An improved discretization of Schrodinger-like radial equations

A new discretization of the radial equations that appear in the solution of separable second order partial differential equations with some rotational symmetry (as the Schrodinger equation in a central potential) is presented. It cures a pathology, related to the singular behaviour of the radial function at the origin, that suffers in some cases the discretization of the second derivative with respect to the radial coordinate. This pathology causes an enormous slowing down of the convergence to the continuum limit when the two point boundary value problem posed by the radial equation is solved as a discrete matrix eigenvalue problem. The proposed discretization is a simple solution to that problem. Some illustrative examples are discussed.Comment: 21 pages, 12 figure

Stability of the skyrmion lattice near the critical temperature in cubic helimagnets

The phase diagram of cubic helimagnets near the critical temperature is obtained from a Landau-Ginzburg model, including fluctuations to gaussian level. The free energy is evaluated via a saddle point expansion around the local minima of the Landau-Ginzburg functional. The local minima are computed by solving the Euler-Lagrange equations with appropriate boundary conditions, preserving manifestly the full nonlinearity that is characteristic of skyrmion states. It is shown that the fluctuations stabilize the skyrmion lattice in a region of the phase diagram close to the critical temperature, where it becomes the equilibrium state. A comparison of this approach with previous computations performed with a different approach (truncated Fourier expansion of magnetic states) is given.Comment: 6 pages, 6 color figure

Nucleation, instability, and discontinuous phase transitions in monoaxial helimagnets with oblique fields

The phase diagram of the monoaxial chiral helimagnet as a function of temperature (T ) and magnetic field with components perpendicular (H x ) and parallel (H z ) to the chiral axis is theoretically studied via the variational mean field approach in the continuum limit. A phase transition surface in the three dimensional thermodynamic space separates a chiral spatially modulated phase from a homogeneous forced ferromagnetic phase. The phase boundary is divided into three parts: two surfaces of second order transitions of instability and nucleation type, in De Gennes terminology, are separated by a surface of first order transitions. Two lines of tricritical points separate the first order surface from the second order surfaces. The divergence of the period of the modulated state on the nucleation transition surface has the logarithmic behavior typical of a chiral soliton lattice. The specific heat diverges on the nucleation surface as a power law with logarithmic corrections, while it shows a finite discontinuity on the other two surfaces. The soliton density curves are described by a universal function of H x if the values of T and H z determine a transition point lying on the nucleation surface; otherwise, they are not universal.Comment: Phase diagram refined, with a new tricritical point located; 9 pages, 8 figures; version shortened, published in Phys. Rev.

Understanding the H-T phase diagram of the mono-axial helimagnet

Some unexpected features of the phase diagram of the monoaxial helimagnet in presence of an applied magnetic field perpendicular to the chiral axis are theoretically predicted. A rather general hamiltonian with long range Heisenberg exchange and Dzyaloshinskii--Moriya interactions is considered. The continuum limit simplifies the free energy, which contains only a few parameters which in principle are determined by the many parameters of the hamiltonian, although in practice they may be tuned to fit the experiments. The phase diagram contains a Chiral Soliton Lattice phase and a forced ferromagnetic phase separated by a line of phase transitions, which are of second order at low T and of first order in the vicinity of the zero-field ordering temperature, and are separated by a tricritical point. A highly non linear Chiral Soliton Lattice, in which many harmonics contribute appreciably to the spatial modulation of the local magnetic moment, develops only below the tricritical temperature, and in this case the scaling shows a logarithmic behaviour similar to that at T=0, which is a universal feature of the Chiral Soliton Lattice. Below the tricritical temperature, the normalized soliton density curves are found to be independent of T, in agreement with the experimental results of magnetorresistance curves, while above the tricritical temperature they show a noticeable temperature dependence. The implications in the interpretation of experimental results of CrNb3S6 are discussed.Comment: 11 pages, 17 figures. Enlarged version, with more details and results. To be publisehd in Phys. Rev.

Thermal fluctuations in the conical state of monoaxial helimagnets

The effect of thermal fluctuations on the phase structure of monoaxial helimagnets with external magnetic field parallel to the chiral axis is analyzed by means of a saddle point expansion of the free energy. The phase transition that separates the conical and forced ferromagnetic phases is changed to first order by the thermal fluctuations. In a purely monoaxial system the pitch of the conical state remains independent of temperature and magnetic field, as in mean field theory, even when fluctuations are taken into account. However, in presence of weak Dzyaloshinskii-Moriya interactions in the plane perpendicular to the chiral axis, thermal fluctuations induce a dependence of the pitch on temperature and magnetic field. This may serve to determine the nature of magnetic interactions in such systems.Comment: 9 pages, 4 figure

