ANNUAL REPORT 2011 - Instituto de Estructura de la Materia

We have investigated transport through ballistic quantum dots in the Coulomb blockade regime. We have shownthrough theoretical arguments based on the shape of the wave functions of chaotic quantum system and numericalcalculations, the emergence of long universal sequences of resonances and phase lapses in the short wavelengthlimit. Our results are in qualitative agreement with experiments on Aharanov-Bohm interferometry. Moreover, ourresults predict the dissapearance of the observed universal regime in the presence of strong magnetic fields and inlarger quantum dots in the diffusive regime.CONTINUUM THREE-BODY WAVE FUNCTIONS: RESONANCES, DECAY MODES, ANDREACTIONSGenerally speaking, our main goal is contribute to the development of the different techniques that permit toinvestigate three-body systems, and to exploit them to study the structure and properties of light nuclei. A propertreatment of the long distance behaviour of the three-body wave function is essential to understand how theresonances of systems like 12 C or 9 Be do actually decay. Another important topic under investigation is the lowenergynuclear reactions involving these nuclei. An important group of them are the radiative capture processes,which play a very important role in the stellar nucleosynthesis processes, in particular, in bridging the waitingpoints. We also investigate pure nuclear reactions, in particular the ones corresponding to a two-body projectilehitting a target. We exploit the particularities of the adiabatic potentials to distinguish in a clean way between thedifferent channels (elastic, inelastic or rearrangement).More specifically, during the last months our work has been focused in two main issues: The first one concerns thetriple-alpha process and its reaction rate at very low temperatures. The fact that at such low temperatures theenergies involved are far below the lowest 0 + resonance in 8 Be implies that a sequential description through thatparticular two-body resonance is not obvious. In fact we have found that a direct capture description of the process,without populating any intermediate two-body state, could increase the reaction rate by up to 7 orders of magnitudefor a temperature of 10 7 GK. The second main topic refers to the description of 1+2 reactions by use of the recentlyderived two integral relations. We have found the method to be very successfull to describe processes below thetwo-body breakup threshold, not only for elastic processes, but also for inelastic and transfer reactions. Applicationof the same method for energies above the breakup threshold is the next step. In this connection, very promising andpreliminary results have recently been obtained.STELLAR WEAK DECAY RATES FOR THE rp PROCESSWeak decay rates under stellar density and temperature conditions holding at the rapid proton-capture process hasbeen studied in neutron-deficient medium-mass waiting-point nuclei extending from Ni up to Sn. Neighboringisotopes to these waiting-point nuclei are also included in the analysis. The nuclear structure part of the problem isdescribed within a deformed Skyrme Hartree-Fock + BCS + quasiparticle random-phase-approximation approach,which reproduces not only the β-decay half-lives but also the available Gamow-Teller strength distributionsmeasured under terrestrial conditions. The various sensitivities of the decay rates to both density (ρ) and temperature(T) have been discussed.There are several distinctions between terrestrial and stellar decay rates caused by the effects of high ρand T. Themain effect of T is directly related to the thermal population of excited states in the decaying nucleus, accompaniedby the corresponding depopulation of the ground states. The weak decay rates of excited states can be significantlydifferent from those of the ground state, and a case-by-case consideration is needed. Another new effect comes fromthe fact that atoms in these scenarios are completely ionized, and consequently, electrons are no longer bound to thenuclei but form plasma obeying a Fermi-Dirac distribution. This opens the possibility for continuum electroncapture, in contrast to the orbital electron capture produced by bound electrons in an atom under terrestrialconditions. Thus, we have studied the decay of the ground and excited states in the parent nucleus by introducing apartition function and statistical weights, as well as the competition between βdecays and continuum electroncaptures. We find a non-negligible contribution from the decay of excited states, which is larger for low-lyingexcited states. We also find that contributions from continuum electron captures are comparable to those from betadecayfor waiting-point nuclei at the rp-conditions. These contributions are usually neglected in present networkcalculations.SHAPE TRANSITIONS IN NEUTRON-RICH ISOTOPESThe study of the nuclear shape evolution as the number of nucleons changes is nowadays a highly topical issue innuclear physics from both theoretical and experimental points of view. Especially interesting are those situationswhere the nuclear structure suffers drastic changes between neighbor nuclides. This is the case of the neutron-richisotopes with masses around A=100. These structural variations lead often to sudden changes of particular nuclearproperties that can be used as signatures of phase/shape transitions.52