9,458 research outputs found
Relativistic N-Boson Systems Bound by Oscillator Pair Potentials
We study the lowest energy E of a relativistic system of N identical bosons
bound by harmonic-oscillator pair potentials in three spatial dimensions. In
natural units the system has the semirelativistic ``spinless-Salpeter''
Hamiltonian H = \sum_{i=1}^N \sqrt{m^2 + p_i^2} + \sum_{j>i=1}^N gamma |r_i -
r_j|^2, gamma > 0. We derive the following energy bounds: E(N) = min_{r>0} [N
(m^2 + 2 (N-1) P^2 / (N r^2))^1/2 + N (N-1) gamma r^2 / 2], N \ge 2, where
P=1.376 yields a lower bound and P=3/2 yields an upper bound for all N \ge 2. A
sharper lower bound is given by the function P = P(mu), where mu =
m(N/(gamma(N-1)^2))^(1/3), which makes the formula for E(2) exact: with this
choice of P, the bounds coincide for all N \ge 2 in the Schroedinger limit m
--> infinity.Comment: v2: A scale analysis of P is now included; this leads to revised
energy bounds, which coalesce in the large-m limi
Oxygen-related traps in pentacene thin films: Energetic position and implications for transistor performance
We studied the influence of oxygen on the electronic trap states in a
pentacene thin film. This was done by carrying out gated four-terminal
measurements on thin-film transistors as a function of temperature and without
ever exposing the samples to ambient air. Photooxidation of pentacene is shown
to lead to a peak of trap states centered at 0.28 eV from the mobility edge,
with trap densities of the order of 10(18) cm(-3). These trap states need to be
occupied at first and cause a reduction in the number of free carriers, i.e. a
consistent shift of the density of free holes as a function of gate voltage.
Moreover, the exposure to oxygen reduces the mobility of the charge carriers
above the mobility edge. We correlate the change of these transport parameters
with the change of the essential device parameters, i.e. subthreshold
performance and effective field-effect mobility. This study supports the
assumption of a mobility edge for charge transport, and contributes to a
detailed understanding of an important degradation mechanism of organic
field-effect transistors. Deep traps in an organic field-effect transistor
reduce the effective field-effect mobility by reducing the number of free
carriers and their mobility above the mobility edge.Comment: 13 pages, 14 figures, to be published in Phys. Rev.
Star formation environments and the distribution of binary separations
We have carried out K-band speckle observations of a sample of 114 X-ray
selected weak-line T Tauri stars in the nearby Scorpius-Centaurus OB
association. We find that for binary T Tauri stars closely associated to the
early type stars in Upper Scorpius, the youngest subgroup of the OB
association, the peak in the distribution of binary separations is at 90 A.U.
For binary T Tauri stars located in the direction of an older subgroup, but not
closely associated to early type stars, the peak in the distribution is at 215
A.U. A Kolmogorov-Smirnov test indicates that the two binary populations do not
result from the same distibution at a significance level of 98%. Apparently,
the same physical conditions which facilitate the formation of massive stars
also facilitate the formation of closer binaries among low-mass stars, whereas
physical conditions unfavorable for the formation of massive stars lead to the
formation of wider binaries among low-mass stars. The outcome of the binary
formation process might be related to the internal turbulence and the angular
momentum of molecular cloud cores, magnetic field, the initial temperature
within a cloud, or - most likely - a combination of all of these. We conclude
that the distribution of binary separations is not a universal quantity, and
that the broad distribution of binary separations observed among main-sequence
stars can be explained by a superposition of more peaked binary distributions
resulting from various star forming environments. The overall binary frequency
among pre-main-sequence stars in individual star forming regions is not
necessarily higher than among main-sequence stars.Comment: 7 pages, Latex, 4 Postscript figures; also available at
http://spider.ipac.caltech.edu/staff/brandner/pubs/pubs.html ; accepted for
publication in ApJ Letter
Energy bounds for the spinless Salpeter equation: harmonic oscillator
We study the eigenvalues E_{n\ell} of the Salpeter Hamiltonian H =
\beta\sqrt(m^2 + p^2) + vr^2, v>0, \beta > 0, in three dimensions. By using
geometrical arguments we show that, for suitable values of P, here provided,
the simple semi-classical formula E = min_{r > 0} {v(P/r)^2 + \beta\sqrt(m^2 +
r^2)} provides both upper and lower energy bounds for all the eigenvalues of
the problem.Comment: 8 pages, 1 figur
Calculating Kaon Fragmentation Functions from NJL-Jet Model
The Nambu--Jona-Lasinio (NJL) - Jet model provides a sound framework for
calculating the fragmentation functions in an effective chiral quark theory,
where the momentum and isospin sum rules are satisfied without the introduction
of ad hoc parameters. Earlier studies of the pion fragmentation functions using
the NJL model within this framework showed qualitative agreement with the
