73 research outputs found
Experimental implementation of a four-level N-type scheme for the observation of Electromagnetically Induced Transparency
A nondegenerate four-level N-type scheme was experimentally implemented to
observe electromagnetically induced transparency (EIT) at the Rb D
line. Radiations of two independent external-cavity semiconductor lasers were
used in the experiment, the current of one of them being modulated at a
frequency equal to the hyperfine-splitting frequency of the excited 5P
level. In this case, apart from the main EIT dip corresponding to the
two-photon Raman resonance in a three-level -scheme, additional dips
detuned from the main dip by a frequency equal to the frequency of the HF
generator were observed in the absorption spectrum. These dips were due to an
increase in the medium transparency at frequencies corresponding to the
three-photon Raman resonances in four-level N-type schemes. The resonance
shapes are analyzed as functions of generator frequency and magnetic field.Comment: 3 pages, 2 figure
A ``Tetris''-like model for the Compaction of Dry Granular Media
We propose a two-dimensional geometrical model, based on the concept of
geometrical frustration, conceived for the study of compaction in granular
media. The dynamics exhibits an interesting inverse logarithmic law that is
well known from real experiments. Moreover we present a simple dynamical model
of planes exchanging particles with excluded volume problems, which allows
to clarify the origin of the logarithmic relaxations and the stationary density
distribution. A simple mapping allows us to cast this Tetris-like model in the
form of an Ising-like spin systems with vacancies.Comment: 4 pages, Latex including 2 PS figures (reference corrected). Subm. to
Phys. Rev. Lett. (1997
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