Microscopic Transport Theory of Nuclear Processes

We formulate a microscopic theory of the decay of a compound nucleus through fission which generalizes earlier microscopic approaches of fission dynamics performed in the framework of the adiabatic hypothesis. It is based on the constrained Hartree-Fock-Bogoliubov procedure and the Generator Coordinate Method, and requires an effective nucleon-nucleon interaction as the only input quantity. The basic assumption is that the slow evolution of the nuclear shape must be treated explicitely, whereas the rapidly time-dependent intrinsic excitations can be treated by statistical approximations. More precisely, we introduce a reference density which represents the slow evolution of the nuclear shape by a reduced density matrix and the state of intrinsic excitations by a canonical distribution at each given shape of the nucleus. The shape of the nuclear density distribution is described by parameters ("generator coordinates"), not by "superabundant" degrees of freedom introduced in addition to the complete set of nucleonic degrees of freedom. We first derive a rigorous equation of motion for the reference density and, subsequently, simplify this equation on the basis of the Markov approximation. The temperature which appears in the canonical distribution is determined by the requirement that, at each time t, the reference density should correctly reproduce the mean excitation energy at given values of the shape parameters. The resulting equation for the "local" temperature must be solved together with the equations of motion obtained for the reduced density matrix.Comment: 33 pages, accepted in Nucl. Phys.

Bloch-Wall Phase Transition in the Spherical Model

The temperature-induced second-order phase transition from Bloch to linear (Ising-like) domain walls in uniaxial ferromagnets is investigated for the model of D-component classical spin vectors in the limit D \to \infty. This exactly soluble model is equivalent to the standard spherical model in the homogeneous case, but deviates from it and is free from unphysical behavior in a general inhomogeneous situation. It is shown that the thermal fluctuations of the transverse magnetization in the wall (the Bloch-wall order parameter) result in the diminishing of the wall transition temperature T_B in comparison to its mean-field value, thus favouring the existence of linear walls. For finite values of T_B an additional anisotropy in the basis plane x,y is required; in purely uniaxial ferromagnets a domain wall behaves like a 2-dimensional system with a continuous spin symmetry and does not order into the Bloch one.Comment: 16 pages, 2 figure

Phase transition in a domain wall

We study a domain wall in a three dimensional XY model as a function of anisotropy (K) and temperature (T). It is shown that the wall undergoes a phase transition in the K, T plane, the order parameter being the chirality. Thus it is a two state system analogous to an Ising system. The phase transition can be of the second order and exhibits soft modes. In the ordered state the wall can exhibit domains of opposite chirality separated by singular lines.On étudie une paroi de domaine dans un modèle XY tridimensionnel en fonction de l'anisotropie K et de la température T. On montre que la paroi présente une transition de phase dans le plan (K, T), le paramètre d'ordre étant la chiralité. C'est un système à deux états analogue à un système d'Ising. La transition peut être du second ordre avec modes mous. Dans l'état ordonné, la paroi peut posséder des domaines de chiralités opposées séparés par des lignes singulières

Effects of a finite screening length on the absorption of electromagnetic waves

When an electromagnetic wave impinges on a semiconductor or ionic conductor having a sizeable screening length, it induces diffusion currents in addition to the ohmic currents, which affects the propagation in heterostructures or composite media involving such materials. In the simple geometries and in the low frequency regime studied here, the absorption may be either enhanced or reduced, depending on the parameters, and effects precluded for metals are predicted: extinction of the reflection by a plane wall, complete absorption of an electric multipolar wave by a sphere, disappearance of the scattering by a small sphere, vanishing of both reflection and transmission coefficients for a slab. If the screening length is larger than the skin depth, a slab with intermediate thickness may have a large transparency, and a thick piece of material is expected to be cooled down by the wave near the interface and overheated deeper inside