empirical parameterizations. Here we extend the NJL-Jet model by including the
strange quark. The corrections to the pion fragmentation functions and
corresponding kaon fragmentation functions are calculated using the elementary
quark to quark-meson fragmentation functions from NJL. The results for the kaon
fragmentation functions exhibit a qualitative agreement with the empirical
parameterizations, while the unfavored strange quark fragmentation to pions is
shown to be of the same order of magnitude as the unfavored light quark's. The
results of these studies are expected to provide important guidance for the
analysis of a large variety of semi-inclusive data.Comment: 9 pages, 14 figure
Energy bounds for the spinless Salpeter equation
We study the spectrum of the spinless-Salpeter Hamiltonian H = \beta
\sqrt{m^2 + p^2} + V(r), where V(r) is an attractive central potential in three
dimensions. If V(r) is a convex transformation of the Coulomb potential -1/r
and a concave transformation of the harmonic-oscillator potential r^2, then
upper and lower bounds on the discrete eigenvalues of H can be constructed,
which may all be expressed in the form E = min_{r>0} [ \beta \sqrt{m^2 +
P^2/r^2} + V(r) ] for suitable values of P here provided. At the critical point
the relative growth to the Coulomb potential h(r)=-1/r must be bounded by dV/dh
< 2\beta/\pi.Comment: 11 pages, 1 figur
Realistic heterointerfaces model for excitonic states in growth-interrupted quantum wells
We present a model for the disorder of the heterointerfaces in GaAs quantum
wells including long-range components like monolayer island formation induced
by the surface diffusion during the epitaxial growth process. Taking into
account both interfaces, a disorder potential for the exciton motion in the
quantum well plane is derived. The excitonic optical properties are calculated
using either a time-propagation of the excitonic polarization with a
phenomenological dephasing, or a full exciton eigenstate model including
microscopic radiative decay and phonon scattering rates. While the results of
the two methods are generally similar, the eigenstate model does predict a
distribution of dephasing rates and a somewhat modified spectral response.
Comparing the results with measured absorption and resonant Rayleigh scattering
in GaAs/AlAs quantum wells subjected to growth interrupts, their specific
disorder parameters like correlation lengths and interface flatness are
determined. We find that the long-range disorder in the two heterointerfaces is
highly correlated, having rather similar average in-plane correlation lengths
of about 60 and 90 nm. The distribution of dephasing rates observed in the
experiment is in agreement with the results of the eigenstate model. Finally,
we simulate highly spatially resolved optical experiments resolving individual
exciton states in the deduced interface structure.Comment: To appear in Physical Review
Discrete spectra of semirelativistic Hamiltonians from envelope theory
We analyze the (discrete) spectrum of the semirelativistic
``spinless-Salpeter'' Hamiltonian H = \beta \sqrt{m^2 + p^2} + V(r), beta > 0,
where V(r) represents an attractive, spherically symmetric potential in three
dimensions. In order to locate the eigenvalues of H, we extend the ``envelope
theory,'' originally formulated only for nonrelativistic Schroedinger
operators, to the case of Hamiltonians H involving the relativistic
kinetic-energy operator. If V(r) is a convex transformation of the Coulomb
potential -1/r and a concave transformation of the harmonic-oscillator
potential r^2, both upper and lower bounds on the discrete eigenvalues of H can
be constructed, which may all be expressed in the form E = min_{r>0} [ \beta
\sqrt{m^2 + P^2/r^2} + V(r) ] for suitable values of the numbers P here
provided. At the critical point, the relative growth to the Coulomb potential
h(r) = -1/r must be bounded by dV/dh < 2 \beta/\pi.Comment: 20 pages, 2 tables, 4 figure
Promotion of Renewable Energy Sources in the European Union
One of the important goals of European energy policy is to increase the share of renewable energy resources in the energy supply. The instruments used in the member states are not fully compatible with the rules of the European internal market. In a theoretical section, this paper analyses possible different instruments for promoting renewable energy. Some countries use feed in tariffs - using fixed prices to increase incentives for producers to invest into renewable energy, other countries use quantity-based systems like quotas that lead to a premium above the market price. In an institutional analysis we show that on the basis of effectiveness and efficiency considerations for the European Union, in the long-term quantity oriented systems of promotion are preferable if combined with elements of a capacity market. The main reason for this conclusion is that price-based systems cannot give enough incentives for backup capacities necessary to cope with intermittent production of renewable sources. In addition price-based systems violate the basic rules of the open internal European market because feed in tariffs are a considerable barrier for trade of renewable energy products
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