2,401 research outputs found
Multiexcitons confined within a sub-excitonic volume: Spectroscopic and dynamical signatures of neutral and charged biexcitons in ultrasmall semiconductor nanocrystals
The use of ultrafast gating techniques allows us to resolve both spectrally
and temporally the emission from short-lived neutral and negatively charged
biexcitons in ultrasmall (sub-10 nm) CdSe nanocrystals (nanocrystal quantum
dots). Because of forced overlap of electronic wave functions and reduced
dielectric screening, these states are characterized by giant interaction
energies of tens (neutral biexcitons) to hundreds (charged biexcitons) of meV.
Both types of biexcitons show extremely short lifetimes (from sub-100
picoseconds to sub-picosecond time scales) that rapidly shorten with decreasing
nanocrystal size. These ultrafast relaxation dynamics are explained in terms of
highly efficient nonradiative Auger recombination.Comment: 5 pages, 4 figures, to be published in Phys. Rev.
Are there plasminos in superconductors?
Hot and/or dense, normal-conducting systems of relativistic fermions exhibit
a particular collective excitation, the so-called plasmino. We compute the
one-loop self-energy, the dispersion relation and the spectral density for
fermions interacting via attractive boson exchange. It is shown that plasminos
also exist in superconductors.Comment: 15 pages, 14 figures, revte
Nonlinear cross-Kerr quasiclassical dynamics
We study the quasiclassical dynamics of the cross-Kerr effect. In this
approximation, the typical periodical revivals of the decorrelation between the
two polarization modes disappear and they remain entangled. By mapping the
dynamics onto the Poincare space, we find simple conditions for polarization
squeezing. When dissipation is taken into account, the shape of the states in
such a space is not considerably modified, but their size is reduced.Comment: 16 pages, 5 figure
MV3: A new word based stream cipher using rapid mixing and revolving buffers
MV3 is a new word based stream cipher for encrypting long streams of data. A
direct adaptation of a byte based cipher such as RC4 into a 32- or 64-bit word
version will obviously need vast amounts of memory. This scaling issue
necessitates a look for new components and principles, as well as mathematical
analysis to justify their use. Our approach, like RC4's, is based on rapidly
mixing random walks on directed graphs (that is, walks which reach a random
state quickly, from any starting point). We begin with some well understood
walks, and then introduce nonlinearity in their steps in order to improve
security and show long term statistical correlations are negligible. To
minimize the short term correlations, as well as to deter attacks using
equations involving successive outputs, we provide a method for sequencing the
outputs derived from the walk using three revolving buffers. The cipher is fast
-- it runs at a speed of less than 5 cycles per byte on a Pentium IV processor.
A word based cipher needs to output more bits per step, which exposes more
correlations for attacks. Moreover we seek simplicity of construction and
transparent analysis. To meet these requirements, we use a larger state and
claim security corresponding to only a fraction of it. Our design is for an
adequately secure word-based cipher; our very preliminary estimate puts the
security close to exhaustive search for keys of size < 256 bits.Comment: 27 pages, shortened version will appear in "Topics in Cryptology -
CT-RSA 2007
Angular performance measure for tighter uncertainty relations
The uncertainty principle places a fundamental limit on the accuracy with
which we can measure conjugate physical quantities. However, the fluctuations
of these variables can be assessed in terms of different estimators. We propose
a new angular performance that allows for tighter uncertainty relations for
angle and angular momentum. The differences with previous bounds can be
significant for particular states and indeed may be amenable to experimental
measurement with the present technology.Comment: 4 pages, 1 figures. Comments welcom
A Large Blue Shift of the Biexciton State in Tellurium Doped CdSe Colloidal Quantum Dots
The exciton-exciton interaction energy of Tellurium doped CdSe colloidal
quantum dots is experimentally investigated. The dots exhibit a strong Coulomb
repulsion between the two excitons, which results in a huge measured biexciton
blue shift of up to 300 meV. Such a strong Coulomb repulsion implies a very
narrow hole wave function localized around the defect, which is manifested by a
large Stokes shift. Moreover, we show that the biexciton blue shift increases
linearly with the Stokes shift. This result is highly relevant for the use of
colloidal QDs as optical gain media, where a large biexciton blue shift is
required to obtain gain in the single exciton regime.Comment: 9 pages, 4 figure
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