Jean Richert

Cluster formation in disordered systems and nuclear fragmentation

Abstract We investigate an approach to the description of nuclear fragmentation relying on the concept of disorder which may characterise a fragmenting system of particles interacting by means of two-body short and long range potentials. We introduce different criteria in order to define clusters of bound nucleons. We work out the fragment content of events generated by means of numerical simulations. We analyse the moments of mass and charge distributions and related observables, as well as the energy content of the system. We discuss the outcome of the calculations in terms of the concept of universality which has been introduced in the field of nuclear fragmentation in the framework of percolation models.

Order parameter fluctuations and thermodynamic phase transitions in finite spin systems and fragmenting nuclei

We show that in small and low density systems described by a lattice gas model with fixed number of particles the location of a thermodynamic phase transition can be detected by means of the distribution of the fluctuations related to an order parameter which is chosen to be the size of the largest fragment. We show the correlation between the size of the system and the observed order of the transition. We discuss the implications of this correlation on the analysis of experimental fragmentation data.

Universal property of multifragmentation in nucleus - nucleus collisions

We have used the bond percolation model to study the multifragmentation of projectiles after collisions with a Cu target at an incident energy of 600 MeV/nucleon. The charge correlations have been analysed. The theoretical results agree well with the experimental data of the ALADIN collaboration. This shows that the percolation model can reasonably describe the universal property of nuclear fragmentation in nucleus - nucleus collisions.

Decoherence and dephasing in Kondo tunneling through double quantum dots

We describe the mechanism of charge-spin transformation in a double quantum dot (DQD) with even occupation, where a time dependent gate voltage v(t) is applied to one of its two valleys, whereas the other one is coupled to the source and drain electrodes. The Kondo tunneling regime under strong Coulomb blockade may be realized when the spin spectrum of the DQD is formed by the ground state spin triplet and two singlet excitations. Charge fluctuations induced by v(t) result in transitions within the spin multiplet characterized by the SO(5) dynamical symmetry group. In a weakly non-adiabatic regime the decoherence, dephasing and relaxation processes affect Kondo tunneling. Each of these processes is caused by a special type of dynamical gauge fluctuations, so that one may discriminate between the decoherence in the ground state of a DQD and dephasing at finite temperatures.

Does localization occur in a hierarchical random-matrix model for many-body states?

Abstract We use random-matrix theory and supersymmetry techniques to work out the two-point correlation function between states in a hierarchical model which employs Feshbachs chaining hypothesis: Classes of many-body states are introduced. Only states within the same or neighboring classes are coupled. We assume that the density of states per class grows monotonically with class index. The problem is mapped onto a one-dimensional non-linear sigma model. In the limit of a large number of states in each class we derive the critical exponent for the growth of the level density with class index for which delocalization sets in. From a realistic modelling of the class-dependence of the level density, we conclude that the model does not predict Fock-space localization in nuclei.

Site-occupation constraints in mean-field approaches to quantum spin systems at finite temperature

We study the effect of site occupation on the description of quantum spin systems at finite temperature and mean-field level. We impose each lattice site to be occupied by a single electron. This is realized by means of a specific prescription. The outcome of the prescription is compared to the result obtained by means of a projection procedure which fixes the site occupation to one particle per site on an average. The comparison is performed for different representations of the Hamiltonian in Fock space leading to different types of mean-field solutions. The behaviour of order parameters is analyzed for each choice of the mean-field and constraint which fixes the occupation rate at each site. Sizable quantitative differences between the outcomes obtained with the two different constraints are observed.

Disorder and universality properties of fragmenting nuclei

Abstract The outcome of the fragmentation of finite nuclei is described in terms of a disordered system of interacting nucleons. Clusters are identified by means of different bond criteria for neighbouring particles. We discuss the possible universality properties of the system which are reflected in the moments of mass distributions and related physical observables. Results are also analyzed with reference to bond percolation approaches which predict these universality properties.

Chiral symmetry restoration in (2+1)-dimensional QED with a Maxwell-Chern-Simons term at finite temperature

We study the role played by a Chern-Simons contribution to the action in the QED{sub 3} formulation of a two-dimensional Heisenberg model of quantum spin systems with a strictly fixed site occupation at finite temperature. We show how this contribution affects the screening of the potential that acts between spinons and contributes to the restoration of chiral symmetry in the spinon sector. The constant that characterizes the Chern-Simons term can be related to the critical temperature T{sub c} above which the dynamical mass goes to zero.

Strict site-occupation constraint in two-dimensional Heisenberg models and dynamical mass generation in QED{sub 3} at finite temperature

We study the effect of site occupation in two-dimensional quantum spin systems at finite temperature in a {pi}-flux state description at the mean-field level. We impose each lattice site to be occupied by a single SU(2) spin. This is realized by means of a specific prescription. We consider the low-energy Hamiltonian which is mapped into a QED{sub 3} Lagrangian of spinons. We compare the dynamically generated mass to the one obtained by means of an average site occupation constraint.

Ageing without detailed balance in the bosonic contact and pair-contact processes: exact results

Ageing in systems without detailed balance is studied in the exactly solvable bosonic contact process and the critical bosonic pair-contact process. The two-time correlation function and the two-time response function are explicitly found. In the ageing regime, the dynamical scaling of these is analysed and exact results for the ageing exponents and the scaling functions are derived. For the critical bosonic pair-contact process, the autocorrelation and autoresponse exponents agree but the ageing exponents a and b are shown to be distinct.