About the freedom of free forms

p. 907-913This paper deals with the arrival of freedom at the world of structures giving birth a new generation of forms: the free forms. Its purpose is to analyze, to discuss and to comment critically this singular fact as well as their implications on the designers' task. It is more a philosophical than a technical paper. For centuries man has imagined new forms for their structures but he has not been always able to analyze and to build them. Before the arrival of the electronic calculus, the representation and analysis of structural forms could be limited to those ones belonging to the Euclidean Geometry. The computers broke those limitations and they gave wide freedom to the designers to conceive a new generation of forms; these new forms were called "free forms". Nowadays any form imagined can be represented, it can be analyzed and it can be built. Nevertheless not any imagined form can become a structural free form. Perhaps it could be a beautiful sculptural form, but not necessarily a structural one. For being a structural form, the inescapable laws of the mechanics must be satisfied. Moreover a structural free form can become an architectural free form just only when aesthetical, functional, environmental and social requirements, among others, are accomplished. Freedom has widened the horizons of creativity for the designers' task. Simultaneously new responsibilities have come altogether with this freedom. Today free form designers face permanent challenges; designers must be familiar with the menus of new and multiple tools created by the modern technology and they must be trained to make the right use of them. They must handle those wide menus in order to select the most appropriated options to generate, to model and to analyze the new free forms. At the same time they must select the most appropriated new materials and techniques to build these free forms. Finally, designer must be fully conscious of the high impact of their engineering and architectural works on the people and physical environment without forgetting their commitment to the society.Andres, OA. (2010). About the freedom of free forms. Editorial Universitat Politècnica de València. http://hdl.handle.net/10251/695

The Vector Curvaton

We analyze a massive vector field with a non-canonical kinetic term in the action, minimally coupled to gravity, where the mass and kinetic function of the vector field vary as functions of time during inflation. The vector field is introduced following the same idea of a scalar curvaton, which must not affect the inflationary dynamics since its energy density during inflation is negligible compared to the total energy density in the Universe. Using this hypothesis, the vector curvaton will be solely responsible for generating the primordial curvature perturbation \zeta. We have found that the spectra of the vector field perturbations are scale-invariant in superhorizon scales due to the suitable choice of the time dependence of the kinetic function and the effective mass during inflation. The preferred direction, generated by the vector field, makes the spectrum of \zeta depend on the wavevector, i.e. there exists statistical anisotropy in \zeta. This is discussed principally in the case where the mass of the vector field increases with time during inflation, where it is possible to find a heavy field (M >> H) at the end of inflation, making the particle production be practically isotropic; thus, the longitudinal and transverse spectra are nearly the same order which in turn causes that the statistical anisotropy generated by the vector field is within the observational bounds.Comment: LaTex file in Aipproc style, 6 pages, no figures. Prepared for the conference proceedings of the IX Mexican School of the DGFM-SMF: Cosmology for the XXIst Century. This work is entirely based on Refs. [23-26] and is the result of Andres A. Navarro's MSc thesi

Dephasing and Hyperfine Interaction in Carbon Nanotubes Double Quantum Dots: Disordered Case

We study theoretically the \emph{return probability experiment}, used to measure the dephasing time $T_2^*$, in a double quantum dot (DQD) in semiconducting carbon nanotubes (CNTs) with spin-orbit coupling and disorder induced valley mixing. Dephasing is due to hyperfine interaction with the spins of the ${}^{13}$C nuclei. Due to the valley and spin degrees of freedom four bounded states exist for any given longitudinal mode in the quantum dot. At zero magnetic field the spin-orbit coupling and the valley mixing split those four states into two Kramers doublets. The valley mixing term for a given dot is determined by the intra-dot disorder and therefore the states in the Kramers doublets belonging to different dots are different. We show how nonzero single-particle interdot tunneling amplitudes between states belonging to different doublets give rise to new avoided crossings, as a function of detuning, in the relevant two particle spectrum, crossing over from the two electrons in one dot states configuration, $(0,2)$, to the one electron in each dot configuration, $(1,1)$. In contrast to the clean system, multiple Landau-Zener processes affect the separation and the joining stages of each single-shot measurement and they affect the outcome of the measurement in a way that strongly depends on the initial state. We find that a well-defined return probability experiment is realized when, at each single-shot cycle, the (0,2) ground state is prepared. In this case, valley mixing increases the saturation value of the measured return probability, whereas the probability to return to the (0,2) ground state remains unchanged. Finally, we study the effect of the valley mixing in the high magnetic field limit; for a parallel magnetic field the predictions coincide with a clean nanotube, while the disorder effect is always relevant with a magnetic field perpendicular to the nanotube axis.Comment: 22 pages, 11 figure