Confinement of two-dimensional excitons in a non-homogeneous magnetic field

The effective Hamiltonian describing the motion of an exciton in an external non-homogeneous magnetic field is derived. The magnetic field plays the role of an effective potential for the exciton motion, results into an increment of the exciton mass and modifies the exciton kinetic energy operator. In contrast to the homogeneous field case, the exciton in a non-homogeneous magnetic field can also be trapped in the low field region and the field gradient increases the exciton confinement. The trapping energy and wave function of the exciton in a GaAs two-dimensional electron gas for specific circular magnetic field configurations are calculated. The results show than excitons can be trapped by non-homogeneous magnetic fields, and that the trapping energy is strongly correlated with the shape and strength of the non-homogeneous magnetic field profile.Comment: 9 pages, 12 figure

Exciton trapping in magnetic wire structures

The lateral magnetic confinement of quasi two-dimensional excitons into wire like structures is studied. Spin effects are take into account and two different magnetic field profiles are considered, which experimentally can be created by the deposition of a ferromagnetic stripe on a semiconductor quantum well with magnetization parallel or perpendicular to the grown direction of the well. We find that it is possible to confine excitons into one-dimensional (1D) traps. We show that the dependence of the confinement energy on the exciton wave vector, which is related to its free direction of motion along the wire direction, is very small. Through the application of a background magnetic field it is possible to move the position of the trapping region towards the edge of the ferromagnetic stripe or even underneath the stripe. The exact position of this 1D exciton channel depends on the strength of the background magnetic field and on the magnetic polarisation direction of the ferromagnetic film.Comment: 10 pages, 7 figures, to be published in J. Phys: Condens. Matte

Conformations of dendrimers in dilute solution

Conformations of isolated homo- dendrimers of G=1-7 generations with D=1-6 spacers have been studied in the good and poor solvents, as well as across the coil-to-globule transition, by means of a version of the Gaussian self-consistent (GSC) method and Monte Carlo (MC) simulation in continuous space based on the same coarse-grained model. The latter includes harmonic springs between connected monomers and the pair-wise Lennard-Jones potential with a hard core repulsion. The scaling law for the dendrimer size, the degrees of bond stretching and steric congestion, as well as the radial density, static structure factor, and asphericity have been analysed. It is also confirmed that while smaller dendrimers have a dense core, larger ones develop a hollow domain at some separation from the centre.Comment: RevTeX, 14 pages, 19 PS figures, Accepted for publication in J. Chem. Phy

Visions of the Empire: Religion, Ontology and the 'International' in Early Modernity

Journal article, translated from Portuguese and published in English.This paper analyzes the relation between basic religious motifs of theoretical thought, general ontology and their specific use in 'international' political theory at the onset on the Modern Era. The analysis is based on Herman Dooyeweerd's reformational philosophy in identifying the basic assumptions on the origin of life, coherence and diversity of reality in several trends of thought. The Greek and Roman classical legacy, in combination with ancient Christian concepts, is emphasized, namely in terms of motifs such as Nature and Grace, guidelines of scholastic worldview, thus influencing its perspective of Christianity, of the Holy Roman Empire and of the Papacy. Reformed Protestantism adopted a more radically Biblical set of assumptions which culminated in a ontologically plural perspective of social authority and political community, as well as of the empire. Christian humanism, and some Protestant thinkers, was also heavily influenced by the motifs of Nature and Grace, but now with a strict separation between both 'logics'. The theorization of an 'internal logic' for each of these spheres gave origin to a reinterpretation of Nature in classical Humanism, according to a 'mechanistic' perspective of reality with its ideal of control. Another religious motif of this secularized form of humanism was the concept of Liberty and of personality. This geometrical theoretical mode influenced ideas on the social contract and its international analogy, leading theoricians to fiery debates on the classification of the Empire

The Potential and the Pitfalls of Metatheory in IR

publication-status: Publishedtypes: ArticleMetatheory is a type of systematic discourse on theory in a given academic discipline. This article further explores this notion of metatheory and critically discusses a number of views against and in favour of metatheory in International Relations (IR). There are 'strong' and 'mild' claims against metatheoretical IR that should not be ignored. Nevertheless, some of the claims against metatheory in IR contradict themselves. Besides, the 'strong' claim that aims for the complete elimination of metatheoretical discourse in IR depends on metatheory as a presupposition. For this reason, it cannot be maintained. 'Mild' claims, on the other hand, acknowledge to some extent the potential of metatheory in IR, whilst pointing out contingent problems in its current form. Some of these issues are acknowledged and contrasted with a number of claims in favour of metatheory, leading to a moderate defence of metatheoretical discourse in IR

On the Role of Metatheory in the Academic Discipline of International Relations

This thesis investigates in three parts the role played by metatheory in the discipline of International Relations (IR). Part one defines metatheory as 'systematic discourse about theory' and classifies it in a typology combining elements internal or external to the discipline with intellectual or contextual aspects of theorising. Each combination has particular functions. They also add to the roles played by several modes of metatheoretical inquiry (hermeneutical, evaluative, corrective, critical and historical). The typology offered in part one clarifies the general roles of metatheory as a constraining and enabling discursive mechanism. This is also discussed in part two, addressing how IR scholars portray metatheory's role in the discipline. Arguments against and in favour of metatheory are scrutinised, leading to a qualified defence of metatheoretical research in IR. Some of the negative impact of metatheorising in IR is acknowledged, but ultimately a stronger case attempting to eliminate it from the field cannot be sustained for analytical reasons. The merits of metatheory, therefore, will depend on how it operates in particular instances. A selection of illustration cases in part three further develops the argument. The first case stresses how metatheoretical directives shaped 17th century views of the Holy Roman Empire. It indicates that metatheory can frame theoretical claims even in a weak disciplinary context. A stronger disciplinary environment frames the second case, analysing a number of IR theories on the impact of the Peace of Westphalia in the European states-system. This discussion often alludes to the notion of hierarchy. The third case examines the interaction between metatheoretical directives and theories of hierarchy. These arguments are not necessarily compatible with the metatheoretical principles argued by their authors. As a mechanism, therefore, metatheory does not relate to theory in a deterministic way. Part three itself is, of course, a metatheoretical study that further illustrates the thesis.Overseas Research SchemeUniversity of Exete

M\"obius and twisted graphene nanoribbons: stability, geometry and electronic properties

Results of classical force field geometry optimizations for twisted graphene nanoribbons with a number of twists $N_t$ varying from 0 to 7 (the case $N_t$=1 corresponds to a half-twist M\"obius nanoribbon) are presented in this work. Their structural stability was investigated using the Brenner reactive force field. The best classical molecular geometries were used as input for semiempirical calculations, from which the electronic properties (energy levels, HOMO, LUMO orbitals) were computed for each structure. CI wavefunctions were also calculated in the complete active space framework taking into account eigenstates from HOMO-4 to LUMO+4, as well as the oscillator strengths corresponding to the first optical transitions in the UV-VIS range. The lowest energy molecules were found less symmetric than initial configurations, and the HOMO-LUMO energy gaps are larger than the value found for the nanographene used to build them due to electronic localization effects created by the twisting. A high number of twists leads to a sharp increase of the HOMO $\to$ LUMO transition energy. We suggest that some twisted nanoribbons could form crystals stabilized by dipolar interactions