Electrodynamics and phase transitions in materials with magnetic monopoles
- Autores: Fernando Ignacio López Bara
- Directores de la Tesis: Fernando López Aguilar (dir. tes.)
- Lectura: En la Universitat Autònoma de Barcelona ( España ) en 2018
- Idioma: inglés
- Tribunal Calificador de la Tesis: David Jou i Mirabent (presid.), Maria del Mar Sánchez López (secret.), Enrique Bronchalo Bronchalo (voc.)
- Programa de doctorado: Programa de Doctorado en Física por la Universidad Autónoma de Barcelona
- Materias:
- Enlaces
- Resumen
The work is addressed to analyze two parts that are intimately related. The first one refers to studying the global states and characteristics of their magnetic structures of the compounds called spin-ices and in the second part the behaviors under the electromagnetic interaction in infinite media and in confined systems (waveguides) are analyzed.
The main novelty in these compounds is the existence of excited global states at low-temperatures in which structural entities that mimic the behavior of magnetic monopoles arise. In the first part, the low temperature excited states or quasiparticles are studied in compounds of the type (REE)2Ti2O7, where REE refers to one of the 15 lanthanides, fundamentally Dy2Ti2O7 and Ho2Ti2O7. At these temperatures (between 0.05 K and 0.17 K) there is a phase transition with characteristics similar to a Bose Einstein condensate whose individual components are in the form of magnetic dipoles (two monopoles, one with positive magnetic charge and the other negative connected by the "Coulomb interaction" and separated by a distance equivalent to the hight of each tetrahedron of the cristaline structure which we described in the text). By increasing the temperature, said dipoles are broken forming a magnetic plasma of free and quasi-free positive and negative magnetic charges whose statistic is of the Fermi-Dirac type. The thermodynamic transition processes are described by analytical models for low energy excitation states and we describe the successive phase transitions. We determine the thermodynamic potentials, specific heat and entropy in which we can show the two possible phase transitions that occur in these compounds.
In the second part, we make an analysis of the modified Maxwell equations as well as the generalized Lorentz force in the presence of these magnetic charges. The solution of these equations allows us to obtain data that may have empirical interest in order to detect magnetic monopoles in other natural compounds. We study the transverse electromagnetic propagation in these materials by adding a strong external electric field with which we deduce the density of monopoles per unit volume and the effective mass of the same.
We deduce the solutions of these dual Maxwell equations in confined media with rectangular and circular symmetries. In these media in the magnetic plasma phase we obtain the non-linear equation of the system order parameter. The characteristics and properties of the solutions of the modified Maxwell equations are determined in the form of TM modes, obtaining magnetic conductivity as a function of frequency (called magnetricity), magnetic susceptibility, as well as peaks in electromagnetic absorption and other data such as the frequencies of precession and the characteristic frequency of plasma or frequency of plasmon. The achieving of these two frequencies allows us to determine the specific mass assigned to these quasiparticles, being physical magnitude is basic for determining and justifying the conduction properties.
The fundamental objective of this part is to perform a systematic analysis to detect in other materials the presence of these possible effective magnetic charges that may appear and have appeared in other artificial compounds even at room temperature, with the practical interest that this novelty may have. The last objective of this second part of the thesis is to make a prospective to study the possibilities of new materials with which to build "magnetronic" devices that allow to transmit energy and information. The reasons why we do the study of two apparently disconnected parts with which we jointly formule a unitary thesis is because although the first global state is similar, not totally equal, to a Bose-Einstein condensate with very clear interest within condensed matter and the second is clearly a state with characteristics of magnetic plasma, only the second has characteristics that can be used directly in circuit applications, which is one of the fundamental objectives of the second part. Therefore, the possible distinguising between these two states requieres a clear experimental mechanism which the electromagnetic propagation allows us to obtain. Another exclusively theoric objective from a classical Physics point of view is the deduction of the solutions of the dual Maxwell equations in waveguides whose knowledge is scarce.
