphs.

cond-mat.stat-mech↗

arXiv2001

Modeling of protein interaction networks

We introduce a graph generating model aimed at representing the evolution of protein interaction networks. The model is based on the hypotesis of evolution by duplications and divergence of the genes which produce proteins. The obtained graphs shows multifractal properties recovering the absence of a characteristic connectivity as found in real data of protein interaction networks. The error tolerance of the model to random or targeted damage is in very good agreement with the behavior obtained in real protein networks analysis. The proposed model is a first step in the identification of the evolutionary dynamics leading to the development of protein functions and interactions.

cond-mat.stat-mech↗

arXiv1999

Slow pulsars from the STScI/NAIC drift scan search

The unique sensitivity of the Arecibo telescope at 430 MHz motivated a drift scan search of the visible sky divided up into eight declination strips. Based on an analysis of the data collected in the region assigned to us, eight new long-period pulsars have so far been discovered.

astro-ph↗

arXiv1998

Dynamic percolation and Slow Relaxation in Glass-like Materials

Glass-like materials are nonequilibrium systems where the relaxation time may exceed reasonable time scales of observations. In the present paper a dynamic percolation model is introduced in order to explain the principal properties of glass-like materials near the dynamic transition. Here, contrary to conventional percolation problems, clusters are groups of particles dynamically correlated. Introducing a size dependent relaxation time and the scaling hypothesis for the distribution of dynamically correlated clusters the two step relaxation predicted by the mode coupling theory and observed in experiments is obtained.

cond-mat↗

arXiv1997

Transition to self-organized criticality in a rice-pile mode

The self-organized critical state is characterized by a power law distribution of cluster sizes and other properties. However experiments with sand and rice piles reveal distributions of avalanche sizes which are not power law distributed. In the present letter a one-dimensional rice-pile model which demonstrate that this is only an apparent contradiction is introduced. Moreover, a robustness of the self-organized critical state is obtained with increasing system size.

cond-mat↗

arXiv1997

Sandpile Model for Relaxation in Complex Systems

The relaxation in complex systems is in general nonexponential. After an initial rapid decay the system relax slowly following a long time tail. In the present paper a sandpile modelation of the relaxation in complex systems is analysed. Complexity is introduced by a process of avalanches in the Bethe lattice and a feedback mechanism which leads to slower decay with increasing time. In this way, some features of relaxation in complex systems: long time tails relaxation, aging, and fractal distribution of characteristic times, are obtained by simple computer simulations.

cond-mat↗