Person: Álvarez Galindo, Gabriel
Universidad Complutense de Madrid
Faculty / Institute
Now showing 1 - 10 of 24
PublicationPhase space and phase transitions in the Penner matrix model with negative coupling constant(IOP Publishing Ltd, 2017-03-24) Álvarez Galindo, Gabriel; Martínez Alonso, Luis; Medina Reus, ElenaThe partition function of the Penner matrix model for both positive and negative values of the coupling constant can be explicitly written in terms of the Barnes G function. In this paper we show that for negative values of the coupling constant this partition function can also be represented as the product of an holomorphic matrix integral by a nontrivial oscillatory function of n. We show that the planar limit of the free energy with 't Hooft sequences does not exist. Therefore we use a certain modification that uses Kuijlaars-McLaughlin sequences instead of 't Hooft sequences and leads to a well-defined planar free energy and to an associated two-dimensional phase space. We describe the different configurations of complex saddle points of the holomorphic matrix integral both to the left and to the right of the critical point, and interpret the phase transitions in terms of processes of gap closing, eigenvalue tunneling, and Bose condensation. PublicationNeutron scattering experiments and simulations near the magnetic percolation threshold of Fe_(x)Zn_(1-x)F_(2)(American Physical Society, 2012-07-12) Álvarez Galindo, Gabriel; Belanger, D.P.; Durand, A.M.; Martín Mayor, Víctor; Montoya, K.; Muro, Y.The low temperature excitations in the anisotropic antiferromagnetic Fe_(x)Zn_(1-x)F_(2) for x = 0.25 and 0.31, at and just above the magnetic percolation threshold concentration x_(p) = 0.25, were measured using inelastic neutron scattering. The excitations were simulated for x = 0.31 using a localized, classical excitation model, which accounts well for the energies and relative intensities of the excitations observed in the scattering experiments. PublicationKinetic dominance and the wave function of the Universe(Amer Physical Soc, 2022-04-06) Álvarez Galindo, Gabriel; Martínez Alonso, Luis; Medina Reus, ElenaWe analyze the emergence of classical inflationary universes in a kinetic-dominated stage using a suitable class of solutions of the Wheeler-DeWitt equation with a constant potential. These solutions are eigenfunctions of the inflaton momentum operator that are strongly peaked on classical solutions exhibiting either or both a kinetic-dominated period and an inflation period. Our analysis is based on semiclassical WKB solutions of the Wheeler-DeWitt equation interpreted in the sense of Borel (to perform a correct connection between classically allowed regions) and on the relationship of these solutions to the solutions of the classical model. For large values of the scale factor the WKB Vilenkin tunneling wave function and the Hartle-Hawking no-boundary wave functions are recovered as particular instances of our class of wave functions. PublicationComplex saddle points in the Gross-Witten-Wadia matrix model(Amer Physical Soc, 2016-11-14) Álvarez Galindo, Gabriel; Martínez Alonso, Luis; Medina, ElenaWe give an exhaustive characterization of the complex saddle point configurations of the Gross-WittenWadia matrix model in the large-N limit. In particular, we characterize the cases in which the saddles accumulate in one, two, or three arcs, in terms of the values of the coupling constant and of the fraction of the total unit density that is supported in one of the arcs, and derive an explicit condition for gap closing associated with nonvacuum saddles. By applying the idea of large-N instanton we also give direct analytic derivations of the weak- coupling and strong-coupling instanton actions. PublicationIntramolecular distances and form factor of cyclic chains with excluded volume interactions(LTD Elsevier Sci Ltd, 2008-01-21) Álvarez Galindo, Gabriel; Rubio, Ana María; Freire, Juan J.Numerical simulations are performed for isolated cyclic, or ring, chains with excluded volume. Data are reported for the form factor, S(x), where x is the reduced scattering variable, and also for averages and distributions of the distance between intramolecular units. The averages of distances are compared with two alternative expressions describing their dependence with the number of segments separating the units. The distribution function results are compared with the des Cloizeaux form. Finally the S(x) data are compared with theoretical functions also derived from the des Cloizeaux expression for the distribution function. Moreover, the low x and asymptotic expansions of these functions are obtained. Based on these expansions, simple formulas are proposed to give a good description of the simulation data in the whole range of values of x. A comparison with similar results for linear chains is also included. (c) 2007 Elsevier Ltd. All rights reserved. PublicationImage-space surface-related multiple prediction(Soc Exploration Geophysicists, 2007-03) Artman, Brad; Álvarez Galindo, Gabriel; Matson, KenA very important aspect of removing multiples from seismic data is accurate prediction of their kinematics. We cast the multiple prediction problem as an operation in the image space parallel to the conventional surface-related multiple-prediction methodology. Though developed in the image domain, the technique shares the data-driven strengths of data-domain surface-related multiple elimination (SRME) by being independent of the earth (velocity) model. Also, the data are used to predict the multiples exactly so that a Radon transform