Exciton states and relaxation dynamics in shallow quantum wells

Il Nuovo Cimento D - Optical properties of GaAs/Al x Ga1−x As shallow quantum wells have been studied in the 2–200K range, both by c.w. and time-resolved experiments. The results agree...     

Laser-induced narrowing of autoionizing resonances studied by the method of parametric generation

It is shown that the autoionizing resonance width and position in atomic nonlinear susceptibility depend on the pumping field intensity. Under certain conditions the narrowing of their width up to radiative one becomes possible. Considered phenomena may be utilized in generation spectroscopy of autoionizing states and for nonlinear susceptibility enhancement.     

The quenched Eguchi-Kawai model

It is argued that spontaneous symmetry breaking occurs in the recently proposed Eguchi-Kawai model. The argument is based on an analytic investigation and on Monte Carlo simulations. A quenched version of the model is proposed which gives good behavior at weak couplings.     

Propagating energy modes in the heisenberg chain: Just plain magnons

We show that the oscillations in the energy density observed in classical simulations on ferromagnetic chains are present in the antiferromagnet as well, and are not due to hydrodynamic “second magnon” modes, but arise from a singularity in the non interacting two spin wave density of states.Quantum occupation number effects change qualitatively the nature of the oscillations, producing a perfectly well defined mode at the spin wave frequency as T→0.     

A dynamic approach to diffusion and permeation measurements

A dynamic method for investigating the mechanism of permeation and diffusion through polymers has been explored. The permeation cell consists of two compartments separated by the membrane. The permeant (gas, vapor, or liquid) is introduced into one compartment; a carrier gas (helium) flows at constant rate through the other and sweeps the permeant which diffuses through the membrane to the thermal conductivity detector. Both compartments are at atmospheric pressure; thus no or little membrane support is required, and leakage problems are minimal. Moreover, the same membrane can be used over a wide temperature range and for diverse permeants. The detector signal is at any instant proportional to the permeation rate. A simple mathematical formalism for deriving the diffusion coefficient from the transient permeation rates has been developed. The measured diffusion and permeability coefficients of CO₂, O₂, and N₂ through low-density polyethylene closely agree with literature values. Permeation of hexane and benzene through polyethylene follows a complex diffusion law, and the rate depends on the thermal history of the system. The dynamic method is particularly suited to the study of transitions in polymers. Changes in permeation rates, usually occurring at transition points, can easily be discovered by slow temperature scanning of the system.     

Neue Methoden zur Untersuchung monomolekularer Filme

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