Heisenberg model in a random field: phase diagram and tricritical behavior

By using the differential operator technique and the effective field theory scheme we study the tricritical behavior of Heisenberg classical model of spin-1/2 in a random field. The phase diagram in the T-h plane on a square and simple cubic lattice for a cluster with two spins is obtained when the random field is bimodal distributed.Comment: 10 pages, 1 figur

Quaternionic differential operators

Motivated by a quaternionic formulation of quantum mechanics, we discuss quaternionic and complex linear differential equations. We touch only a few aspects of the mathematical theory, namely the resolution of the second order differential equations with constant coefficients. We overcome the problems coming out from the loss of the fundamental theorem of the algebra for quaternions and propose a practical method to solve quaternionic and complex linear second order differential equations with constant coefficients. The resolution of the complex linear Schrodinger equation, in presence of quaternionic potentials, represents an interesting application of the mathematical material discussed in this paper.Comment: 25 pages, AMS-Te

Probing the physical and mathematical structure of f(R)f(R) gravity by PSR J0348+0432J0348+0432

There are several approaches to extend General Relativity in order to explain the phenomena related to the Dark Matter and Dark Energy. These theories, generally called Extended Theories of Gravity, can be tested using observations coming from relativistic binary systems as PSR $J0348+0432$. Using a class of analytical $f(R)$-theories, one can construct the first time derivative of orbital period of the binary systems starting from a quadrupolar gravitational emission. Our aim is to set boundaries on the parameters of the theory in order to understand if they are ruled out, or not, by the observations on PSR $J0348+0432$. Finally, we have computed an upper limit on the graviton mass showing that agree with constraint coming from other observations.Comment: 6 pages, 1 figure, accepted in International Journal of Geometric Methods in Modern Physic

Energy Fluctuations in One Dimensional Classical Magnets

The time- and frequency dependent energy fluctuations in the Heisenberg chain are studied by means of a continued fraction representation. In a broad wave vector and temperature range, the energy fluctuations are found to display dominant oscillatory behavior.

Code of good research practices

Podeu consultar la versió en català a: http://hdl.handle.net/2445/28542 i en castellà a: http://hdl.handle.net/2445/28543As stated in its Statutes, one of the University of Barcelona's (UB) priority objectives is to carry out the highest level of research. This quality research should contribute to the following: progress in all knowledge areas, qualityof-life improvements, environmental preservation and improvement, the promotion of peace, the elimination of social and economic inequalities between individuals and peoples, and scientific and artistic progress in general. The equal opportunities of women and men are respected in all of these areas. The University does not participate in research projects that are incompatible with this objective. In particular, it does not take part in projects that could contribute to the arms race. The UB ensures that all of its research is of a high quality. To achieve this, it evaluates studies undertaken by individuals, research groups and any other forms of research collaboration (Article 100.6, University of Barcelona Statutes). The entities in the UB Group are responsible for ensuring that all of their research is undertaken in accordance with the current legislation and using good scientific practices