Radioactive ion beams for solid state research

Radioactive isotopes are widely used in many research fields. In some applications they are used as tracers after diffusion or after activation in the material itself through nuclear reactions. For research in solid state physics, the ion implantation technique is the most flexible and convenient method to introduce the radioactive isotopes in the materials to be studied, since it allows the control of the ion dose, the implantation depth and the isotopic purity. The on-line coupling of isotope separators to particle accelerators, as is the case of the ISOLDE facility at CERN, allows the obtention of a wide range of high purity short lived isotopes. Currently, the most stringent limitation for some applications is the low acceleration energy of 60 keV of the ISOLDE beam. In this communication a short review of the current applications of the radioactive beams for research in solid state physics at ISOLDE is done. The development of a post-accelerator facility for MeV radioactive ions is introduced and the advantages of energetic radioactive beams are discussed

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

Direct evidence for stability of tetrahedral interstitial Er in Si up to 900∘^{\circ}C

Conversion electron emission channeling from the isotope $^{167m}$Er (2.28 s), which is the decay product of radioactive $^{167}$Tm (9.25 d), offers a means of monitoring the lattice sites of Er in single crystals. We have used this method to determine the lattice location of $^{167m}$Er in Si directly following room temperature implantation of $^{167}$Tm, after subsequent annealing steps, and also in situ during annealing up to 900°C. Following the recovery of implantation damage around 600°C, about 90% of Er occupies near-tetrahedral interstitial sites in both FZ and CZ Si. While in FZ Si $^{167m}$Er was found to be stable on these sites even at 900°C, the tetrahedral Er fraction in CZ Si decreased considerably after annealing for 10 min at 800°C and above

Valores, actitudes y comportamientos sociopolíticos de los estudiantes portugueses de economía

Previous research on the Portuguese case confirmed free-riding, free-marketer and right-wing political inclinations among economics students. Further scrutiny was endeavored here, perceiving also a considerably increased interest for politics, notwithstanding the lack of concern for public problems. Various aspects of free-riding proclivity were distinguished and discussed, relating them with youngsters’ risk-loving and sensation-seeking tendencies, but also with a number of other facets specific to economics students, including: prevalence of achievement values, contextual influences on morals, peer pressure, perceived group status and social expectations regarding the economic profession.Pesquisas anteriores sobre o caso português confirmaram inclinações políticas de direita e pró-mercado entre os estudantes de economia, a par duma maior tendência para o free-riding. Investigação subsequente, ora exposta, permitiu perceber também uma atração acrescida pela política, não obstante a falta de interesse pelos problemas públicos. Diversos aspetos da inclinação free-rider foram distinguidos, relacionando-os com propensão juvenil para o risco e a busca de sensações, mas também com traços específicos dos estudantes de economia, incluindo: prevalência de valores de realização, influências contextuais na moralidade, pressão dos pares, perceção de status e expetativas associadas à profissão de economista.Des études antérieures ont confirmé la prévalence de tendances politiques de droite et pro-marché parmi les étudiants d’économie au Portugal, parallèlement à une inclination accrue pour le free-riding. Des recherches subséquentes, ici exposés, ont détecté aussi une intense attraction par la politique, malgré l’indifférence vers la vie publique. Plusieurs aspects de la pente vers le free-riding ont été distingués, en les rapportant à la propension des jeunes pour le risque et la recherche de sensations, mais également à des traits spécifiques des étudiants d’économie, nommément: valeurs d’accomplissement, influences contextuelles sur la moralité, pression des pairs, perception de statut et attentes relatives à la profession d’économiste.Pesquisas anteriores en el caso portugués confirmaron inclinaciones políticas de derecha y pro-mercado entre los estudiantes de economía, en paralelo con una mayor tendencia al free-riding. Investigación subsiguiente, aquí expuesta, permitió percibir también una mayor atracción por la política, pero sin interés por asuntos públicos. Diversos aspectos de la inclinación free-rider fueron distinguidos, relacionándolos con la propensión juvenil hacia el riesgo y la búsqueda de sensaciones, pero también con aspectos específicos de los estudiantes de economía: prevalencia de valores de realización, influencias contextuales en la moralidad, presión de los pares, percepción de status y expectativas relativas a la profesión de economista

Percolative phase transition on ferromagnetic insulator manganites: uncorrelated to correlated polaron clusters