Work Statistics, Loschmidt Echo and Information Scrambling in Chaotic Quantum Systems

Characterizing the work statistics of driven complex quantum systems is generally challenging because of the exponential growth with the system size of the number of transitions involved between different energy levels. We consider the quantum work distribution associated with the driving of chaotic quantum systems described by random matrix Hamiltonians and characterize exactly the work statistics associated with a sudden quench for arbitrary temperature and system size. Knowledge of the work statistics yields the Loschmidt echo dynamics of an entangled state between two copies of the system of interest, the thermofield double state. This echo dynamics is dictated by the spectral form factor. We discuss its relation to frame potentials and its use to assess information scrambling.Comment: 11+6pp, 5 figures. v3: version accepted for publication in Quantu

Filmando teorías políticas: dependencia y liberación en La hora de los hornos

Numerosos estudios se han dedicado a analizar el film La hora de los hornos (Fernando Solanas y Octavio Getino, 1968) como una obra de importancia para el desarrollo del cine latinoamericano en su representación de los problemas sociales y los conflictos políticos. Pero no se han presentado análisis profundos atendiendo a los conceptos de teorías políticas y a los ensayos que influenciaron a los directores. Propongo hacer una lectura narrativa del film desde los aportes de la Teoría de la dependencia (Theotonio Dos Santos, Fernando Henrique Cardoso y Enzo Faletto, entre otros) y del movimiento descolonizador de la negritud (Frantz Fanon).Numerous studies have been devoted to analyzing the film The Hour of the Furnaces (Fernando Solanas and Octavio Getino, 1968) as a work of importance in the representation of the social and political conflicts for the development of Latin American cinema. But there have been no indepth analysis taking into account the concepts of Political Theories and political essays that influenced the directors. I propose a reading of the film narrative from the contributions of Dependency Theory (Theotonio Dos Santos, Fernando Henrique Cardoso and Enzo Faletto, among others) and the decolonization movement of Negritude (Frantz Fanon).Fil: Campo, Javier Alberto. Universidad de Buenos Aires. Facultad de Ciencias Sociales. Instituto de Investigaciones ; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Cine documental: tratamiento creativo (y político) de la realidad

Este trabajo se propone como un recorrido por los nudos conceptuales problemáticos de la historia del cine documental que decantan en uno de sus vínculos más celebres: con la política. Se repondrán diferentes perspectivas y definiciones sobre el cine documental, como registro y discurso, sus funciones sociales y, finalmente, las teorizaciones que han intentado develar los motivos de la pregnancia de la política en las imágenes y sonidos documentales en su siglo de vida.Este trabalho se propõe como um recorrido pelos nós conceituais problemáticos da história do cinema documentário que decantam em um de seus vínculos mais célebres: com a política. Recolocar-se- ão diferentes perspectivas e definições sobre o cinema documentário, como registro e discurso, suas funções sociais e, finalmente, as teorizações que tentaram desvelar os motivos da pregnância da política nas imagens e sons documentais em seu século de vida.This work proposed as a tour of the problematic conceptual knots in the history of documentary cinema, related to one of his most famous links: politics. Different perspectives and definitions of documentary cinema as recording and discourse, social functions, and finally the theories that have attempted to uncover the reasons for the pregnancy of the political documentary images and sounds in his century of life will be restored.Fil: Campo, Javier Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

A la vez acogedor y hostil: El cine etnográfico en sus obras y realizadores canónicos (1960-1990).

El objetivo de este artículo es hacer un repaso por las obras y directores considerados emblemáticos del cine etnográfico en sus años de visibilización y transformación estética, la década del sesenta hasta los noventa del siglo veinte. Los films de Robert Gardner, John Marshall, Jean Rouch, David MacDougall y Timothy Asch, entre otros, serán analizados como tramando un fondo de experiencia para uno de los realizadores de cine etnográfico más prolíficos de América Latina: el argentino Jorge Prelorán. Director vernáculo reconocido por investigadores de este tipo de cine como perteneciente al reducido grupo de los “pioneros”. Este artículo forma parte de una investigación amplia sobre las obras, la carrera, ideas y legado de Prelorán como cineasta.The aim of this article is to review the works and directors considered emblematic of ethnographic cinema in their years of visualization and aesthetic transformation, from the Sixties to the Nineties of the Twentieth Century. The films of Robert Gardner, John Marshall, Jean Rouch, David MacDougall and Timothy Asch, among others, will be analyzed as plotting an experience fund for one of the most prolific ethnographic filmmakers in Latin America: the Argentinian Jorge Prelorán. Local director recognized by researchers of this type of cinema as belonging to the small group of “pioneers”. This article is part of a broad investigation about the works, career, ideas and legacy of Prelorán as a filmmaker.Fil: Campo, Javier Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tandil; Argentin