need not be designed to separate the two types of events. The cost of the prediction is approximately the same as that of data-space methods, though it can be computed during the course of migration. The additional cost is not significant compared to that incurred by shot-profile migration, though split-spread gathers must be used. Image-space multiple predictions are generated by autoconvolving the traces in each shot-gather at every depth level during the course of a shot-profile migration. The prediction in the image domain is equivalent to that produced by migrating the data-space convolutional prediction. Adaptive subtraction of the prediction from the image is required. Subtraction in the image domain, however, provides the advantages of focused energy in a smaller domain since extrapolation removes some of the imperfections of the input data. PublicationSeparatrices in the Hamilton-Jacobi formalism of inflaton models.(American Institute of Physics, 2020-04-01) Álvarez Galindo, Gabriel; Martínez Alonso, Luis; Medina Reus, Elena; Vázquez, Juan LuisWe consider separatrix solutions of the differential equations for inflaton models with a single scalar field in a zero-curvature Friedmann-Lemaitre-Robertson-Walker universe. The existence and properties of separatrices are investigated in the framework of the Hamilton-Jacobi formalism, where the main quantity is the Hubble parameter considered as a function of the inflaton field. A wide class of inflaton models that have separatrix solutions (and include many of the most physically relevant potentials) is introduced, and the properties of the corresponding separatrices are investigated, in particular, asymptotic inflationary stages, leading approximations to the separatrices, and full asymptotic expansions thereof. We also prove an optimal growth criterion for potentials that do not have separatrices. PublicationGeneralised Asymptotic Solutions for the Inflaton in the Oscillatory Phase of Reheating(MDPI, 2021-10-19) Álvarez Galindo, Gabriel; Martínez Alonso, Luis; Medina, ElenaWe determine generalised asymptotic solutions for the inflaton field, the Hubble parameter, and the equation-of-state parameter valid during the oscillatory phase of reheating for potentials that close to their global minima behave as even monomial potentials. For the quadratic potential, we derive a generalised asymptotic expansion for the inflaton with respect to the scale set by inverse powers of the cosmic time. For the quartic potential, we derive an explicit, two-term generalised asymptotic solution in terms of Jacobi elliptic functions, with a scale set by inverse powers of the square root of the cosmic time. In the general case, we find similar two-term solutions where the leading order term is defined implicitly in terms of the Gauss hypergeometric function. The relation between the leading terms of the instantaneous equation-of-state parameter and different averaged values is discussed in the general case. Finally, we discuss the physical significance of the generalised asymptotic solutions in the oscillatory regime and their matching to the appropriate solutions in the thermalization regime. PublicationErythrocyte rouleau formation under polarized electromagnetic fields(American Physical Society, 2005-09) Sebastián Franco, José Luis; Muñoz San Martín, Sagrario; Sancho Ruíz, Miguel; Miranda Pantoja, José Miguel; Álvarez Galindo, GabrielWe study the influence of an external electromagnetic field of 1.8 GHz in the formation or disaggregation of long rouleau of identical erythrocyte cells. In particular we calculate the variation of the transmembrane potential of an individual erythrocyte illuminated by the external field due to the presence of the neighboring erythrocytes in the rouleau, and compare the total electric energy of isolated cells with the total electric energy of the rouleau. We show that the polarization of the external electromagnetic field plays a fundamental role in the total energy variation of the cell system, and consequently in the formation or disaggregation of rouleau. PublicationPolarizability of shelled particles of arbitrary shape in lossy media with an application to hematic cells(American Physical Society, 2008-11) Sebastián Franco, José Luis; Muñoz San Martín, Sagrario; Sancho Ruíz, Miguel; Álvarez Galindo, GabrielWe show that within the dipole approximation the complex polarizability of shelled particles of arbitrary shape can be written as the volume of the particle times a weighted average of the electric field in the particle, with weights determined by the differences in permittivities between the shells and the external, possibly lossy media. To calculate the electric field we use an adaptive-mesh finite-element method which is very effective in handling the irregular domains, material inhomogeneities, and complex boundary conditions usually found in biophysical applications. After extensive tests with exactly solvable models, we apply the method to four types of hematic cells: platelets, T-lymphocytes, erythrocytes, and stomatocytes. Realistic shapes of erythrocytes and stomatocytes are generated by a parametrization in terms of Jacobi elliptic functions. Our results show, for example, that if the average polarizability is the main concern, a confocal ellipsoid may be used as a model for a normal erythrocyte, but not for a stomatocyte. A comparison with experimental electrorotation data shows quantitatively the effect of an accurate geometry in the derivation of electrical cell parameters from fittings of theoretical models to the experimental data.