SDG fermion-pair algebraic SO(12) and Sp(10) models and their boson realizations

It is shown how the boson mapping formalism may be applied as a useful many-body tool to solve a fermion problem. This is done in the context of generalized Ginocchio models for which we introduce S-, D-, and G-pairs of fermions and subsequently construct the sdg-boson realizations of the generalized Dyson type. The constructed SO(12) and Sp(10) fermion models are solved beyond the explicit symmetry limits. Phase transitions to rotational structures are obtained, also in situations where there is no underlying SU(3) symmetry.Comment: 25 LaTeX pages, 4 uuencoded postscript figures included, Preprint IFT/8/94 & STPHY-TH/94-

Microscopic structure of fundamental excitations in N=Z nuclei

Excitation energies of the $T$=1 states in even-even as well as $T$=0 and $T$=1 states in odd-odd $N$=$Z$ nuclei are calculated within the mean-field approach. It is shown that the underlying structure of these states can be determined in a consistent manner only when both isoscalar and isovector pairing collectivity as well as isospin projection, treated within the iso-cranking approximation, are taken into account. In particular, in odd-odd $N$=$Z$ nuclei, the interplay between quasiparticle excitations (relevant for the case of $T$=0 states) and iso-rotations (relevant for the case of $T$=1 states) explains the near-degeneracy of these fundamental excitations.Comment: 4 pages, 4 figure

Compressive creep of Fe3Al-type iron aluminide with Zr additions

High-temperature creep ofa Fe3Al-type iron aluminide alloyed by zirconium was studied in the temperature range 873-1073 K. The alloy contained (wt.%) 31.5% Al, 3.5% Cr, 0.25% Zr, 0.19% C (Fe balance). It was tested in two states: (i) as received after hot rolling and (ii) heat treated (1423 K/ 2 h/air). Creep tests were performed in compression at constant load with stepwise loading: in each step, the load was changed to a new value after steady state creep rate had been established. Stress exponent and activation energy of the creep rate were determined and possible creep mechanisms were discussed in terms ofthe threshold stress concept. A rapidfall ofthe stress exponent and ofthe threshold stress with the increasing temperature indicates that creep is impeded by the presence of precipitates only at temperature 873 K. The results were compared with the results oflong-term creep tests in tension performed recently on the same alloy. Изучена высокотемпературная ползучестьИзучена высокотемпературная ползучесть алюминида железа типа Fe3Al, легирован­ного цирконием в диапазоне температур 873...1073 К. Сплав содержал (ат.%) 31,5 Al, 3,5 Сг, 0,25 Zг, 0,19 С (остальное Fе). Испытания проводили в двух состояниях: в состоя­нии поставки после горячей прокатки и после термообработки (1423 К/2 ч/воздух). Испытания на ползучесть выполняли при постоянной сжимающей нагрузке при сту­пенчатом нагружении: на каждой ступени использовали нагрузку другой величины после того, как наступала стадия установив­шейся скорости ползучести. Определена экс­понента напряжения и энергия активации для скорости ползучести, обсуждены возмож­ные механизмы ползучести с позиций кон­цепции порогового напряжения. Резкое умень­шение экспоненты напряжения и порогового напряжения при увеличении температуры свидетельствует о том, что наличие вторич­ных фаз снижает скорость ползучести толь­ко при температуре 873 К. Полученные ре­зультаты сравнивали с данными длительных испытаний на ползучесть при растяжении, выполненных на том же сплаве

Estimation of anisotropy of mechanical properties in Mg alloys by means of compressive creep tests