In this work, we report an atomic scale study on the ferromagnetic insulator manganite LaMnO$_{3.12}$ using $\gamma-\gamma$ PAC spectroscopy. Data analysis reveals a nanoscopic transition from an undistorted to a Jahn-Teller-distorted local environment upon cooling. The percolation thresholds of the two local environments enclose a macroscopic structural transition (Rhombohedric-Orthorhombic). Two distinct regimes of JT-distortions were found: a high temperature regime where uncorrelated polaron clusters with severe distortions of the Mn$^{3+}$O$_{6}$ octahedra survive up to $T \approx 800 K$ and a low temperature regime where correlated regions have a weaker JT-distorted symmetry.Comment: 4 pages, 4 Figures, submitted to PRL, new version with more data, text reformulate

Ab-initio study of the relation between electric polarization and electric field gradients in ferroelectrics

The hyperfine interaction between the quadrupole moment of atomic nuclei and the electric field gradient (EFG) provides information on the electronic charge distribution close to a given atomic site. In ferroelectric materials, the loss of inversion symmetry of the electronic charge distribution is necessary for the appearance of the electric polarization. We present first-principles density functional theory calculations of ferroelectrics such as BaTiO3, KNbO3, PbTiO3 and other oxides with perovskite structures, by focusing on both EFG tensors and polarization. We analyze the EFG tensor properties such as orientation and correlation between components and their link with electric polarization. This work supports previous studies of ferroelectric materials where a relation between EFG tensors and polarization was observed, which may be exploited to study ferroelectric order when standard techniques to measure polarization are not easily applied.Comment: 9 pages, 6 figures, 5 tables, corrected typos, as published in Phys. Rev.

Modeling of the performance of scintillator based x-ray detectors

Over the last years several prototypes of detectors for digital radiography have been developed. One of the several approaches is based on scintillating crystals. Generally, these prototypes have been developed in a purely experimental basis, with a lack of mathematical and physical support. In the present work we have performed a systematic study of the various interactions and processes essential to the construction of x-ray sensors based in scintillator crystals. These results are an important help in the simulation of the performance of the whole sensor before its construction, which allows a better selection of the materials, the dimensions, shape and fabrication process

Multiple energetic injections in a strong spike-like solar burst

An intense and fast spike-like solar burst was built up of short time scale structures superimposed on an underlying gradual emission, the time evolution of which shows remarkable proportionality between hard X-ray and microwave fluxes. The finer time structure were best defined at mm-microwaves. At the peak of the event, the finer structures repeat every 30x60ms. The more slowly varying component with a time scale of about 1 second was identified in microwave hard X-rays throughout the burst duration. It is suggested that X-ray fluxes might also be proportional to the repetition rate of basic units of energy injection (quasi-quantized). The relevant parameters of one primary energy release site are estimated both in the case where hard X-rays are produced primarily by thick-target bremsstrahlung, and when they are purely thermal. The relation of this figure to global energy considerations is discussed

Dynamic capabilities: Antecedents and implications for firms' performance

Purpose: This paper aims to explore a new causal link between learning, market and entrepreneurial orientations and firms' performance by introducing dynamic capabilities and competitive advantages as mediator variables. Design/methodology/approach: The mediating role of dynamic capabilities and competitive advantages is tested using a sample of 1,190 Portuguese firms, and structural equation models. Findings: It is shown that dynamic capabilities mediate the relationship between the three orientations–learning, market and entrepreneurial–and competitive advantages of differentiation and cost leadership, and both competitive advantages lead to firm's performance. It is also shown that learning orientation is an antecedent of market orientation and entrepreneurial orientation. Practical implications: This research shows that firm's performance depends on the capacity of firms to learn, innovate, be proactive, take risks and collect the best market data. Indeed, by optimizing the internal management and knowledge dissemination, firms will develop a set of capabilities and competitive advantages that lead to an appropriate response to market challenges. Originality/value: This study tests the relationship between strategic orientations and firm's performance by taking the mediating effects of dynamic capabilities and competitive advantages into account. This research was conducted in Portugal.info:eu-repo/semantics/acceptedVersio

Corrections to the Boltzmann mean free path in disordered systems with finite size scatterers

The mean free path is an essential characteristic length in disordered systems. In microscopic calculations, it is usually approximated by the classical value of the elastic mean free path. It corresponds to the Boltzmann mean free path when only isotropic scattering is considered, but it is different for anisotropic scattering. In this paper, we work out the corrections to the so called Boltzmann mean free path due to multiple scattering effects on finite size scatterers, in the s-wave approximation, ie. when the elastic mean free path is equivalent to the Boltzmann mean free path. The main result is the expression for the mean free path expanded in powers of the perturbative parameter given by the scatterer density.Comment: 12 page