The MSSM from Scherk-Schwarz Supersymmetry Breaking

We present a five-dimensional model compactified on an interval where supersymmetry is broken by the Scherk-Schwarz mechanism. The gauge sector propagates in the bulk, two Higgs hypermultiplets are quasilocalized, and quark and lepton multiplets localized, in one of the boundaries. The effective four-dimensional theory is the MSSM with very heavy gauginos, heavy squarks and light sleptons and Higgsinos. The soft tree-level squared masses of the Higgs sector can be negative and they can (partially) cancel the positive one-loop contributions from the gauge sector. Electroweak symmetry breaking can then comfortably be triggered by two-loop radiative corrections from the top-stop sector. The fine tuning required to obtain the electroweak scale is found to be much smaller than in the MSSM, with essentially no fine-tuning for few TeV gaugino masses. All bounds from direct Higgs searches at LEP and from electroweak precision observables can be satisfied. The lightest supersymmetric particle is a (Higgsino-like) neutralino that can accomodate the abundance of Dark Matter consistently with recent WMAP observations.Comment: 23 pages, 3 figure

A investigação adequada e ética sobre nutrição é um direito humano

Editorial Dr. Maria Isabel T. D. Correi

Quantum modelling of poly(vinylidene fluoride)

Although extensive studies have been conducted on poly(vinylidene fluoride) (PVDF) because of its ferroelectric, pyroelectric and piezoelectric properties, the effects of inverted monomer units, on the molecular scale properties of this polymer are not fully understood. Therefore, we have used a method combining molecular dynamics with a self-consistent semi-empirical quantum mechanical method to study the effects of both chain length and monomer inversion on the electronic properties of individual PVDF chains, such as the dipole moment and the polarizability. The effects of monomer inversion on the infrared spectra are also discussed. Our results suggest that alpha and beta polymorphs of PVDF have dipole moment per monomer unit that varies (increases for beta-PVDF and decreases for alpha-PVDF) with the chain length but converges to a nearly constant value for chain lengths greater than a certain value, whereas chain length does not seem to produce any significant effect on molecular polarizability of both polyrnorphs. Our calculations also suggest a decrease of the dipole moment with increasing inversion monomer defect concentration but no significant effect has been predicted for molecular polarizability, except when two or more isolated defects are present in the same chain. The presence of monomer inverted defects decreases the intensity of the infrared peaks attributable to defect-free chains and gives rise to new peaks. The number and frequency of the defect peaks depends both on the isolated defect concentration and molecular chain structure.Comunidade Europeia (CE). Fundo Europeu de Desenvolvimento Regional (FEDER).Fundação para a Ciência e a Tecnologia (FCT) – Programa Operacional “Ciência, Tecnologia, Inovação” - POCTI/CTM/41574/2001, SFRH/BD/11231/2002

What can we learn from vibrational analysis calculations of defective polymer chains?

The possibility of using infrared (IR) spectroscopy to determine the concentration of inversion monomer defects in polymers depends on the knowledge of the relationship between the spectral properties and the polymer microstructure. This can easily be achieved by performing vibrational analysis. In order to investigate the changes in IR spectra of poly(vinylidene fluoride) resulting from the presence of monomeric units in “head-to-head” and “tail-to-tail” positions, we calculated the frequencies and intensities of IR-active vibrations for individual molecules in alpha and beta form with a defect concentration up to 15% and compared them with the ones obtained for a defect-free molecule.Fundação para a Ciência e a Tecnologia (FCT) – Programa Operacional “Ciência , Tecnologia, Inovação” – POCTI/CTM/41574/2001, CONC-REEQ/443/2001 e SFRH/BD/11231/2002.Comunidade Europeia (CE). Fundo Europeu de Desenvolvimento Regional (FEDER)

Modeling charge transport properties of cyano-substituted PPV

In recent years, poly (p-phenylenevinylene) (PPV) and its derivatives have attracted much interest due to their applications in light-emitting diodes (LEDs). One of the issues that determine device performance is the transport of charge carriers along the polymer strands. For that reason, we investigate the influence of cyano substitution on geometry and electronic behaviour of PPV chains using self-consistent quantum molecular dynamics simulations. Our results suggest that substitution by cyano groups induce distortion in the PPV chains and a charge rearrangement among the polymer atoms. Specifically addressed is the issue concerning estimates of charge (electron and hole) mobility by computer experiments. Significant differences have been found both in the strength of the electric field needed to move positive and negative charge carriers along the polymer chain as well as in charge mobility.Fundação para a Ciência e a Tecnologia (FCT) - Programa Operacional "Ciência, Tecnologia e Inovação" (POCTI) - POCTI/CTM/41574/2001

Modelling the effect of structure and base sequence on DNA molecular electronics

DNA is a material that has the potential to be used in nanoelectronic devices as an active component. However, the electronic properties of DNA responsible for its conducting behaviour remain controversial. Here we use a self-consistent quantum molecular dynamics method to study the effect of DNA structure and base sequence on the energy involved when electrons are added or removed from isolated molecules and the transfer of the injected charge along de molecular axis when an electric field is applied. Our results have shown that the DNA molecules of poly(C)-poly(G) on B-form and poly(A)-poly(T) on A-form have the highest energy released when one electron is added or removed from them and their Z-form has the lowest energy released. Besides, when an electric field is applied to a charged DNA molecule along its axis there is electron transfer through the molecule, regardless of the number and sign of the injected charge, the molecular structure and the base sequence. Results from these simulations provide useful information that is hard to obtain from the experiments and needs to be considered for a further modelling aiming to improve charge transport efficiency in nanoelectronic devices based on DNA.Programa Operacional “Ciência ,Tecnologia, Inovação” – POCTI/CTM/41574/2001Fundação para a Ciência e a Tecnologia (FCT)REEQ/443/EEI/2005 e SFRH/BD/11231/200

Quantum modelling of photo-excited processes

In the framework of quantum field theory and the dipole approximation, a self-consistent quantum molecular dynamics method is used to investigate the effect of chain length on the probability of formation or decay of both singlet and triplet excitons due to photon absorption or emission in isolated poly( p-phenylene vinylene) (PPV) chains. We found that the probability of the photoinduced intra-molecular singlet exciton formation and decay increases linearly with chain length and the probability for triplet exciton formation and decay does not depend on the chain length. Polymers with long chains have thus an advantage over small molecules in solar cell and light-emitting diode (LED) applications because their efficiency depends on the number of intramolecular singlet excitons formed or emitted in the device, which is expected to increase with the conjugation length.Comunidade Europeia (CE). Fundo Europeu de Desenvolvimento Regional (FEDER)Fundação para a Ciência e a Tecnologia (FCT) – Programa Operacional “Ciência, Tecnologia, Inovação” - POCTI/CTM/41574/2001, SFRH/BD/11231/2002