A detailed knowledge of dependence of mechanical properties on orientation in materials prepared by directional processes may present an important factor influencing the design of construction parts. Toward this end, the compressive creep testing of short specimens may be useful. Three different magnesium-based materials were subjected to this testing: (i) pure magnesium, (ii) magnesium matrix composite reinforced with 10 vol.% of titanium, and (iii) magnesium alloy WE54. All three materials were prepared through a powder metallurgical route with final hot extrusion. The specimensfor creep tests were cut in such a way that their longitudinal axis (i.e., the direction of compressive creep stress) and the axis of extruded bar contained a predestined angle. Two extreme cases can be observed: In pure Mg and in Mg-Ti composite, the dependence of the creep rate is very sensitive to the orientation especially at small inclinations from extrusion axis. The greatest creep resistance is observed in specimens with stress axis parallel to the extrusion axis, the lowest at declinationsfrom 45 to 90°. On the other hand, in WE54 no orientation dependence was observed. Possible explanations of the behaviour based on microstructural observations are discussed.Глубокое понимание зависимости механи­ческих свойств от ориентации волокон в материалах, полученных методами направ­ленного воздействия, может быть важным фактором в создании структурных элемен­тов. Исходя из этого, испытания коротких образцов на ползучесть при сжатии могут дать полезную информацию. Испытания проводили на трех различных материалах на основе магния: чистый магний; композит с магниевой матрицей, упрочненный 10 об.% титана; магниевый сплав WES4. Все три материала были получены методами порош­ковой металлургии и горячей экструзии. Образцы вырезали таким образом, чтобы их продольная ось (т.е. направление напряже­ния ползучести при сжатии) и ось экстру­дированного образца имели заданный угол. Для чистого магния и Mg-Ti композита за­висимость скорости ползучести существен­ но зависит от ориентации, особенно при не­ большом отклонении от оси экструзии. Наи­большее сопротивление ползучести имели образцы с осью напряжений, параллельной оси экструзии, наименьшее - при отклонении на 45°-90°. В сплаве WES4 не наблю­далось зависимости от ориентации. Подоб­ное поведение может быть связано с микро­структурой материала

An Introduction to Nuclear Supersymmetry: a Unification Scheme for Nuclei

The main ideas behind nuclear supersymmetry are presented, starting from the basic concepts of symmetry and the methods of group theory in physics. We propose new, more stringent experimental tests that probe the supersymmetry classification in nuclei and point out that specific correlations should exist for particle transfer intensities among supersymmetric partners. We also discuss possible ways to generalize these ideas to cases where no dynamical symmetries are present. The combination of these theoretical and experimental studies may play a unifying role in nuclear phenomena.Comment: 40 pages, 11 figures, lecture notes `VIII Hispalensis International Summer School: Exotic Nuclear Physics', Oromana, Sevilla, Spain, June 9-21, 200

A schematic model for QCD at finite temperature

The simplest version of a class of toy models for QCD is presented. It is a Lipkin-type model, for the quark-antiquark sector, and, for the gluon sector, gluon pairs with spin zero are treated as elementary bosons. The model restricts to mesons with spin zero and to few baryonic states. The corresponding energy spectrum is discussed. We show that ground state correlations are essential to describe physical properties of the spectrum at low energies. Phase transitions are described in an effective manner, by using coherent states. The appearance of a Goldstone boson for large values of the interaction strength is discussed, as related to a collective state. The formalism is extended to consider finite temperatures. The partition function is calculated, in an approximate way, showing the convenience of the use of coherent states. The energy density, heat capacity and transitions from the hadronic phase to the quark-gluon plasma are calculated.Comment: 33 pages, 11 figure

A schematic model for QCD I: Low energy meson states

A simple model for QCD is presented, which is able to reproduce the meson spectrum at low energy. The model is a Lipkin type model for quarks coupled to gluons. The basic building blocks are pairs of quark-antiquarks coupled to a definite flavor and spin. These pairs are coupled to pairs of gluons with spin zero. The multiplicity problem, which dictates that a given experimental state can be described in various manners, is removed when a particle-mixing interaction is turned on. In this first paper of a series we concentrates on the discussion of meson states at low energy, the so-called zero temperature limit of the theory. The treatment of baryonic states is indicated, also.Comment: 29 pages, 6 figures. submitted to Phys. Rev.

Harnessing person-generated health data to accelerate patient-centered outcomes research: the Crohn’s and Colitis Foundation of America PCORnet Patient Powered Research Network (CCFA Partners)

The Crohn’s and Colitis Foundation of America Partners Patient-Powered Research Network (PPRN) seeks to advance and accelerate comparative effectiveness and translational research in inflammatory bowel diseases (IBDs). Our IBD-focused PCORnet PPRN has been designed to overcome the major obstacles that have limited patient-centered outcomes research in IBD by providing the technical infrastructure, patient governance, and patient-driven functionality needed to: 1) identify, prioritize, and undertake a patient-centered research agenda through sharing person-generated health data; 2) develop and test patient and provider-focused tools that utilize individual patient data to improve health behaviors and inform health care decisions and, ultimately, outcomes; and 3) rapidly disseminate new knowledge to patients, enabling them to improve their health. The Crohn’s and Colitis Foundation of America Partners PPRN has fostered the development of a community of citizen scientists in IBD; created a portal that will recruit, retain, and engage members and encourage partnerships with external scientists; and produced an efficient infrastructure for identifying, screening, and contacting network members for participation in research