Monte Carlo determination of the phase diagram of the double-exchange model

We study the phase diagram of the double exchange model, with antiferromagnetic interactions, in a cubic lattice both at zero and finite temperature. There is a rich variety of magnetic phases, combined with regions where phase separation takes place. We identify phases, intrinsic to the cubic lattice, which are stable for realistic values of the interactions and dopings. Some of these phases break chiral symmetry, leading to unusual features.

Reconfigurable computing for Monte Carlo simulations: results and prospects of the Janus project

We describe Janus, a massively parallel FPGA-based computer optimized for the simulation of spin glasses, theoretical models for the behavior of glassy materials. FPGAs (as compared to GPUs or many-core processors) provide a complementary approach to massively parallel computing. In particular, our model problem is formulated in terms of binary variables, and floating-point operations can be (almost) completely avoided. The FPGA architecture allows us to run many independent threads with almost no latencies in memory access, thus updating up to 1024 spins per cycle. We describe Janus in detail and we summarize the physics results obtained in four years of operation of this machine; we discuss two types of physics applications: long simulations on very large systems (which try to mimic and provide understanding about the experimental non equilibrium dynamics), and low-temperature equilibrium simulations using an artificial parallel tempering dynamics. The time scale of our non-equilibrium simulations spans eleven orders of magnitude (from picoseconds to a tenth of a second). On the other hand, our equilibrium simulations are unprecedented both because of the low temperatures reached and for the large systems that we have brought to equilibrium. A finite-time scaling ansatz emerges from the detailed comparison of the two sets of simulations. Janus has made it possible to perform spin glass simulations that would take several decades on more conventional architectures. The paper ends with an assessment of the potential of possible future versions of the Janus architecture, based on state-of-the-art technology.

Tethered Monte Carlo: computing the effective potential without critical slowing down

We present Tethered Monte Carlo, a simple, general purpose method of computing the effective potential of the order parameter (Helmholtz free energy). This formalism is based on a new statistical ensemble, closely related to the micromagnetic one, but with an extended configuration space (through Creutz-like demons). Canonical averages for arbitrary values of the external magnetic field are computed without additional simulations. The method is put to work in the two-dimensional Ising model, where the existence of exact results enables us to perform high precision checks. A rather peculiar feature of our implementation, which employs a local Metropolis algorithm, is the total absence, within errors, of critical slowing down for magnetic observables. Indeed, high accuracy results are presented for lattices as large as L = 1024.

Mean-value identities as an opportunity for Monte Carlo error reduction

In the Monte Carlo simulation of both lattice field theories and of models of statistical mechanics, identities verified by exact mean values, such as Schwinger-Dyson equations, Guerra relations, Callen identities, etc., provide well-known and sensitive tests of thermalization bias as well as checks of pseudo-random-number generators. We point out that they can be further exploited as control variates to reduce statistical errors. The strategy is general, very simple, and almost costless in CPU time. The method is demonstrated in the twodimensional Ising model at criticality, where the CPU gain factor lies between 2 and 4.

Tethered Monte Carlo: computing the effective potential without critical slowing down

We present Tethered Monte Carlo, a simple, general purpose method of computing the effective potential of the order parameter (Helmholtz free energy). This formalism is based on a new statistical ensemble, closely related to the micromagnetic one, but with an extended configuration space (through Creutz-like demons). Canonical averages for arbitrary values of the external magnetic field are computed without additional simulations. The method is put to work in the two-dimensional Ising model, where the existence of exact results enables us to perform high precision checks. A rather peculiar feature of our implementation, which employs a local Metropolis algorithm, is the total absence, within errors, of critical slowing down for magnetic observables. Indeed, high accuracy results are presented for lattices as large as L = 1024.

Usos y aplicaciones del tratado de Andrea Pozzo, perspectiva pictorum architectorum (Roma 1693-1700) en España: docencia científica y práctica artística en la primera mitad del siglo XVIII

La influencia del arquitecto, cuadraturista y constructor de aparatos efímeros Andrea Pozzo (Trento 1642 – Viena 1709) en las artes españolas a través de la difusión de su tratado Perspectiva Pictorum Architectorum (Roma 1693-1700), es un tema que ha aparecido con cierta frecuencia en la historiografía dedicada a nuestro arte barroco. Esta popularidad no necesariamente ha significado la comprensión del fenómeno, pues en la mayoría de ocasiones, se ha tratado de manera fragmentaria y en base a tópicos. Esta tesis doctoral se centra en explicar las funciones que cumplió el tratado del jesuita Pozzo en la cultura artística en la primera mitad del siglo XVIII español así como su presencia en la educación científico-técnica de la Compañía de Jesús. El presente trabajo analiza cuáles fueron las vías de la influencia pozzesca en nuestro país, cómo y en qué campos se verificó concretamente la adopción de sus modelos, y cuáles fueron las razones del fin de su vigencia en la práctica artística. El periodo que acota nuestra investigación se abre en la última década de la dinastía austriaca en España, cuando se comenzaban a apreciar los síntomas de una dispersa renovación intelectual y cultural; y termina con la implantación del centralismo administrativo en el campo de la educación y el ejercicio artístico, consolidado por la fundación de la Academia de Bellas Artes de San Fernando en Madrid (1752). Nuestra investigación parte de un examen detallado de la figura de Andrea Pozzo y del estudio pormenorizado de su tratado de perspectiva. Nuestro mayor aporte en este punto es la atención a su perfil político y social para contextualizar su formación, que le caracteriza como producto del ambiente milanés filoespañol. Destacamos así mismo la relación instrumental que mantuvo con la Orden jesuita, de la que formó parte dedicado exclusivamente a labores artísticas, propagandísticas y docentes, como hermano lego; y cómo sus excelentes relaciones con los estamentos Habsburgo consiguieron que, una vez alcanzado el éxito en el ambiente romano, pudiera continuar su ascenso trasladándose a la Corte de Viena, donde obtuvo un gran reconocimiento al servicio del emperador José I y los Liechtenstein...

Optimized Monte Carlo method for glasses

A new Monte Carlo algorithm is introduced for the simulation of supercooled liquids and glass formers, and tested in two model glasses. The algorithm thermalizes well below the Mode Coupling temperature and outperforms other optimized Monte Carlo methods.