5,360 research outputs found
Heavy atom quantum diffraction by scattering from surfaces
Typically one expects that when a heavy particle collides with a surface, the
scattered angular distribution will follow classical mechanics. The heavy mass
assures that the de Broglie wavelength of the incident particle in the
direction of the propagation of the particle (the parallel direction) will be
much shorter than the characteristic lattice length of the surface, thus
leading to a classical description. Recent work on molecular interferometry has
shown that by increasing the perpendicular coherence length, one may observe
interference of very heavy species passing through a grating. Here we show,
using quantum mechanical simulations, that the same effect will lead to quantum
diffraction of heavy particles colliding with a surface. We find that the
effect is robust with respect to the incident energy, the angle of incidence
and the mass of the particle. It may also be used to verify the quantum nature
of the surface and its fluctuations at very low temperatures.Comment: 9 pages, 3 figure
The glass transition and the Coulomb gap in electron glasses
We establish the connection between the presence of a glass phase and the
appearance of a Coulomb gap in disordered materials with strongly interacting
electrons. Treating multiparticle correlations in a systematic way, we show
that in the case of strong disorder a continuous glass transition takes place
whose Landau expansion is identical to that of the Sherrington-Kirkpatrick spin
glass. We show that the marginal stability of the glass phase controls the
physics of these systems: it results in slow dynamics and leads to the
formation of a Coulomb gap
Nonexponential Relaxations in a Two-Dimensional Electron System in Silicon
The relaxations of conductivity have been studied in a strongly disordered
two-dimensional (2D) electron system in Si after excitation far from
equilibrium by a rapid change of carrier density n_s at low temperatures T. The
dramatic and precise dependence of the relaxations on n_s and T strongly
suggests (a) the transition to a glassy phase as T->0, and (b) the Coulomb
interactions between 2D electrons play a dominant role in the observed
out-of-equilibrium dynamics.Comment: 5 pages, 5 figure
History-dependent relaxation and the energy scale of correlation in the Electron-Glass
We present an experimental study of the energy-relaxation in
Anderson-insulating indium-oxide films excited far from equilibrium. In
particular, we focus on the effects of history on the relaxation of the excess
conductance dG. The natural relaxation law of dG is logarithmic, namely
dG=-log(t). This may be observed over more than five decades following, for
example, cool-quenching the sample from high temperatures. On the other hand,
when the system is excited from a state S_{o} in which it has not fully reached
equilibrium to a state S_{n}, the ensuing relaxation law is logarithmic only
over time t shorter than the time t_{w} it spent in S_{o}. For times t>t_{w}
dG(t) show systematic deviation from the logarithmic dependence. It was
previously shown that when the energy imparted to the system in the excitation
process is small, this leads to dG=P(t/t_{w}) (simple-aging). Here we test the
conjecture that `simple-aging' is related to a symmetry in the relaxation
dynamics in S_{o} and S_{n}. This is done by using a new experimental procedure
that is more sensitive to deviations in the relaxation dynamics. It is shown
that simple-aging may still be obeyed (albeit with a modified P(t/t_{w})) even
when the symmetry of relaxation in S_{o} and S_{n} is perturbed by a certain
degree. The implications of these findings to the question of aging, and the
energy scale associated with correlations are discussed
Hopping models and ac universality
Some general relations for hopping models are established. We proceed to
discuss the universality of the ac conductivity which arises in the extreme
disorder limit of the random barrier model. It is shown that the relevant
dimension entering into the diffusion cluster approximation (DCA) is the
harmonic (fracton) dimension of the diffusion cluster. The temperature scaling
of the dimensionless frequency entering into the DCA is discussed. Finally,
some open questions about ac universality are mentioned.Comment: 6 page
Extreme(ly) mean(ingful): Sequential formation of a quality group
The present paper studies the limiting behavior of the average score of a
sequentially selected group of items or individuals, the underlying
distribution of which, , belongs to the Gumbel domain of attraction of
extreme value distributions. This class contains the Normal, Lognormal, Gamma,
Weibull and many other distributions. The selection rules are the "better than
average" () and the "-better than average" rule, defined as
follows. After the first item is selected, another item is admitted into the
group if and only if its score is greater than times the average score
of those already selected. Denote by the average of the first
selected items, and by the time it takes to amass them. Some of the key
results obtained are: under mild conditions, for the better than average rule,
less a suitable chosen function of converges almost surely
to a finite random variable. When ,
and , then
is of approximate order . When , the asymptotic results for
are of a completely different order of magnitude. Interestingly,
for a class of distributions, , suitably normalized, asymptotically
approaches 1, almost surely for relatively small , in probability
for moderate sized and in distribution when is large.Comment: Published in at http://dx.doi.org/10.1214/10-AAP684 the Annals of
Applied Probability (http://www.imstat.org/aap/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Dielectric susceptibility of the Coulomb-glass
We derive a microscopic expression for the dielectric susceptibility
of a Coulomb glass, which corresponds to the definition used in classical
electrodynamics, the derivative of the polarization with respect to the
electric field. The fluctuation-dissipation theorem tells us that is a
function of the thermal fluctuations of the dipole moment of the system. We
calculate numerically for three-dimensional Coulomb glasses as a
function of temperature and frequency
Electroreflectance spectroscopy in self-assembled quantum dots: lens symmetry
Modulated electroreflectance spectroscopy of semiconductor
self-assembled quantum dots is investigated. The structure is modeled as dots
with lens shape geometry and circular cross section. A microscopic description
of the electroreflectance spectrum and optical response in terms of an external
electric field () and lens geometry have been considered. The field
and lens symmetry dependence of all experimental parameters involved in the
spectrum have been considered. Using the effective mass formalism
the energies and the electronic states as a function of and dot
parameters are calculated. Also, in the framework of the strongly confined
regime general expressions for the excitonic binding energies are reported.
Optical selection rules are derived in the cases of the light wave vector
perpendicular and parallel to . Detailed calculation of the Seraphin
coefficients and electroreflectance spectrum are performed for the InAs and
CdSe nanostructures. Calculations show good agreement with measurements
recently performed on CdSe/ZnSe when statistical distribution on size is
considered, explaining the main observed characteristic in the
electroreflectance spectra
The sources of the deep water of the Eastern Mediterranean Sea
The deep water in the two basins of the eastern Mediterranean Sea, the Levant and the Ionian Sea, is identical in temperature and salinity though differing slightly in oxygen content. Contrary to Nielsen\u27s thesis, none of this water is formed in the southern Aegean Sea...
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