40,584 research outputs found
Surface Phonons and Other Localized Excitations
The diatomic linear chain of masses coupled by harmonic springs is a
textboook model for vibrational normal modes (phonons) in crystals. In addition
to propagating acoustic and optic branches, this model is known to support a
``gap mode'' localized at the surface, provided the atom at the surface has
light rather than heavy mass. An elementary argument is given which explains
this mode and provides values for the frequency and localization length. By
reinterpreting this mode in different ways, we obtain the frequency and
localization lengths for three other interesting modes: (1) the surface
vibrational mode of a light mass impurity at the surface of a monatomic chain;
(2) the localized vibrational mode of a stacking fault in a diatomic chain; and
(3) the localized vibrational mode of a light mass impurity in a monatomic
chain.Comment: 5 pages with 4 embedded postscript figures. This paper will appear in
the American Journal of Physic
Multiphoton resonance in a three-level system with nearly degenerate excited states
An analytic study is presented of the efficient multiphoton excitation and
strong harmonic generation in three-level systems specified by a pair of nearly
degenerate, strongly dipole-coupled excited states. Such systems are physically
formed by the three lowest states in, e.g., the hydrogen atom or evenly charged
homonuclear diatomic molecular ions under reasonably chosen laser intensities.
As a detailed analytic result, we found that the laser pulse of photon energy
, duration and intensity is able to produce complete inversion of
the initial population in the hydrogen atom through the 5-photon excitation. At
the same photon energy, the pulse of duration and intensity
was found to produce the
same effect in the molecular ion but through the 9-photon excitation. We show
that the accompanying scattering of light has very rich spectrum differing
substantially from that of the two-level system.Comment: 9 pages, 5 figures,submitted to Phys. Rev. A, comments welcom
An approach to the multidimensional assessment of food security and environmental sustainability: a vulnerability framework for the Mediterranean region
Poster presented at First International Conference on Global Food Security. Noordwijkerhout (The Netherlands), 29 Sep - 2 Oct 201
Multidimensional assessment of food security and environmental sustainability: a vulnerability framework for the Mediterranean Region [Poster]
Poster presented at Tropentag 2013. International Research on Food Security, Natural Resource Management and Rural Development. "Agricultural development within the rural-urban continuum". Stuttgart-Hohenheim (Germany), Sep 17-19 2013
An approach to the multidimensional assessment of food security and environmental sustainability: a vulnerability framework for the Mediterranean region
Poster presented at First International Conference on Global Food Security. Noordwijkerhout (The Netherlands), 29 Sep - 2 Oct 201
Spinor Parallel Propagator and Green's Function in Maximally Symmetric Spaces
We introduce the spinor parallel propagator for maximally symmetric spaces in
any dimension. Then, the Dirac spinor Green's functions in the maximally
symmetric spaces R^n, S^n and H^n are calculated in terms of intrinsic
geometric objects. The results are covariant and coordinate-independent.Comment: 7 page
Optical pumping of the electron spin polarization in bulk CuCl
In CuCl bulk crystal negatively charged excitons (trions ) can be
induced by the resonant optical excitation of extra electrons in conduction
band minimum. In the case of light polarization and due to the top valence band
structure of CuCl only the electrons with spin antiparallel to the direction of
the light propagation contribute to the formation of , while the emerging
can recombine into both possible electron states, with spin parallel and
antiparallel to the direction of light propagation. We propose to use this
mechanism for optical electronic spin pumping. We describe the dynamics of
pumping in terms of density matrix formalism. The coherent pumping laser pulse
propagating through the sample is described by Maxwell wave equation coupled to
the density matrix evolution equations. The results of our approximate simple
model calculations suggest that spin polarization close to 100% can be achieved
in time shorter than 100ps.Comment: new extended version, 7 pages, 4 figure
Thermally activated breakdown in a simple polymer model
We consider the thermally activated fragmentation of a homopolymer chain. In
our simple model the dynamics of the intact chain is a Rouse one until a bond
breaks and bond breakdown is considered as a first passage problem over a
barrier to an absorbing boundary. Using the framework of the Wilemski-Fixman
approximation we calculate activation times of individual bonds for free and
grafted chains. We show that these times crucially depend on the length of the
chain and the location of the bond yielding a minimum at the free chain ends.
Theoretical findings are qualitatively confirmed by Brownian dynamics
simulations
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