456 research outputs found
Physisorption of positronium on quartz surfaces
The possibility of having positronium (Ps) physisorbed at a material surface
is of great fundamental interest, since it can lead to new insight regarding
quantum sticking and is a necessary first step to try to obtain a Ps
molecule on a material host. Some experiments in the past have produced
evidence for physisorbed Ps on a quartz surface, but firm theoretical support
for such a conclusion was lacking. We present a first-principles
density-functional calculation of the key parameters determining the
interaction potential between Ps and an -quartz surface. We show that
there is indeed a bound state with an energy of 0.14 eV, a value which agrees
very well with the experimental estimate of eV. Further, a brief
energy analysis invoking the Langmuir-Hinshelwood mechanism for the reaction of
physisorbed atoms shows that the formation and desorption of a Ps molecule
in that picture is consistent with the above results.Comment: 5 pages, 3 figures, submitte
Deuteron Momentum Distribution in KD2HPO4
The momentum distribution in KD2PO4(DKDP) has been measured using neutron
Compton scattering above and below the weakly first order
paraelectric-ferroelectric phase transition(T=229K). There is very litte
difference between the two distributions, and no sign of the coherence over two
locations for the proton observed in the paraelectric phase, as in KH2PO4(KDP).
We conclude that the tunnel splitting must be much less than 20mev. The width
of the distribution indicates that the effective potential for DKDP is
significantly softer than that for KDP. As electronic structure calculations
indicate that the stiffness of the potential increases with the size of the
coherent region locally undergoing soft mode fluctuations, we conclude that
there is a mass dependent quantum coherence length in both systems.Comment: 6 pages 5 figure
Melting temperature of screened Wigner crystal on helium films by molecular dynamics
Using molecular dynamics (MD) simulation, we have calculated the melting
temperature of two-dimensional electron systems on \AA-\AA helium
films supported by substrates of dielectric constants
at areal densities varying from cm to cm. Our results are in good agreement with the available
theoretical and experimental results.Comment: 4 pages and 4 figure
A new electromagnetic mode in graphene
A new, weakly damped, {\em transverse} electromagnetic mode is predicted in
graphene. The mode frequency lies in the window
, where is the chemical potential, and can be
tuned from radiowaves to the infrared by changing the density of charge
carriers through a gate voltage.Comment: 5 pages, 4 figure
Electrostatic traps for dipolar excitons
We consider the design of two-dimensional electrostatic traps for dipolar
indirect excitons. We show that the excitons dipole-dipole interaction,
combined with the in-plane electric fields that arise due to the trap geometry,
constrain the maximal density and lifetime of trapped excitons. We derive an
analytic estimate of these values and determine their dependence on the trap
geometry, thus suggesting the optimal design for high density trapping as a
route for observing excitonic Bose-Einstein condensation.Comment: 5 pages, 3 figures. This 2nd version contains a revised Fig.3 + minor
revisions to the discussion and abstrac
The healing mechanism for excited molecules near metallic surfaces
Radiation damage prevents the ability to obtain images from individual
molecules. We suggest that this problem can be avoided for organic molecules by
placing them in close proximity with a metallic surface. The molecules will
then quickly dissipate any electronic excitation via their coupling to the
metal surface. They may therefore be observed for a number of elastic
scattering events that is sufficient to determine their structure.Comment: 4 pages, 4 figures. Added reference
Hybrid phase at the quantum melting of the Wigner crystal
We study the quantum melting of the two-dimensional Wigner crystal using a
fixed node quantum Monte-Carlo approach. In addition to the two already known
phases (Fermi liquid at large density and Wigner crystal at low density), we
find a third stable phase at intermediate values of the density. The third
phase has hybrid behaviors in between a liquid and a solid. This hybrid phase
has the nodal structure of a Slater determinant constructed out of the bands of
a triangular lattice.Comment: 5 pages, 4 figure
Detection of Noble Gas Scintillation Light with Large Area Avalanche Photodiodes (LAAPDs)
Large Area Avalanche Photodiodes (LAAPDs) were used for a series of
systematic measurements of the scintillation light in Ar, Kr, and Xe gas.
Absolute quantum efficiencies are derived. Values for Xe and Kr are consistent
with those given by the manufacturer. For the first time we show that argon
scintillation (128 nm) can be detected at a quantum efficiency above 40%.
Low-pressure argon gas is shown to emit significant amounts of non-UV
radiation. The average energy expenditure for the creation of non-UV photons in
argon gas at this pressure is measured to be below 378 eV.Comment: 16 pages, 7 figure
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